Soil Erosion and Sediment Transport in the Morogoro River Catchment, Tanzania

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Abstract
The catchment covers an area of 19.1 km2. It has a range of altitude from 550 m to 2138 m a.s.l. on the northern slopes of the Uluguru mountains. The uppermost 40% of the catchment is covered by a rainforest reserve. On aerial photos from June 1970 cleared land with grass or bush fallow covered 44%, and cultivation 10% on slopes as steep as 26°–42°. The soils are sandy loams, derived from weathering of the Precambrian metamorphic bedrock. The annual rainfall is 2400 mm at 1450 m a.s.l., and 890 mm at 530 m a.s.l., where June—October are dry months with less than 50 mm rainfall. Suspended sediments in the Morogoro river were sampled during three rainy seasons: March—May 1969, 1970 and 1971. The flow is flashy with very rapid rises and short-lasting peaks, which make manual sampling difficult. Highest concentration of suspended sediments measured was 10.6 g/lit. Mean annual sediment transport in 1966–1970 was calculated as 7500 tons based on streamflow data and sediment rating curve from the sampling period. It corresponds to about 390 tons or 260 m3 of annual sediment yield per km2 of drainage area. The steep slopes and stream channels make it likely that the suspended material sampled in the river mainly consists of soil particles eroded and brought directly from the slopes by the same flood water. Sheet wash from the cultivated 10% of the catchment is thought to supply the main flow of sediments, occasionally increased by small landslides and mud-flows. Rills occur only to a minor extent and steep-sided gullies not at all in these valleys. The absence of gullies is probably due to a high infiltration rate and thin soils. The main hazard of cultivating such steep slopes, beside the rapid exhaustion of the soils, is the danger of catastrophic erosion during extremely intensive rains with several years interval. See Temple & Rapp 1972 (this volume). A programme of chemical analyses of soils, runoff water and sediments was planned but could not be carried out. Continued research in this catchment should give high priority to analyses of the role played by solution losses. The considerable losses of soil and water documented in our studies of the Morogoro river catchment can probably be reduced to acceptable proportions by simple conservation measures such as the following. Planting of tree belts on critical slope sections to reduce the danger of landsliding (cf. Temple & Rapp 1972). Extended use of grass barriers, trash bunds and mulching on bare fields to reduce splash and sheet wash (cf. Temple & Murray-Rust 1972). Manuring of fields to permit longer periods of cultivation and hence longer periods of grass and bush fallow on a larger proportion of the cleared slopes.

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