Channel bed adjustment along mine-affected rivers of northeast Tasmania

Abstract

Forty million m 3 and 1 million m 3 of mining waste are estimated to have been supplied respectively to the Ringarooma and George Rivers between 1875 and 1984. Given the volumes involved and the fact that much of the input was less than 5 mm in diameter, the size composition of the bed material changed from gravel to sand as the rivers progressively aggraded their beds downstream. Increases in bed height of over 10 m and 1 m are predicted respectively for the two rivers, with change becoming more gradual further downstream. With upstream supplies becoming depleted first, degradation followed the same pattern as aggradation — progressing downstream. Maximum rates of almost 0.5 m yr −1 were measured. Degradation has returned the George River to its pre-1875 level but has yet to reach the downstream reaches of the Ringarooma, where sediment waves continue to pass over a slightly aggrading bed. Where degradation has been occurring long enough, the bed material has again become gravelly through re-exposure of the original bed and/or lag concentration of coarser fractions within the introduced load. Such armouring should improve bed stability and slow the rate of degradation. However, at Herrick, which is representative of intermediate reaches with only a thin gravel veneer, the annual amount of degradation is reasonably well correlated with flow conditions, suggesting that high discharges can strip the surface armour and continue to degrade actively. Incision has not only changed the vertical position of the rivers but also their planimetric and cross-sectional geometry. In particular, non-uniform lowering of the bed has narrowed the channel by up to 60%, changes which can be predicted quite accurately by Chang's quantitative model of channel adjustment. At least another 50 years will be needed for degradation to cleanse the Ringarooma of mining debris.