Geomorphic adjustment, sensitivity, and connectivity of the lower Mara River and its floodplain wetlands, Tanzania
Date
2024
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Abstract
The Mara River in semiarid East Africa traverses one of the world’s most important wilderness conservation regions. Like many other dryland rivers, the Mara sustains wetland and terrestrial ecosystems that provide essential ecosystem services to local human and wildlife populations. However, the hydrological and geomorphological diversity and complexity of dryland rivers, including the Mara, makes comprehensive understanding of their development and prediction of future biophysical changes difficult, representing major challenges in fluvial geomorphology and wetland management.
This thesis examines the key geomorphic controls that drive dryland river morphology and floodplain wetland formation, as well as the intrinsic processes that lead to channel and wetland adjustment, within the Mara system. Morphometric analysis based on satellite imagery mapping and discharge modelling is used to assess the valley-scale controls and hydrogeomorphic forms and processes of the river and its palaeochannels, as well as recent historical geomorphic adjustments (last 32 years) along the lower Mara River. The river and palaeochannels have marked downstream declines in channel capacity and changes in sinuosity due to declines in discharge and stream power, coincident with a reduction in valley confinement. While several large palaeochannels extended further downstream than the modern river, likely driven by higher-energy hydrological regimes in the past, the river now exhibits a nonequilibrium response to rapidly declining fluvial energy resulting in avulsions, channel diminution, and channel breakdown in the wetlands.
Geomorphic sensitivity analysis of the lower Mara River for the recent historical period indicates that the upper and lower reaches are ‘Passive sensitive’, the former with limited adjustments, the latter with numerous cut-offs and palaeochannels, and both reaches having low migration rates. In contrast, the middle reach is ‘Fragile’ with numerous lateral migration features, cut-offs, and higher migration rates. The lower reach appears to be close to a threshold and may switch to a Fragile state where avulsions become dominant, while the other reaches may be more resilient and maintain their present states. These complex geomorphic responses are likely related to spatiotemporal patterns of hydrogeomorphic connectivity in the catchment. Modelling of contributary hydrogeomorphic connectivity indicates that the upper catchment (source zone) is consistently well-connected and is responsible for the greatest inputs of water and sediment to the fluvial network. The middle 4 section (transfer zone) has lower and more variable seasonal, inter-annual and decadal connectivity, while the lowland reaches (deposition zone), where large floodplain wetlands occur, have relatively consistent, low connectivity. These trends are associated with downstream sedimentation resulting in river and wetland adjustments and avulsive behaviour.
Together, these findings provide novel insights into dryland river character and behaviour, as well as the valley- and catchment-scale conditions that contribute to geomorphic adjustments in internationally significant fluvial systems. Under increasing pressure from anthropogenic and climate change impacts the Mara River and its wetlands are likely to undergo future geomorphic changes, and thus an understanding of the biophysical controls, forms, and processes of river adjustment as well as states of sensitivity and patterns of connectivity are imperative for guiding conservation and management efforts.
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2023
Keywords
Floodplain, Geomorphic controls