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

Résumé

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.

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