The influence of geology and land use, land cover impacts on the river nyando basin, Kenya

Abstract

The topography of a catchment evolves as the product of tectonic construction interacting with weathering and erosional processes intimately tied to climate, composition of vegetation cover and the hydrological regimes. However, Unsustainable land management practices result in high load sediment and nutrients being discharged into the natural water, affecting land production and water quality. Studies of environmental factors ecology which provide an integrated assessment of the driving factors and consequences in the Nyando River Basin are still in their infancy. The interactions between geology and land use were studied in the River Nyando catchment., by combining Geographical Information System (GIS), Remote Sensing, and analysis of time series data on river flow and rainfall. The principle objective of this study was to evaluate the changes in the land cover over the last 14 years (1986-2000) and the hydrological changes over a period of 37 years (1961- 1997). The study area has been identified as a major source of sediment and nutrient flow into the lake. The watershed geomorphology comprises; hills, scarps, plateaus and a plain area with different types of erosional features; The spatial and temporal change in patterns of land use were quantified by interpreting remote Sensing (RS) data and used a geographical information system (GIS). Landsat images (1986, and 2000) were used to classify the catchment into five land use classes: bare ground, water body, agricultural land, dense forest and sparse forest using supervised classification in ENVI. The resulting quantitative relationships were assessed with respect to their influence on soil degradation The classification of the data used in the integration and the information used in the GIS gave three main divisions of the landscape: highlands, midlands and lowlands, which have unique geology, land use and degradation problems specific to them. The slope inclination of the highlands is 14-30%, mid-lands are 7-13% and the lowlands are 0-6%. 58% of the Nyando catchment is lowland area, highland and mid land areas are 18% and 23% respectively. Upland (highland and mid land) soils are shallow to moderately deep. Lowland soils are deep to very deep. The greater part of the Nyando catchment comprises of soda rich volcanics which occupy the eastern portion of the catchment. These rocks characterize the highland areas, together with nephelinites, phonolitic nephelinites, anal cites, basanites and tepherites, trachytes and quartz trachytes and basalt and mafic basalts. Defining the Kavirondo Rift are dolerites, granites granitoid gneisses, migmatites and limestones. The soils that overly these rocks are Ferrasols, Nitisols, Cambisols, Planosols, Luvisols, Vertisols and Solonetz. Cambisols are abundant, occupying 29% of the catchment. The sharp increase in the amount of surface runoff at the beginning of the rainy season (MAM) after a dry period (JF) causes severe soil erosion because there is minimum vegetation cover. The upland areas are prone to sheet and rill erosion while the lowland areas experiences, gullying and waterlogging. Various types of erosion phenomena observed in the study area: badlands, stripped bedrock slopes with some sheet and rill erosion in the upland areas of the catchment, gullying and badlands are common in the Lake plain area which represent different scales of erosion in time, magnitude and space. From the erosion hazard analysis, it is observed that the area with high to severe soil erosion is 72% (394,320 hectares), and the remaining 28% (96,543 hectares) of the catchment area experiences low to medium erosion. With the rate of loss from gullying, these are compelling reasons for improving the way we manage the soils. Therefore this study points out factors that in combination makes the Nyando river basin susceptible to degradation.