Structural analysis of the drainage basin of Kenyan rift valley lakes within the aberdare detachment, using satellite data, GIS and field observations
Onywere, S. M.
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In addressing the problem of lake-level changes in the drainage basin of the Kenyan Rift Valley lakes (Bogoria, Nakuru, Elmenteita, Naivasha) of the Aberdare Detachment System, a study of the watershed of the basin is made. The study deals with information extraction and representation using digital techniques based on ERDAS image analysis programs and Intergraph, Geographic Information System (GIS) based on Microstation software. A GIS database is designed, and through modelling, has provided the cartographic ability to address the questions posed by the present problem. Because of variation in data types and sources, raster-based GIS and vector-based GIS are used. At first approach, analysis is based on visual interpretation of photographic products. In designing raster-based GIS, satellite imagery is initially digitally enhanced and filtered. Analysis of Landsat-5 TM and SPOT imagery is made and used for interpretation and classification, facilitating analysis of land use and land cover characteristics and patterns. Good ground resolution, and characteristics of the data reveal significant details of the land use cover patterns and linear patterns of fault scarps, with the synoptic view suggesting NNE, NE and NW trending scarps and associated grabens. The spectral analysis has allowed for: (1) Image classification of Nakuru-Menengai area using supervised image classification programs and Principal Component Analyzed (PCA) image data. (2) Lineament analysis and drainage pattern analysis using peA and directionally filtered image data. (3) Generation of raster GIS data files that are integrated and modelled with vector GIS data files. III The image classification identifies 17 land cover types that show a marked increase in agricultural activity in the study area in the period July 1984 to present. Increased irrigation around lake Naivasha in this period has resulted in increased direct consumption of water from the lake. There is also increased land use in the catchment areas of the lakes, affecting the morphology of the delicate soil structure and the forest cover, and subsequently run-off into the lakes. The loss of vegetation cover on the soils has resulted in decreased infiltration and thus ground discharge into the lakes. This has affected the water levels in the lakes. For the vector GIS, information concerning five themes, namely structures (faults), lithology, drainage patterns, land use patterns, and topography, is extracted from topographic maps, geologic maps, and tables and reports of the study area. The various digitized elements are given attribute values through Look-up-Tables and referenced to the analyzed satellite data. In the referencing, the imagery products are draped on the vector data and incorporated into the database. The area is divided into three tectonic blocks on the basis of recognized trends of fault patterns. A detailed analysis of selected areas in the three divisions is made, basing the interpretation on a number of measurable and recognizable features in the images, ground truth data, and integration with digitized data sets. Stored GIS information allows for graphic presentation of the morphotectonic structures of the Aberdare Detachment. Comparison between vector data and raster data, reveal a strong structural control and tectonic involvement on the drainage patterns and the lake basins. There is also strong structural control into the geothermal reservoirs. Step-fault ramps channel water from the marginal escarpments and the platform areas into the rift floor, largely directing the waters away from the nearby lake basins into widely separated basins. Thus, the drainage into lakes Bogoria, Nakuru, Elmenteita and Naivasha cover small areas, poorly recharging these lakes.