Contrasting groundwater flow systems in comparable African Mountains-role of permeability structure

Topography is widely recognized as a primary control on groundwater flow organization in mountain settings along with Recharge (R) and Hydraulic conductivity (K), yet groundwater simultaneously modifies topography through seepage-driven erosion and sapping. This bidirectional coupling implies that landscapes should record the architecture of subsurface flow. However, the diagnostic value of geomorphic indicators remains poorly constrained and rarely used as a proxy for understanding groundwater flow organization in mountain systems. We evaluate whether contrasting geomorphic signatures are consistent with differences in groundwater-flow organization in two African mountain systems selected to be broadly comparable in first-order climatic and topographic setting, while differing in geological and subsurface architecture: an East African mountain system (Jema) and the Drakensberg–uThukela (UU) transect in South Africa. Using 30 m DEMs and consistent terrain analysis, we quantify complementary geomorphic indicators: amphitheatre-headed valley distribution, slope–area scaling, and channel sinuosity within wetland systems. The analyses show systematic differences in geomorphic organization. In the East African sector, we observe laterally organized sapping features, elevated upper slope–area envelopes (for equivalent drainage area), geomorphic features consistent with spatially focused groundwater discharge—consistent with deeper, regionally integrated groundwater circulation. In the South African UU region, we find dense, highly integrated drainage, limited development of organized sapping related landforms, broad wetland-dominated valley floors, and elevated channel sinuosity—consistent with shallower, more compartmentalized groundwater flow and stronger local groundwater–surface-water coupling. This divergence inverts expectations based on relief alone: the higher-relief UU terrain exhibits shallower flow organization. The geomorphic interpretation is compared with published hydrogeological evidence: environmental isotopes (δ 18 O, δ 2 H, 3 H, 14 C), groundwater residence times (young <10 3 yrs. in the East African system vs. old >10^4 yrs. in the Karoo), salinity distribution, hydrochemical facies evolution, regional piezometry, and structural mapping of dolerite intrusion networks. These observations provide an independent consistency check, although their spatial scales and locations differ. These independent lines of evidence are broadly consistent with the geomorphic interpretation: the East African volcanic aquifers sustain active regional flow including inter-basin connectivity, whereas Karoo aquifers are structurally compartmentalized, depth-disconnected, and dominated by slow moving, brackish groundwater. We hypothesize that dense dolerite intrusions in the Karoo suppress lateral connectivity, whereas layered, interbedded volcanic sequences in the East African system sustain permeability with depth. Differences in depth-integrated permeability architecture provide a plausible explanation for the contrasting flow organization, with geomorphic expression as an emergent, independently assessed landscape-scale signature. The approach is proposed as an exploratory, hypothesis-generating framework for screening groundwater-flow organization in data-scarce mountain environments rather than as a uniquely diagnostic method. These different flow patterns imply need for adaptive groundwater management approaches- basin wide for East African mountain and catchment approaches for UU. The novelty lies not in treating individual geomorphic features as diagnostic of groundwater, but in evaluating whether the combined spatial organization of complementary geomorphic metrics can distinguish contrasting styles and scales of groundwater-flow organization.

There is growing global interest in mountain–valley groundwater systems, especially how excess water over evapotranspiration and groundwater recharge in elevated areas moves to springs, wetlands, rivers, and discharge zones in adjacent lowlands (Condon et al., 2020; Gleeson and Manning, 2008; Manning, 2011; Manning and Solomon, 2005; Markovich et al., 2019; Meybeck et al., 2001; Somers and McKenzie, 2020; Viviroli et al., 2007, 2020; Viviroli and Weingartner, 2004). This is because mountains serve as natural water infrastructure supporting downstream ecosystems, agriculture, hydropower, cities, and livelihoods.

In tropical and subtropical Africa, this role is especially important because many mountain regions are densely populated, agriculturally productive, and ecologically significant1. Understanding groundwater availability and flow in these settings is therefore essential not only for downstream water security, but also for local water supply, wetlands, and ecosystem functioning within the mountain environments themselves.

Groundwater flow theory shows that water can move through the subsurface in shallow, intermediate, and deep flow paths, carrying recharge from higher areas such as mountains toward lower discharge areas such as valleys (Tóth, 1963). In practice, however, identifying these flow paths is difficult because key information is often missing, including how permeability changes with depth, the local geology, water-table levels, and recharge patterns. Field data and modelling evidence suggest lateral groundwater contribution to valley aquifers ranges anywhere from 5 to 50% of recharge waters (Markovich et al., 2019). Recent studies have improved understanding of the key controls on groundwater movement in mountain systems and its connection to adjacent valley aquifers. Groundwater flow in mountain systems is governed by the interaction of hydraulic conductivity (K), recharge (R), and topography, with additional control from geological structure and boundary conditions (Gleeson and Manning, 2008). In the presence of favourable K and R, geology (faults, dykes) can still impose barriers to the lateral connection and limit mountain-valley subsurface connectivity (Belay and Kebede, 2025; Kebede et al., 2008).

K and R control the depth, extent, and persistence of mountain groundwater flow paths. Where K remains high and recharge are relatively low, groundwater can circulate over longer distances, and regional flow component dominates the flows. In contrast, low K and a rapid decline in K with depth, combined with high recharge, tends to restrict flow to shallow, localized flow paths (Carroll et al., 2025; Fan, 2015). Topography controls the hydraulic gradients (i) that drive groundwater flow and determines both the direction of flow and the connection between mountains and adjacent valleys.

However, the role of topography in regional groundwater connectivity is arguable. In hydrogeology, incision-generated topography is widely recognized as a key control on groundwater flow because it creates hydraulic gradients, valley positions, and basin geometry that organize local to regional flow systems (Gleeson and Manning, 2008; Markovich et al., 2019). On the other hand, coupled hydrologic–geomorphic models, show that groundwater and landform co-evolve through feedback among subsurface flow, surface runoff, and erosion processes (Litwin et al., 2022; Zeng et al., 2018; Zhang et al., 2016). These models demonstrate that groundwater influences the long-term development of hillslopes, relief, and drainage structure, a theory supported by a growing body of field-based studies (Chignell et al., 2019; Gomes et al., 2023; Graves et al., 2025; Hilgendorf et al., 2020; Irwin et al., 2014; Mazurek, 2011; Migoń et al., 2023; Miguez-Macho and Fan, 2012; Mohammad et al., 2025; Nash, 1996; Schumm et al., 1995; Tooth, 2018).

For example, groundwater induced sapping (incision) has been evoked as a process that modifies valleys and leave lasting geomorphic features in the landscape (Mohammad et al., 2025; Nash, 1996; Schumm et al., 1995; de Sordi et al., 2022). This means that topography should be viewed not only as a static control on groundwater flow, but also as part of a coupled hydrologic–geomorphic system that is shaped by sub surface flow. This dual role of topography—as both a control on and a product of groundwater flow—provides a theoretical basis for using geomorphic indicators such as slope–area relationships, drainage density, channel sinuosity, valley form, wetlands, and amphitheatre-shaped landforms to infer groundwater-flow patterns. However, the applicability and limitations of these indicators still require broader evaluation across different geological and climatic terrains.

This approach is relevant for mountain groundwater systems, where direct subsurface data on hydraulic conductivity, recharge, water-table depth, and flow pathways are limited. The problem is especially acute in African mountains, where regional hydrogeological characterization remains sparse. Topography and geomorphic indicators therefore offer a practical framework for inferring groundwater flow patterns and their environmental significance where direct measurements are unavailable. The objective is not to prove regional groundwater flow from geomorphology alone, but to assess whether independent geomorphic indicators, interpreted together with existing hydrogeological knowledge, are consistent with contrasting groundwater-flow organization in two comparable African mountain systems. This study does not attempt to directly quantify groundwater fluxes but instead evaluates groundwater system organization through integrated geomorphic signatures, compared against independent hydrogeological literature evidence.

Using geomorphic indicators, this study compares two stratigraphically and climatologically comparable African mountain systems in Ethiopia and South Africa to understand groundwater-flow organization and investigate controls the flow patterns influence lateral hydraulic connectivity.

The contribution of this study is not the identification of individual geomorphic features associated with groundwater, many of which are already well recognized and are non-unique when considered alone. Rather, we test whether their combined spatial organization provides a coherent landscape-scale signature of groundwater-flow organization. Specifically, we evaluate whether an ensemble of sapping-related landforms, slope–area behaviour, drainage density and channel sinuosity can distinguish a more laterally integrated groundwater system from one dominated by shallow, locally coupled and compartmentalized flow. The approach is intended as a first-pass, hypothesis-generating framework rather than a uniquely diagnostic method.

Theory to field application

Theoretically, the co-evolutionary perspective of hydrology and geomorphology provides a theoretical basis for using geomorphology to infer groundwater-system organization in mountain terrains. For example, Litwin et al. (2022) and Zeng et al. (2018) show that groundwater flow can strongly influence drainage morphology and landscape organization, supporting the use of geomorphic indicators to infer subsurface processes. Accordingly, features such as sapping, slope–area relationships, channel planform, sinuosity, channel spacing, and drainage density can be treated as first-order indicators of groundwater influence on landscape development, and therefore as indirect evidence for groundwater-flow patterns.

Theatre shaped valleys: One recognizable landform shaped by groundwater sapping or undercutting of the aquifers is theatre shaped valleys. In the classic drainage evolution by headward erosion and concurrent seepage–undermining model, groundwater issues from a seepage face at a scarp or valley head, undercuts the slope, and causes collapse and headward retreat, producing prominent amphitheatre-shaped valleys with steep headwalls (Abotalib et al., 2016; Mastronuzzi and Sansò, n.d; Mazurek, 2011; Mohammad et al., 2025; Nash, 1996; Schumm et al., 1995; de Sordi et al., 2022; Hilgendorf et al., 2020). Typical sapping features (Figure 1a) carved by groundwater include abrupt amphitheatre-headed valleys, steep bedrock sidewalls, flat floors, constant valley width, sparse tributary development, and the presence of springs, seepage zones, or alcoves near valley heads. Well-developed and laterally organized sapping morphology suggests groundwater systems capable of sustained and spatially focused discharge, implying stronger subsurface connectivity and more regionalized flow organization than would be expected from shallow flow systems. In contrast, surface runoff–and shallow groundwater dominated systems are characterized by dense and highly integrated drainage networks, closely spaced tributaries, uniformly dissected slopes, and V-shaped valleys produced by distributed fluvial incision rather than localized headward retreat (Figure 1b).

Slope-area relation: Slope–area relationships help distinguish surface runoff-dominated from groundwater-influenced erosion. In this framework, local slope reflects incision and sediment-transport processes, while contributing area represents accumulated flow. Coupled hydrologic–geomorphic models show that subsurface flow can modify slope–area scaling by delaying runoff generation, focusing erosion at seepage zones, and maintaining steeper slopes for a given drainage area. Thus, groundwater-influenced landscapes may show elevated slope–area envelopes, lower drainage density, and selective incision, whereas runoff-dominated systems tend to show more uniform slope decline and denser drainage networks (Litwin et al., 2022; Zhang et al., 2016).

Sinuosity in wetland streams: In river–wetland systems, channel sinuosity is not governed solely by discharge and sediment dynamics but emerges from the coupled interaction between surface flow and groundwater exchange. Wetlands represent zones of persistent surface–groundwater interaction where high water tables, lateral connectivity, and delayed storage modify bank strength, floodplain saturation, and flow continuity (Tooth, 2018; Tooth et al., 2002; Tooth and McCarthy, 2007; van Balen et al., 2008). Within such settings, groundwater contributes to meander development by enhancing bank erodibility through seepage, focusing erosion at outer bends, and sustaining baseflow that allows gradual lateral migration. At the same time, the spatial organization of groundwater flow—controlled by aquifer properties and floodplain geometry—can influence the characteristic wavelength, depth and sinuosity of meanders by selectively amplifying bends at scales comparable to the groundwater flow system (Wittmann, 2022).

Consequently, groundwater-dominated wetlands with long hydroperiods are typically expressed as broad, low-gradient, weakly incised landscapes with diffuse drainage, shallow channels, backswamps, oxbows, abandoned channels, and depressions that remain wet because of persistent saturation and lateral groundwater exchange (Figure 2a). When the water table is naturally deep or falls far below the wetland surface, these landforms lose saturation, wetlands contract, drainage becomes more clearly channelized, and incision, gully formation, or channel simplification may dominate (Figure 2b).

In summary, the four indicators were selected because they represent complementary expressions of groundwater–landscape interaction at different scales: amphitheatre-headed valleys represent localized seepage-related headward erosion; slope–area relationships characterize systematic modification of channel gradients; drainage density represents catchment-scale partitioning between surface and subsurface drainage; and wetland-channel morphology reflects shallow groundwater–surface water coupling. Metrics such as hypsometry, valley width and lineament density were not included because they are less directly linked to the groundwater–geomorphic mechanisms tested here or primarily characterize geological structure rather than its integrated landscape response.

Study-site selection and comparative framework

The study compares groundwater-related geomorphic features in two African mountain systems: the Ethiopian Highlands and the Drakensberg–Indian Ocean transect. These regions were selected because they are broadly comparable in relief (Figure 3), stratigraphy, and geologic origin (Figure 4). Both have been shaped by mantle plume–related uplift, volcanism, and multi-stage denudation (Decker et al., 2013; Gilfillan et al., 2019; Halldõrsson et al., 2014; Moore et al., 2009; Mulugeta et al., 2007; Pik et al., 2006a; Stewart and Rogers, 1996; van der Beek et al., 2002), and both contain basement rocks, layered sedimentary units, and volcanic plateau sequences (Figure 4). The purpose is not to isolate one controlling variable, but to evaluate whether contrasting subsurface architecture is reflected in contrasting groundwater related geomorphic expression.

To reduce influence of climate-driven recharge differences on geomorphic features, the comparison was restricted to subregions with similar aridity index, rainfall regime, relief, and broad stratigraphic setting (Figures 3 and 4). Areas strongly affected by dense rift faulting or large shield volcanoes were avoided to minimize major tectonic contrasts. Aridity index values from (Zomer et al., 2022) were used as a proxy for climatic water balance and first-order recharge conditions. Since the balance between recharge (R) and hydraulic conductivity (K) is a primary control on groundwater-flow organization (Gleeson and Manning, 2008; Markovich et al., 2019), constraining recharge allows differences in geomorphic expression to be interpreted more directly in terms of subsurface controls, including K, lithology, geological structure, and flow barriers.

Based on this approach, the Jema basin and surrounding central to south-central Ethiopian Highlands were selected as the closest analogue to the humid-semi arid, dissected Drakensberg escarpment–to–Indian Ocean transect encompassing the uThukela–uMngeni (UU) region in southeastern South Africa. Under these broadly comparable climatic, stratigraphic, and relief conditions, differences in drainage form, valley morphology, wetland stream sinuosity, springs, and sapping-related landforms are interpreted as integrated geomorphic expressions of contrasting groundwater-flow organization.

Subsurface contrast between the two mountains

The key contrast is that the Ethiopian Highlands are younger, more recently (Cenozoic) uplifted, and more deeply dissected, whereas the mountains of South Africa are older, and generally only moderately dissected (Meybeck et al., 2001). This difference is also reflected in denudation rates, which are relatively high in Ethiopia (Abbate et al., 2014, 2015; Ismail and Abdelsalam, 2012) but markedly lower in South Africa (Decker et al., 2013; van der Beek et al., 2002; Ismail and Abdelsalam, 2012).

In the Ethiopian Highlands, the volcanic basalt cap is typically thick, internally layered, and lithologically variable (Pik et al., 2006b). It includes stacked lava flows with abundant flow contacts, scoriaceous basalts, volcanic ash, ignimbrites, and interbedding of inter-trappean sediments (Abbate et al., 2014; Zemelak et al., 2024). On the other hand, the Karoo–Drakensberg basalt succession is composed of thick, stacked and laterally extensive lava flows, with relatively uniform upper basalt units overlying more heterogeneous lower lava units. Individual flows commonly contain massive interiors, chilled margins, and repeated flow contacts, while major sedimentary interbeds and well-developed weathering horizons appear limited within the main lava pile (Harris et al., 2026a).

In the South African Drakensberg–UU region, the sedimentary formation dominated mainly by interbedded shale, mudstone, siltstone, and sandstone units (Harris et al., 2026b). While the Ethiopia sedimentary formation is predominantly limestones and sandstones with minor shale (Abbate et al., 2015).

Another major contrast concerns the abundance of dolerite sills and dykes. The African Karoo Basin contains an exceptionally dense and laterally extensive network of dolerite dykes and sills, locally reaching up to 30% by volume. These intrusions are highly interconnected and anastomosed, forming a shallow stockwork-like intrusive system rather than isolated bodies (Chevallier and Woodford, 1999). In Ethiopia, by contrast, dyke emplacement occurs mainly as discrete plume-related swarms rather than as a continuous intrusive network. Although locally concentrated, cumulative dyke dilation is generally around 1% or slightly higher (Mège & Korme, 2004).

Geomorphic indices: sapping, sinuosity, slope-area ratio determinations

All geomorphic indicators were derived using Python from the same SRTM 30 m DEM source and spatial resolution for both study areas to ensure methodological consistency. The DEMs were projected to the appropriate metric coordinate system (UTM) and hydrologically conditioned using identical sink-filling, flow-direction, and flow-accumulation procedures. The 30 m resolution is appropriate for the regional-scale drainage and major valley-head morphology targeted here, although it cannot resolve small seepage faces, narrow wetlands, or fine-scale headwall features; the analysis therefore focuses on landscape-scale geomorphic organization rather than individual seepage features. Potential sapping-related landforms were identified from DEM-derived slope and curvature by selecting high-slope areas (≥90th percentile) and strong concave–convex curvature (≥80th percentile), with spatial continuity evaluated within a 9 × 9-cell neighbourhood and small isolated patches excluded. These extracted forms are interpreted as potential sapping-related landforms based on their morphology and spatial organization, rather than direct evidence of active groundwater discharge, and are evaluated together with the other geomorphic and independent hydrogeological evidence. Drainage networks were extracted using a common contributing-area threshold of 1 km 2 (~1,111 cells), and drainage density was calculated as total stream length per unit catchment area. Channel sinuosity was calculated from vectorized stream segments as the ratio of channel length to straight-line end-to-end distance, excluding segments <200 m. Slope–area relationships were derived from channel pixels only, with contributing area converted to m 2 and very low slopes (<0.001) removed before plotting and analysis.

Sapping features: The DEM-derived extraction of potential sapping features (Figure 5a) highlights a clear contrast between the UU and Jema regions in both spatial distribution and geomorphic expression. In the UU region, identified features are sparse, fragmented, and largely confined to upper catchment zones. These occur as small, isolated concave headwater hollows and short, disconnected tributaries that do not significantly influence the broader drainage structure. The overall drainage network in UU is dense and well-integrated, with closely spaced channels and uniform headwater development, indicating that runoff-driven fluvial incision dominates landscape evolution. The limited extent and discontinuity of extracted features suggest that groundwater-related sapping processes are localized and subordinate, operating primarily at small scales near seepage zones rather than controlling escarpment morphology.

In contrast, the Jema Valley sector (Figure 5b) exhibits more prominent and spatially organized sapping features. The extracted patterns define a series of laterally aligned, deeply indented valley heads along the northern plateau margin, forming a discontinuous but coherent zone of headward retreat. These features are associated with steep escarpment slopes, low tributary density, and pronounced amphitheatre-like embayment, consistent with focused erosion at discrete points rather than uniform fluvial dissection. The relative sparsity of the drainage network, combined with the concentration of concave headwall features, indicates selective headward erosion likely driven by groundwater discharge along lithologic or structural contacts.

Overall, the comparison shows that while UU is characterized by widespread fluvial dissection with only minor, localized groundwater expression, Jema displays more developed and laterally organized sapping features. This suggests a stronger role of groundwater in controlling slope retreat and valley evolution in Jema, consistent with a more connected subsurface flow system, whereas in UU groundwater influence remains spatially restricted and secondary to surface runoff processes.

Slope-area relationship: This interpretation follows coupled hydrologic–geomorphic modelling studies showing that subsurface flow can modify slope–area scaling by influencing incision, saturation thresholds, and channel initiation. The channel-only slope–area distributions (Figure 6) for both catchments define a broadly similar envelope but show a systematic offset in their upper bounds. For a given contributing area, the Jema catchment consistently exhibits higher channel slopes than the UU system, expressed as an upward displacement of the slope–area cloud. This difference is most pronounced in the intermediate drainage-area range (~10 6 –10 8 m 2), where Jema displays a distinct population of high-slope channel segments that are largely absent or less developed in UU. Rather than forming a separate cluster, this groundwater signal appears as an expansion of the upper envelope, indicating more frequent occurrence of steep channel segments under comparable drainage conditions.

This upward shift is consistent with theoretical expectations for landscapes influenced by subsurface flow, where groundwater emergence can promote localized incision and sustain steeper gradients. The elevated upper envelope in Jema is interpreted in this framework, where groundwater influence is expected to appear not as a separate class of points, but as a systematic modification of the slope–area relation through seepage-enhanced incision and delayed surface runoff initiation. In contrast, the UU distribution remains more tightly constrained, with slopes generally declining more uniformly with increasing drainage area, reflecting a system dominated by runoff-driven incision and diffusive hillslope processes. Notably, although UU exhibits higher overall relief, it does not show a comparable extension into higher slope values at equivalent contributing areas, indicating that relief alone does not control slope–area behaviour; rather, process controls associated with groundwater influence are required to explain the observed divergence.

The slope–area patterns are consistent with the broader drainage characteristics observed in the DEMs. The uMngeni–uThukela region exhibits a dense and highly integrated drainage network, with closely spaced tributaries and widespread dissection across the plateau–escarpment surface. In contrast, the Jema sector is characterized by wider interfluves and more selective incision focused along major valley margins. Together, these patterns indicate that while UU is dominated by pervasive surface runoff processes, Jema shows a more organized subsurface influence, expressed through localized steepening and concentration of incision—consistent with a stronger role of groundwater in shaping channel and valley development.

Wetlands geomorphology and stream sinuosity: The comparison of wetland and channel features based on sinuosity computed at each stream order level (Figure 7) and image of typical wetlands taken from GoogleEarth™ (Figures 8A,B) show a clear contrast between the two study areas. In the South African UU region, wetlands are laterally extensive and occupy broad, low-gradient valley floors associated with shallow, weakly incised, and highly sinuous channels. The presence of floodplain depressions abandoned meanders, oxbow-like forms, and relict channels indicates active floodplain storage and strong surface–groundwater interaction.

In the Jema region, wetlands are more limited and discontinuous. They occur mainly as small depressions or localized wet valley-floor pockets, while the main channels are more confined and show fewer abandoned meanders or oxbow features. This indicates less lateral floodplain connectivity compared with UU.

Overall, the UU system shows a shallow, laterally connected groundwater–surface water system expressed through wetlands, high sinuosity, and relict floodplain channels. In contrast, Jema shows more discrete wetland expression and more focused groundwater discharge, consistent with deeper or more localized groundwater connectivity rather than broad floodplain saturation.

Taken together, the geomorphic indicators point to two different expressions of groundwater influence. In the Jema sector, organized sapping features, selective incision, and elevated slope–area envelopes suggest focused groundwater discharge and stronger lateral subsurface connectivity. In the UU region, dense drainage, laterally extensive wetlands, high channel sinuosity, and limited sapping expression suggest a shallower groundwater system more closely coupled to local valley floors, wetlands, and runoff processes. The contrast is therefore not between groundwater presence and absence, but between different groundwater architectures and their geomorphic expression.

Geological and structural controls on groundwater-flow organization

The combined geomorphic and hydrogeological evidence suggests that the primary distinction between the two mountain systems is not simply groundwater occurrence, but the degree of lateral subsurface connectivity and the extent to which groundwater circulation is organized at regional scales. Although both terrains share a broadly similar stratigraphic arrangement—basement rocks at depth, multilayered sedimentary units in the middle, and volcanic sequences above—the observed differences in geomorphic expression appear more consistent with contrasts in volcanic architecture, intrusive structure, weathering, and subsurface connectivity than with broad lithologic differences alone. Thus, depth-integrated permeability architecture is not treated here as independent of lithology and structure, but as their resulting hydrogeological expression, reflecting differences in emplacement, interflow development, fracturing, weathering, and intrusive architecture.

These geomorphic contrasts are not considered uniquely diagnostic of groundwater, as drainage and valley morphology may also reflect lithology, tectonics, denudation history, climate, and vegetation. The groundwater interpretation is therefore based on the convergence of multiple geomorphic indicators and is subsequently evaluated against independent hydrogeological evidence (Section 4.2), rather than being inferred from any individual landform characteristic.

In the South African Drakensberg–Karoo terrain, the volcanic succession consists of thick and relatively uniform flood-basalt flows intruded by an extensive network of dolerite dykes and sills (Duncan et al., 1997). Massive flow interiors, limited interflow sedimentary horizons, extensive intrusive networks, and widespread alteration of basaltic units together suggest a comparatively compartmentalized subsurface structure. Hydrogeological studies from the Karoo further indicate that groundwater is commonly concentrated within shallow weathered and fractured zones, while dolerite intrusions frequently subdivide the aquifer system into smaller hydraulic compartments. Although dolerite contacts may locally enhance permeability through baking, shrinkage fracturing, and shear-related openings, the intrusions may also act as barriers across strike, particularly at depth where fracture apertures decline. The resulting groundwater system is therefore generally interpreted as shallow, discontinuous, and structurally segmented (Belay and Kebede, 2025; Fourie et al., 2020).

The limited development of organized sapping features in the Karoo terrain is also consistent with groundwater flow being largely restricted to shallow levels and with permeability decreasing rapidly with depth. Tectonic quiescence and low denudation rates (Decker et al., 2013) imply limited exposure of fresh rock and relatively low overall weathering intensity, producing a weathering-limited system. Under such conditions, permeability is expected to be strongly controlled by the depth of weathering, increasing near the surface but declining rapidly downward as weathering effects diminish and fracture apertures close under compression (Worthington, 2024). Consequently, groundwater flow is inferred to remain concentrated within shallow weathered zones and discrete secondary structures such as bedding-plane fractures, rather than forming laterally extensive deep flow systems (Belay and Kebede, 2025). Localized zones of enhanced permeability may nevertheless develop where weathering and fracturing are concentrated, for example within weathered dolerite sills beneath wetlands where alteration can extend beyond ~7 m depth (Edwards et al., 2016). Such settings may support shallow diffuse groundwater flow and contribute to wetland persistence and meandering channel development, but these effects appear spatially restricted and bounded by more resistant rock units.

This interpretation is broadly consistent with the geomorphic patterns observed in the UU region. The dense and integrated drainage network, limited development of laterally organized sapping features, and prevalence of wetlands associated with shallow valley-floor saturation suggest that groundwater influence is expressed mainly through shallow and localized flow systems rather than through extensive regional circulation. The near absence of regionally organized amphitheatre-headed valleys further supports the interpretation that groundwater discharge is spatially restricted and insufficient to sustain large-scale seepage-driven escarpment retreat.

In contrast, the Ethiopian volcanic succession is composed of layered flood basalts interbedded with scoriaceous flow tops, vesicular horizons, volcanic ash, ignimbrites, and local intertrappean sediments (Abbate et al., 2014; Ayalew et al., 2021). The volcanic pile is therefore structurally more heterogeneous and internally layered than the Karoo succession. Laterally extensive interflow zones and pyroclastic horizons may provide pathways for more laterally organized groundwater movement (Kebede et al., 2005). In addition, weakly welded pyroclastic materials are susceptible to piping and seepage-related erosion, processes that are consistent with the more pronounced amphitheatre-headed valleys and selective slope retreat observed in the Jema sector.

Intercomparison of groundwater flow within the underlying sedimentary formations of the two terrains remains difficult because the Ethiopian sedimentary units are largely buried beneath the volcanic pile and direct hydrogeological evidence remains limited. Nevertheless, the contrasting geomorphic signatures of the two terrains are broadly consistent with differing styles of groundwater-flow organization. In the Ethiopian case, the combination of layered volcanics, interflow zones, and comparatively open structural connectivity appears compatible with more laterally connected groundwater circulation and stronger groundwater influence on valley development. In the South African terrain, by contrast, the dense intrusive network and shallow weathering-controlled groundwater occurrence appear more consistent with compartmentalized flow, localized wetland formation, and runoff-dominated dissection.

The role of dolerite intrusions is particularly important in this regard. In the Karoo terrain, dolerites appear to function both as local conduits and as cross-cutting barriers that reorganize groundwater flow paths. Recent remote-sensing analyses from the Main Karoo Basin suggest that barrier-controlled behaviour is more common than conduit behaviour along dolerite dykes, indicating that many intrusions act primarily to deflect and compartmentalize groundwater flow rather than sustain vertically integrated regional circulation. Under such conditions, wetlands and shallow wet zones do not necessarily imply discharge from deep regional systems, but may instead reflect shallow groundwater storage, perched flow systems, and localized redistribution along fractures and intrusive contacts.

Overall, these geological differences are likely to translate into contrasting effective transmissivity at the landscape scale. Permeability architecture is not directly measured in this study and is therefore treated as an inferred hydrogeological control. Its interpretation is based on the geological and structural characteristics of the two systems and is subsequently evaluated against published hydrogeological evidence in Section 4.2. The layered volcanic architecture of the Ethiopian Highlands is consistent with relatively higher transmissivity and more laterally connected flow paths, allowing groundwater to circulate at greater depth and over longer distances. In contrast, the massive basalt flows, shallow weathering profile, and dense dolerite intrusion network of the South African Karoo terrain are consistent with lower effective transmissivity and stronger compartmentalization. The net effect is inferred to be a deeper and more regionalized groundwater-flow system in Ethiopia, compared with a shallower, more tightly coupled groundwater–surface water system in South Africa.

Hydrogeological evidence consistent with geomorphic inference

The evidence evaluated below is derived from published site- to regional-scale hydrogeological studies within the two study settings and is independent of the geomorphic analysis undertaken here. These studies differ in spatial coverage, analytical methods, and observation periods and are therefore used to assess consistency with the geomorphic interpretation. The geomorphic analysis presented above provides a first-order interpretation of groundwater-flow organization in the two study areas. To assess the robustness of this interpretation, the inferred patterns are compared here with independent hydrogeological evidence from the literature, including isotope studies, hydrochemistry, piezometric observations, groundwater age dating, and groundwater–surface water interaction studies. The aim is not to comprehensively review these systems, but rather to evaluate whether existing hydrogeological observations are broadly consistent with the geomorphic patterns identified in this study.

The hydrogeological literature from Ethiopia generally supports the interpretation of relatively strong regional groundwater connectivity. In the classical Tóthian framework, regional groundwater systems are characterized not only by deeper and longer flow paths, but also by the tendency for groundwater flow to become partly decoupled from local topography and surface drainage. In Ethiopia’s volcanic and tectonically integrated terrains, multiple studies indicate that groundwater commonly moves beyond local catchment boundaries through laterally continuous volcanic aquifers and structurally controlled pathways (Azagegn et al., 2015a; Bretzler et al., 2011c; Darling et al., 2026; Kebede et al., 2005; Yitbarek et al., 2012). Investigations from the Upper Awash Basin and the Main Ethiopian Rift suggest that groundwater flow may occur both transverse and longitudinal to the rift, and that inter-basin transfer across surface-water divides can occur through connected volcanic aquifers (Azagegn et al., 2015b; Yitbarek et al., 2012) documented groundwater transfer from the Blue Nile side toward the Awash Basin through basaltic aquifers, while (Bretzler et al., 2011a) showed that deep rift-floor waters are largely tritium-free and regionally connected along faulted volcanic blocks. More recent work from the Upper Awash similarly indicates that groundwater systems regional flow paths often several tens of kilometres extending beyond local surface divides (Darling et al., 1996).

These hydrogeological observations are broadly consistent with the geomorphic evidence from the Jema sector, including selective incision, organized sapping features, and more spatially focused groundwater expression. Together, they suggest a groundwater system in which lateral subsurface connectivity plays a significant role in organizing valley development and groundwater discharge patterns.

Regional groundwater connectivity also appears to influence hydrologic and ecological processes within the Ethiopian Rift system more broadly. Several groundwater modelling studies have suggested that long distance groundwater circulation contributes to the persistence of springs far away from recharge site, sustains springs and wetlands in structurally favourable discharge zones (Belay et al., 2023; Abera et al., 2016), and locally buffers hydrologically closed basins and thereby serving as hydro-refugia playing role in human evolution and dispersal (Cuthbert et al., 2017). In parts of the Awash Basin, isotopic and piezometric evidence shows groundwater and surface water appear partly decoupled (Ayenew et al., 2008), with the river functioning mainly as a losing system while groundwater follows longer regional pathways before re-emerging several tens of kilometres away (Kebede et al., 2021). Studies from the Bale Mountains further indicate that although upper headwater channels may be ephemeral, distant groundwater discharge contributes to the maintenance of perennial wetlands and downstream channels (Chignell et al., 2019). Isotopic and Geochemical evidence also shows the presence of deep regional flows contributing to deep groundwater outflow from Lakes and contributing to maintaining unusually low lake salinity for closed lakes under high evaporation (AET > 1.5 xPPT) (Craig et al., 1977; Darling et al., 1996).

In contrast, the hydrogeological literature from the South African Karoo more commonly emphasizes compartmentalization and localized flow systems. Flow is mainly matrix-fed and fracture-controlled, with important vertical leakage from low-permeability rock matrix into bedding-parallel fractures (Botha et al., 1998). Flow is confined to thin shallow weathered zone, inclined sheets and discrete fractures, with low discharge springs and seeps commonly emerging where these structures intersect the surface (Chevallier et al., 2004). Because these fractures are sparse and weakly connected, lateral hydraulic connectivity is limited. Groundwater systems are therefore often more tightly coupled to local topography, wetlands, springs, and stream baseflow than to distant regional discharge zones. Recent work from the Eastern Karoo explicitly characterizes groundwater flow as discontinuous and structurally compartmentalized, flow lines mirroring local topography all signs of lack of regional connectivity, with limited spatial coherence across topographic gradients (Fourie et al., 2020).

This interpretation is broadly consistent with the geomorphic patterns observed in the UU region, where wetlands, shallow valley-floor saturation, dense drainage networks, and limited sapping development collectively suggest dominance of shallow groundwater circulation and strong groundwater–surface water coupling at local scales. The prevalence of laterally extensive wetlands and meandering floodplain systems further supports the interpretation of shallow storage-dominated groundwater systems rather than deeply integrated regional flow systems.

A further contrast is provided by groundwater residence time and groundwater salinity. In the Karoo, deeper and sub-thermal groundwaters are commonly old, with low radiocarbon, little or no tritium, and residence times that may extend to tens of thousands of years (upto 20,000 years; Murray et al., 2015). This suggests sluggish renewal and slow vertical exchange between deep and shallow groundwater systems. The occurrence of high-salinity groundwater at depth further supports long residence times and limited flushing. In contrast, groundwater from comparable or even deeper volcanic aquifers in the central Ethiopian plateau is generally much younger, commonly on the order of hundreds to a few thousand years, and remains relatively fresh (Bretzler et al., 2011b; Darling et al., 2026; Hailu et al., 2023; Kebede et al., 2008). This implies faster groundwater circulation, greater lateral connectivity, and more effective flushing in the Ethiopian volcanic system. Thus, the contrast in groundwater age and salinity is consistent with the geomorphic interpretation: deeper regional flow is more active and laterally integrated in Ethiopia, whereas deep Karoo groundwater is older, more sluggish, and more weakly connected at deeper level to active recharge.

Implications for groundwater management and conceptualization

The management significance of the comparison lies in the distinction between local groundwater occurrence and regional groundwater organization. Both mountain systems contain groundwater, springs, wetlands, and groundwater–surface water interaction. However, the scale at which groundwater systems are connected appears to differ. This distinction matters because the appropriate scale of groundwater assessment, monitoring, and management depends on whether recharge, storage, and discharge are organized locally, at sub-catchment scale, or through longer regional pathways.

The contrasting geomorphic and hydrogeological patterns identified in this study suggest fundamentally different styles of groundwater organization in the Ethiopian and South African mountain systems. In the Ethiopian Highlands, structurally connected volcanic aquifers appear to support more laterally integrated groundwater circulation, allowing recharge to move beyond local surface-water divides and re-emerge far from recharge areas. In contrast, the South African Karoo terrain appears more strongly compartmentalized by dolerite intrusions, with groundwater systems more tightly coupled to local wetlands, streams, and topographic gradients.

These differences have important implications for groundwater assessment and management. In Ethiopia, groundwater availability may depend not only on local recharge, but also on mountain-block recharge and long-distance lateral inflow through connected volcanic aquifers. Under such conditions, local abstraction may partly depend on recharge originating tens of kilometres away, implying that groundwater sustainability cannot be evaluated solely from local water balances. Distinguishing between local and regional recharge components therefore becomes important for assessing long-term resource resilience and avoiding overestimation of locally renewable groundwater resources.

In the South African Karoo, by contrast, aquifer compartmentalization suggests that groundwater systems are more strongly controlled by local recharge, shallow storage, and catchment-scale groundwater–surface water interaction. Dolerite intrusions commonly restrict lateral connectivity and reinforce localized groundwater circulation. Under such conditions, groundwater management may be more effectively approached through local to sub-catchment water balances, recharge estimation, wetland protection, and monitoring of streamflow response to abstraction.

The contrasting groundwater architectures also imply different modelling and monitoring requirements. Ethiopian volcanic systems may require regional-scale conceptualization that incorporates structural controls, inter-basin groundwater transfer, and separation of shallow and deeper flow systems, supported by isotopic and hydrochemical evidence. In contrast, compartmentalized Karoo systems may be more appropriately represented using local to intermediate-scale models focused on groundwater–surface water coupling, storage behaviour, and structurally bounded flow compartments.

More broadly, the findings support a conceptual framework in which groundwater influence on landscape evolution depends on the interaction among recharge, transmissivity, structural pathways, weathering, and relief. These factors influence whether groundwater systems remain shallow and tightly coupled to local topography, or evolve into more laterally connected regional systems capable of exerting stronger geomorphic influence through selective incision, sapping, and focused discharge. In this sense, geomorphic expression may provide an integrated indicator of groundwater-system organization in mountain environments where direct subsurface information remains limited.

Limitations and transferability

This study provides a qualitative, first-pass interpretation of groundwater-flow organization from landscape-scale geomorphic expression and does not directly quantify groundwater fluxes, hydraulic conductivity, transmissivity, water-table depth, or flow-path depth. The interpretation is therefore indirect and is evaluated against existing hydrogeological evidence rather than new field measurements. The common 30 m DEM provides a consistent basis for regional comparison but cannot resolve small seepage faces, narrow wetlands, or fine-scale valley-head morphology, and the automatically identified amphitheatre-like features are therefore treated as potential sapping-related landforms rather than confirmation of active groundwater discharge. The number and geometry of extracted potential sapping-related features may also vary with the selected slope, curvature, neighbourhood, and minimum-area thresholds, although identical criteria were applied to both regions to maintain comparative consistency. Although the comparative site-selection approach constrains first-order differences in aridity, relief, and broad geological architecture, differences in rainfall seasonality, extreme events, evapotranspiration, vegetation, tectonic and denudational history, and internal geological structure cannot be completely controlled.

As with other indirect hydrogeological tools, including geophysics, environmental isotopes, and hydrochemistry, geomorphic indicators are non-unique when interpreted in isolation. Their value lies in providing an additional, spatially extensive line of evidence that can be integrated with geological and hydrogeological observations to constrain plausible groundwater-flow organization and identify areas or processes requiring further investigation. The approach should therefore be regarded as a hypothesis-generating and first-pass diagnostic framework, particularly useful in data-scarce mountain regions, rather than as a universally diagnostic method. Its application to other settings requires calibration against the local geological, climatic, geomorphic, and hydrogeological context.

This study demonstrates the value of geomorphology as a first-order screening tool for interpreting groundwater-flow organization in data-scarce mountain environments. Consistent with the theory-to-field framing of the paper, features such as sapping, slope–area relationships, drainage density, wetlands, and channel sinuosity provide useful indirect evidence of how groundwater systems may be organized where direct subsurface information is limited.

The comparison between the Ethiopian Highlands and the South African UU region suggests contrasting groundwater-flow styles under broadly comparable aridity, relief, and stratigraphic settings. The Ethiopian system is consistent with stronger regional groundwater organization, expressed through selective incision and sapping-related morphology. In contrast, the South African system is more consistent with shallow, compartmentalized groundwater closely coupled to wetlands, local streams, and runoff-dominated dissection.

The study is qualitative rather than quantitative and does not directly measure hydraulic conductivity, transmissivity, recharge, water-table depth, or groundwater fluxes. The geomorphic signatures may also reflect both long-term landscape evolution and present-day hydrological forcing, including palaeoclimate and inherited landforms, making the interpretations partly non-unique. In addition, the use of slope–area scaling as a groundwater indicator remains more strongly supported by theory and modelling than by broad field testing, so it is treated here as supporting evidence rather than a stand-alone diagnostic. Future work should test these interpretations using dated geomorphic markers, palaeoclimate data, field hydrogeology, tracers, geophysics, and numerical modelling. Nevertheless, the results reinforce the value of viewing groundwater both as a driver of landscape evolution and as a system whose organization can be partly inferred from landscape form where direct data remain sparse.

The principal contribution of this study is therefore not the use of any single geomorphic feature as a groundwater indicator, but the evaluation of multiple complementary geomorphic signatures as an integrated expression of groundwater-flow organization. Their combined spatial organization provides a testable basis for distinguishing contrasting styles and scales of groundwater circulation, while recognizing that individual indicators remain non-unique. This ensemble approach offers a simple first-pass framework for generating hydrogeological hypotheses in data-scarce mountain environments that can subsequently be tested using field hydrogeology, tracers, geophysics and modelling.