Rapid, Human-Induced Siltation of the Macro-Tidal Ord River Estuary, Western Australia

Abstract

Field studies on the sediment dynamics of the two arms of Cambridge Gulf, tropical Western Australia, were undertaken. The system is macro-tidal with 8m tides. The West Arm is the estuary of the Durack and Pentecost rivers, which are largely untouched by human activities. The East Arm is the estuary of the Ord River affected by land clearing which started 100 years ago, and by two river dams constructed 30 years ago. Prior to damming of the Ord River, the natural flow in all rivers was highly seasonal, characterized by small, at times negligible, flow during the dry season, which lasts eight months, and occasional river floods during the wet season, which lasts four months. The river flow has been regulated in the East Arm as a result of damming; this resulted in suppressing large river floods. The salinity intrusion length has decreased by about 50% in the East Arm following river damming. The East Arm has silted measurably over the last 30 years, with the stream cross-sectional area decreasing by about 50%. Field and numerical studies suggest that this is due to the dam-induced suppression of large river floods, which has in turn allowed a greater amount of tidal pumping of sediment into the East Arm from Cambridge Gulf. Since tidal pumping is increasing as the system is silting, the East Arm appears to now be geomorphologically unstable as a result of human activities. Numerical studies suggest it may take as little as 100 years for the Ord River to reach a new equilibrium. Modelling suggests that this equilibrium may be characterized by a salinity intrusion length half that before damming, a channel width and depth reduced by 70%, and a much stronger tidal asymmetry characterized by an increasing size of the tidal bore. The West Arm appears to be at equilibrium since there has been no net loss or gain of sediment for the last 111 years. Tidal dynamics and river floods appear responsible for self-scouring this channel.