Suspended sediment load in the tidal zone of an Indonesian river

Forest clearing for reasons of timber production, open pit mining and the establishment of oil palm planta- tions generally results in excessively high sediment loads in tropical rivers. The increasing sediment loads pose a threat to coastal marine ecosystems, such as coral reefs. This study presents observations of suspended sediment loads in the Berau River (Kalimantan, Indonesia), which debouches into a coastal ocean that is a preeminent center of coral diver- sity. The Berau River is relatively small and drains a moun- tainous, still relatively pristine basin that receives abundant rainfall. In the tidal zone of the Berau River, flow velocity was measured over a large part of the river width using a horizontal acoustic Doppler current profiler (HADCP). Sur- rogate measurements of suspended sediment concentration were taken with an optical backscatter sensor (OBS). Av- eraged over the 6.5 weeks covered by the benchmark sur- vey period, the suspended sediment load was estimated at 2 Mt yr 1 . Based on rainfall-runoff modeling though, the river discharge peak during the survey was supposed to be moderate and the yearly averaged suspended sediment load is most likely somewhat higher than 2 Mt yr 1 . The conse- quences of ongoing clearing of rainforest were explored us- ing a plot-scale erosion model. When rainforest, which still covered 50-60 % of the basin in 2007, is converted to pro- duction land, soil loss is expected to increase with a factor between 10 and 100. If this soil loss is transported seaward as suspended sediment, the increase in suspended sediment load in the Berau River would impose a severe stress on this global hotspot of coral reef diversity.