Using sediment fingerprinting to identify erosion hotspots in a sub-catchment of Lake Kivu, Rwanda.

Abstract

Sedimentation of water bodies affects water quality and biotic communities of aquatic ecosystems. Understanding the causes and origin of sediments is crucial for planning watershed management activities and safeguarding aquatic biodiversity and critical ecosystem services. Rwanda, as a hilly country, experiences increased sedimentation due to unsustainable land use practices in upstream catchment areas which negatively affects irrigation, fishing and hydropower generation. We used a sediment fingerprinting technique to determine sources of sedimentation and identifying hotspots of soil erosion in Sebeya River Catchment (area of 357km2), a sub-catchment of Lake Kivu located in Northwest Rwanda. Five soil samples were collected from each of the six geological classes, and 34 suspended sediment samples were taken within key locations of the hydrological network in the catchment. X-Ray Spectrometry was used to determine the geochemical composition of suspended sediments and soil. A multi-step statistical procedure with a Bayesian mixing model was used to determine the contribution of each geologic group and sub-catchment to the suspended sediments in the river. Erosion hotspots were classified based on the underlying land use and their contribution to the suspended sediments. The resulting erosion hotspot map shows that about 70.9% of the Sebeya Catchment area contributes at least 50% of sediment load in the river and currently experiences unsustainable land use and land cover. The erosion hotspots identified and culpable factors should be used to guide best land use practices, prioritizing the areas with high contribution to the river sedimentation in Sebeya Catchment.