Dry season suspended sediment concentration and sedimentation in the Richmond River estuary, northern NSW, Australia

Dry season suspended sediment concentration and sedimentation in the Richmond River catchment were investigated during 2 hydrological years (1994-96). Longitudinal suspended sediment transects during low flow months showed that the Richmond River estuary remained well mixed and maintained <30 mg/L suspended sediment concentration without any visible turbidity maximum zone all along the estuary. During the entire dry season, there is very little exchange of suspended sediment between the upper and lower estuaries because of very small input from the upper catchment. The estuary receives net sediment input from the continental shelf during the dry months under normal tidal circulation, and marker horizon core samples confirmed that most of these imported sediments were deposited in the lower estuary; during the 2 dry seasons, lower estuary sedimentation rate varied about 0.84 ± 0.31 cm to 0.48 ± 0.3 cm. Flushing times of the Richmond River estuary show that all point and non-point source inputs of sediments and pollutants into the estuary can be flushed out during one dry season.