Valley-scale controls and hydrogeomorphic processes driving channel breakdown in a dynamic floodplain wetland system: The Mara River, Tanzania

Abstract

The character and behaviour of dryland rivers are driven by complex interactions between extrinsic and intrinsic controls. A long-term perspective is necessary to assess the sensitivity of dryland rivers to extrinsic forcing, such as hydroclimatic changes, and to contextualise the threshold channel responses. The semi-arid Mara River in northern Tanzania is the only perennial river in the UNESCO World Heritage Serengeti-Mara ecosystem and provides water resources for human and wildlife populations. Satellite imagery and discharge modelling were used to assess the valley-scale controls and hydrogeomorphic processes driving the morphological adjustments of the lower Mara River and its floodplain wetland. Both the modern and palaeochannels undergo downstream morphological and hydrological changes as they traverse the broad, alluvial floodplain. In response to marked downstream declines in discharge (from ~300 m3·s−1 to ~45 m3·s−1) and stream power (from ~40 W·m−2 to ~0.5 W·m−2), the modern channel exhibits a nonequilibrium river response resulting in downstream declines in channel size (from ~60 m to ~10 m wide) and eventual channel breakdown in the floodplain wetland. Palaeochannels in the unconfined reaches are typically larger with higher palaeodischarges than the modern channel. While some palaeochannels exhibit similar downstream trends to the modern channel, others may have maintained a continuous channel farther into the wetland. Comparison of the Mara River to other dryland river systems highlights the importance of valley setting and hydroclimate as long-term controls that influence geomorphic river response over time. The lower Mara River represents a nonequilibrium river response to declining discharges related to transmission losses that leads to channel bed aggradation and avulsion, constituting an inherent condition in many dryland alluvial rivers. An improved understanding of the interplay between intrinsic and extrinsic controls is important for the assessment of river character, behaviour, and floodplain wetland development, particularly under future projections of more variable or extreme climate conditions.