Contemporary channel adjustment and geomorphic sensitivity of the lower Mara River and its floodplain wetlands, Tanzania

Abstract

An understanding of river adjustment processes, previous landform changes, and sensitivity to flow regimes are prerequisites for predicting how rivers may respond to future climate change and anthropogenic disturbance. Geomorphic river sensitivity is an important concept that can be used to assess the capacity of adjustment for a river type and forecast river responses to hydroclimatic change over time. In this study, we use satellite imagery to characterise and quantify recent geomorphic river adjustment in a dryland river in East Africa, over the past 32 years (1988–2020). The natural or expected river behaviour (i.e., behavioural regime) and capacity for adjustment (i.e., valley confinement) are quantified to provide a measure of the reach-scale geomorphic sensitivity of the lower Mara River, Tanzania. Two behavioural river sensitivity classes are identified. The upper reach is classified as Passive sensitive, exhibiting a limited range of bend adjustments and low migration rates (0.89 m a−1) owing to valley confinement and capacity to recover from disturbance events. The middle reach is classified as Fragile and shows the greatest capacity to adjust in response to intrinsic fluxes. This reach is characterised by numerous lateral migration features and cut-offs (0.2/km) on the floodplain and exhibits the highest migration rates (1.39 m a−1). The lower reach is also categorised as Passive sensitive and undergoes bend and boundary adjustments within its behavioural regime and exhibits the lowest migration rates (0.63 m a−1). However, this reach appears to be operating close to a threshold state and if subjected to a threshold-breaching disturbance event it may ‘switch’ to a Fragile system where avulsions are dominant. The Mara River shows that geomorphic sensitivity is an important consideration in assessing system response to short-term variability and forecasting future river behaviour and will be key in guiding the management and rehabilitation efforts of other dryland wetlands under different land use and climate change scenarios.