Complex response of a sand-bed stream to upstream impoundment



Mangrove Creek Dam, the eighth highest dam in NSW, Australia has induced a complex response of the downstream, sand-bed channel. Since dam closure on 1 October 1981, mean daily flows and the peak instantaneous discharge of floods have been reduced greatly (up to 94 per cent) and nearly 100 per cent of the incoming sediment load has been trapped behind the Dam. The magnitude of the hydrologic effects decreases with distance downstream. River response varies in direct proportion to the magnitude of the altered hydrologic regime and includes alternating but localized bed aggradation and degradation as well as channel contraction. Contraction has occurred by a combination of in-channel bench construction, the formation and bank attachment of longitudinal bars and bank deposition. Leptospermum polygalifolium has rapidly colonized these in-channel deposits and will result in the stabilization of benches. Armouring has not been a significant feedback process to date because of the limited degradation and low gravel supply. It is postulated that the above trends will continue, thus converting the former large, straight, active sand-bed channel into a small, sinuous, well- vegetated sand-bed stream.