Human Impact on Sediment Flows in the Lake Kivu Region

This study is carried out in the western (DRC) and eastern  (Rwanda and its surrounding) part of Lake Kivu region, located in the humid tropical zone; in the Kivu Highlands region, between 1460 m at the outlet (Lake Kivu) and over 2500 m altitude at the highest point. Accelerated soil erosion is a serious problem in this particular region and the world, with high economic and environmental impacts. Many human activities, such as mining, uncontrolled construction, and agricultural activities, disrupt watersheds can cause erosion. Across the world, land affected by soil degradation due to erosion is estimated at 1100 million ha by water erosion and 550 million ha by wind erosion. Suspended fine sediments are abundant in river systems for a variety of reasons including their impact on aquatic habitats and biogeochemical flow. The increase in suspended sediment is related to land cover and land use. Suspended sediment production increases as vegetation cover decreases and grazed farmland increases.
Collect of suspended sediment in the Kihira rivers in the western part of Lake Kivu. Photo: Bagalwa Rukeza Montfort

Photo: Matti Barthel

In the Lake Kivu region, these phenomena of soil erosion and sediment flow are catastrophic. The main physical factor and the most important of these phenomena is the rain. The rainfall erosivity in the RUSLE equation expressed through the R factor quantifies the effect of raindrop impact and reflects the amount and rate of runoff likely associated with rainfall. Erosive rains, falling on steep slopes with fragile soils, carry enormous amounts of sediment. Calculating the value of the R factor in the RUSLE equation to predict related soil loss may be possible to analyze the variability of rainfall erosivity with long time series of precipitation data. The estimate of soil erosivity in this region is essential.
This study began with socio-economic surveys of the inhabitants of the northwestern part of the Lake Kivu region, 72% of the surveyed population indicated that erosions followed by sedimentation in the region have become more catastrophic since the years ‘90. In the region, the meteorological stations are insufficient, temporarily operational and do not cover the study period, from 1934 to 2017. To obtain the data available and adapted to this study, for the calculation of erosivity, we have used rainfall data of 30 minutes resolution, daily and monthly resolution of 26 stations in the western and eastern part of the Lake Kivu region. The results obtained indicate the variation of erosivity in the western part of the Lake Kivu region, from 1496 to 5907.8 MJ mm ha-1 h-1 yr -1.
Installation of Shasha weather station DRC. Photos: Bagalwa Rukeza Montfort
Since July 2016, suspended sediments have been collected once a week in the selected rivers in the northwestern part of the Lake Kivu region to measure the mass of sediments in grams per liter transported and deposited in Lake Kivu. Water samples are taken once a week in one liter polyethylene bottles. The preliminary results obtained for these analyzes indicate a total concentration of suspended sediments of 292.08 gl-1 day-1 for the Ngoleko River, 119.788 gr l-1 day-1 for the Kihira River, 2.0329 gr l-1 day-1 for the river Renga and 1.003 gr l-1 day-1 for the Shasha River.
Installation of philips tubes in the Kihira and Renga rivers. Photos: Bagalwa Rukeza Montfort
Installation of DIVER in the Kihira and Renga rivers. Photos: Bagalwa Rukeza Montfort
Since November 2011, water samples are collected once a month in the identified rivers of the study area, using the polyethylene bottles and Philips tube, to analyze the Compounds Specific Stable Isotope, in order to analyze the concentration of isotope, organic matter and then other trace elements contained in suspended fine sediments. For this study, we collect once a month sample of water in the one liter plastic bottle and we installed the Philips tubes for collect of fine sediments.
The main objective of this study is to analyze the human impact on sediment flows and the rainfall erosivity in the Lake Kivu region.