Land use and vegetation change in response to river basin development in the lower Tana Basin of Eastern Kenya

Abstract

This study describes the impacts of river development projects on land use and vegetation in a floodplain that includes old-growth forest and an important primate habitat located in eastern Kenya. River basin development activities include the construction of hydro-electric dams in the upper river basin, and an irrigation scheme, the Bura Irrigation and Settlement Project, in the lower basin. Through flood frequency analysis for both the pre- and post-dam period, I demonstrate that there has been a significant (p < 0.01) reduction in floods with a recurrence interval of 5 years or greater. A hydrological simulation model is used to estimate the frequency and duration of flooding of 73 vegetation sample plots for the pre- and post-dam period. Four of the plots, lying less than 1.25 m above dry season river level, show a slight increase in days flooded, whereas the rest show a significant decline in days flooded from the pre- to the post-dam period. Detailed descriptions of the structure and dynamics of the Tana riverine forest, and exploration into the influence of abiotic variables to species composition, are made using ordination and classification techniques. The three canopy levels examined, and the regeneration layer, had different species compositions. Many of the upper canopy species are not regenerating. Results of detailed land cover and change detection mapping using remotely sensed data reveal significant change. Forest cover declined slightly (about 2%) between 1975 and 1984. However, between 1989 and 1996, there was a 27% decline in riverine forest, while cultivated area within the forest increased by 45%. Over the same period, area of exposed soil increased by 112%. Several landscape measures are given and all indicate significant fragmentation of riverine forest. The extent of riverine forest along the active river channel declined by about 200 m between 1989 and 1996. Human disturbance now represents the greatest threat to continued survival of the forest. Results of a dendrochronologie investigation reveal that a number of species produce growth rings. Four species identified as offering the best chance for developing a ring-width chronology are; Acacia elatior, Acacia robusta, Tamarindus indica, and Newtonia hildebrandtii.