IMPACTS OF LAND USE/COVER CHANGES IN SOUTHWEST MAU ON
THE DOWNSTREAM SILTATION OF RIVER SONDU
The study area is Sondu River basin, which drains into the southern part of Lake
Victoria (Kenyan region). It is bound by latitudes 0017'S and 0022'S, longitude
34004'E and 34049'E. The river has its catchment in southwest Mau Forest that is
characterized by diverse anthropogenic activities such as agriculture, altering the
natural state of the land and hence affecting the water quality and quantity.
Accurate and timely information on the distribution of vegetation in Southwest
Mau, therefore, is a requisite for understanding how changes in land cover affect
phenomenon as diverse as the siltation in the Lake. The main aims of the study
were to determine the land use/cover changes in southwest Mau over a period of
27 years (1973-2000) and to evaluate the changes in siltation levels downstream
over a period of 35 years (1976-2011).
The data used for the study include satellite images from landsat. org, daily rainfall
and discharge values, all obtained from WRMA and DEM obtained from USGS.
The satellite images were processed using Arcmap 10.1. The catchment was
classified into three groups: bareland and settlement, farmland and forest using
supervised classification technique. Similarly, the downstream of the river was
classified into five classes: bareland, scattered vegetation, dense vegetation, turbid
water and clear water. The two classifications above showed the extent of forest
conversion at the catchment and sediment plume in the Lake (respectively),
making the correlation of LUCC and siltation levels possible.
Time series analysis between rainfall and discharge were done using Microsoft
excel to try to determine any relationship between the two parameters. The
decrease in time lag during the rainy seasons (MAM) between 1976 and 1986
increased the rate of soil erosion, adding more sediment into the river. This proved
that runoff is a function of LUCC given other parameters such as shape of the
basin, geology and relief constant. The sensors used; MSS, TM and ETM+ have
varied spectral resolution. This presented a challenge in the comparison of Landsat
images (for both siltation and LUCC). Therefore, this is an area for further studies,
that is, to quantify the suspended sediments within the Lake and to map the extent
of land occupied by natural forest within the watersheds.