GEOMORPHOLOGIC CHARACTERITICS OF NZOIA RIVER BASIN
Keywords:Stream order, bifurcation ratio, form factor, drainage density
Relationships between geomorphologic characteristics are important in understanding runoff response of many drainage basins and other related flow driven basin phenomena like erosion potential. In Kenya, geomorphologic studies are few and therefore inadequate. Nzoia basin in which this study is based is no exception. Nzoia River basin is the largest sub-basin in the Lake Victoria North basin of Kenya. However, geomorphology as a component of the basin’s general physical characteristics has not been studied and its linkage to flooding established. This study aimed at finding a geomorphologic explanation for the run-off response in the Nzoia River basin. The methodology involved is characterizing the basin using indices like: relief ratios, stream orders, bifurcation ratios, drainage density and form factor. The study concluded that there is no consistent relationship between relief ratio and basin order in this basin due to high variance in relief on the divide line. However, high relief ratios of above 0.03 are observed as characteristics of upland subbasins and low value of about 0.01 for subbasins at the lowlands. This change has explained the flooding phenomena at the lowlands. Bifurcation ratios ranged from 2.8 and 3.3 which are within Strahler’s range but average value of 3.1 is closer to the lower bound value of 2. On the basis of bifurcation ratio, Nzoia basin flows may experience delayed time to peak and this is a good property for planning flood response strategies. Analysis of drainage density distribution has shown uniform conveyance efficiency with a mean value of 0.24-km, however, theoretically, the value lacks control values to base judgment. Overall form factor has classified the basin as fern-shaped with possibility of delayed high peaked hydrographs which is also important for flood evacuation planning.