Soil erosion is the biggest source of nonpoint pollution in watersheds worldwide, with fine sediment being the most common pollutant (eg. Gurgen 2003, Yanda & Munishi 2007, Davis & Fox 2009). In Rwanda and other areas within the Nile Basin, suspended sediments have been sharply increasing in water bodies since the 1990s (Probst & Suchet 1992, Odado & Olaga 2007, REMA 2009). The State of the Environment Report (REMA 2009) mentions that the Nyabarongo river system carries 51 kg/second of soil at Nyabarongo-Kigali, 44 kg/s at Nyabarongo-Kanzenze and 26 kg/s at AkageraRusumo. Increasing sediment loads in rivers leads to the deterioration of water quality, a condition that affects freshwater ecosystems and their capacity to deliver the critical freshwater ecosystem services upon which human populations depend in a timely and cost-effective way. For instance, sediment settles on streambeds and fill up the gaps underneath stones, thus removing habitat for aquatic macroinvertebrates (insect larvae) which feed on detritus, thus maintain water quality and constitute food for stream fishes. Sediment deposition in river channels and reservoirs also reduces volume capacity that worsens flooding during periods of high rainfall.
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