Structural analysis of the drainage basin of Kenyan rift valley lakes within the aberdare detachment, using satellite data, GIS and field observations
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Date
1997-03
Authors
Onywere, S. M.
Journal Title
Journal ISSN
Volume Title
Publisher
Nairobi University
Abstract
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.
Description
A Thesis Submitted to the Faculty of Science, in Fulfilment of the Requirements for the Award of the Degree of Doctor of Philosophy (Geology) Department of Geology, University of Nairobi. March 1997