The Rufiji River, Tanzania Hydrology and Sediment Transport

Abstract

Sediment transport by large tropical rivers has generally not received adequate study due to deficiencies in recorded data on discharge and sediment load. This paper is a case study of sediment transport in the Rufiji river, which drains a varied catchment of 156,600 km2, and is the largest river in Tanzania.Data on suspended sediment load spans the period 1955/56 to 1961/62, while discharge data covers the period 1954/55 through 1969/70. During this period, daily discharges ranged between 6637 m3/sec and 69 m3/sec. Floods with a discharge of 5000 m3/sec have a return period of approximately 5 years. Suspended sediment load data fortunately covers the extremes of the river record as well as more normal conditions. Suspended sediment load data indicates a maximum daily transport of approximately 1,000.000 tons during the investigated period. Sediment-rating-curve and duration-curve analyses indicate that some 250–300 mill, tons of suspended sediment have passed through Stiegler's gorge in the 16 years covered by discharge records. To this figure should be added the bed load transport and probably also a certain increase due to inaccuracy in the sampling and analysis procedures.Heaviest loads of sediment are related to sudden flash floods in the rising limb of the annual hydro- graph. The suspended sediment peaks are out of phase with the flood peaks, indicating an early annual flushing of sediment from the system with the onset of the flood season. The sources of this sediment and the characteristics of the tributary basins in terms of relief, geology, seasonal rainfall regimes, and vegetation are discussed.Geomorphological and economic implications are discussed in general but will be covered more fully in a later report.