RAINFALL AND SEDIMENT DISCHARGE IN SELECTED RIVERS IN INDONESIA

Estimates of the amount of bed load and the degree of fluvial fill of estuaries, and the nature of stream channels (meandering or braided) were used to evaluate sediment discharge in river systems in tropical regions that include Sumatra, Borneo, and Seram, and Timor. In addition, reconnaissance-level stream sampling (suspended sediment concentrations, solute concentrations, and pH) was also conducted in rivers in all study areas. The factors that influence fluvial sediment discharge in tropical climates (catchment basin size, relief, gradient, bed rock lithology, and rainfall) can be readily evaluated in these Indonesian fluvial systems where rainfall is the primary variable. In equatorial regions of Sumatra, Borneo, and Seram, rainfall (>2.4 m/yr) exceeds evapotransporation for all months of the year (aseasonal, perhumid climates) whereas, in Timor (10 degrees south), 85 percent of all rainfall (1.4 m/yr) is restricted to a four month rainy season (dry subhumid seasonal climate).

Sparse bed loads, the lack of a fluvial fill in estuaries, and very low sediment concentrations (10 mg/l suspended and 10mg/l dissolved) indicate that fluvial sediment discharge is very low in the perhumid regions. In contrast, braided stream bed loads, the complete fluvial fill of estuaries, and high sediment concentrations during the rainy season are indicative of high fluvial sediment discharge in the dry subhumid climates. The dominant variable affecting modern fluvial sediment discharge among the islands of Indonesia, therefore, appears to be the degree of seasonality in rainfall regardless of tectonic setting, relief, bedrock geology, or catchment basin size. Sediment discharge during the Pleistocene may have been much greater in equatorial regions as indicated by placer tin deposits in bed loads of low-stand river valleys on the submerged Sunda shelf. The apparent high bed load of these ancient rivers indicates that erosion increased coincident with drier and more seasonal rainfall climatic conditions during glacial intervals.