Special study on sediment discharge and Its consequences (SedSS)
Date
1999
Authors
Sichingabula, Henry
Journal Title
Journal ISSN
Volume Title
Publisher
United Nations Development Programme/Global Environment Facility (UNDP/GEF)
Abstract
Lake Tanganyika is among Zambia's important fisheries which require protection from adverse impacts. Sedimentation is one of the threats to Lake Tanganyika's biodiversity. This is because deposition of sediment clogs streams and reduces their capacities. Pollution by sediment is also one of the major factors causing deterioration in quality of streams and lakes. The sediment deposited into streams, lakes and reservoirs destroys the habitat for fish and other species. In aquatic environments, sediment impairs the dissolved oxygen balance and obscures the light needed for aquatic growth, both of which are detrimental to aquatic life forms. Additionally, heavier sediment particles blanket fish spawning areas and cover food supplies for many species. Pesticides and nutrients origination from agricultural lands are carried off the land by sediment in surface runoff and add to the pollution of downstream waters. For example, phosphates are adsorbed by soil colloids and move into streams and lakes through erosion of soil particles on which it is adsorbed. Nitrogen fertilisers together with phosphorus are the major cause of eutrophication in lakes and rivers which is detrimental to aquatic species. Therefore, the prevention of soil erosion by good conservation practices are the most efficient means of controlling pollution from agricultural lands. The physical characteristics of Lake Tanganyika basin are to a large extent controlled by the geology which is dominated by meta-quartzites of Precambrian ages. These rocks with the high rainfall regime experienced in the area have given rise to the high leached sandveldt soils which characterise the plateau area. These features of the physical environment in the lake Tanganyika basin are discussed in detail. This report documents the activities of the Zambia Special Sediment Study undertaken in almost a period of one year. Methods of data collection an errors inherent in methods used are also discussed. The determination of volumes of discharge and sediment deposited in the lake employed on various statistical models in the development of rating curves. The models selected were tested for accuracy before they were applied to monitored water level data. The study found that mean flow discharges on rivers draining into Lake Tanganyika ranged from 1.426 m3 s-1 on Izi River to 98.563 m3 s-1 on Lufubu River. The lowest and maximum discharges ranged from 0.183 m3 s-1 on Lucheche River to 346.68 m3 s-1 per day on Lufubu River. Similarly, quantities of clastic suspended sediment deposited by the five rivers into the lake between September, 1998 and May, 1999 were found to range from 0.082 tonnes on Kalambo River to 1,539.634 tonnes per day on Lufubu River. The mean values of sediment deposited into Lake Tanganyika were found to range from 1.248 tonnes on Izi River to 208.603 tonnes per day on Lufubu River. Magnitude frequency analysis was used in the determination of the flow ranges which transport the most load in a single year. The effective discharge on the studied rivers was variable ranging from 0.78 m3 s-1 on Izi River to 258 m3 s-1 on the Lufubu River. The durations of these class-based effective discharges were found to range from 2.1% corresponding to the highest discharge, to 54.5% of the time on Izi River where the lowest event transported the most load. The sediment-discharge regimes for rivers in southern Lake Tanganyika basin were characterised by a uniform histogram having a well defined mode and a relatively frequent effective discharge. This was exemplified by Lucheche River. In cases where the regime in which the lower level flows were the effective discharge this was exemplified by Lunzua River
Description
Technical Report
Keywords
Sediment monitoring--Zambia