The impact of damming the Ord River on the fine sediment budget in Cambridge Gulf, northwestern Australia

Abstract

Cambridge Gulf is a turbid embayment about 50 km long, with a mean water depth of about 12 m. The spring tidal range is 8 m. At its apex, the gulf divides into two estuaries, namely the West Arm that drains the mostly undisturbed Durack and Pentecost rivers, and the East Arm that drains the Ord River. As a result of a dam constructed in 1970 on the Ord River, the river flood regime has been greatly affected within the East Arm, having silted by an average of 3 m. By contrast the West Arm bathymetry has remained practically unchanged for the last 100 years. Oceanographic studies suggest that the West Arm exports fine sediment at a rate of about 40,000 ton d 1, and that most of that sediment is now imported into the East Arm and does not reach Cambridge Gulf. Negligible net sediment flow was measured at the mouth of the gulf. This suggests that the pre-dam Cambridge Gulf received a total sediment inflow of about 100,000 ton [d.sup.-1], and about zero at present. Sediment is currently being redistributed within the gulf, the west coast may be accreting and the east coast receding.