| dc.description.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. | en |
