Changes in benthic sediment in the thomson river, Southeastern Australia, during construction of the thomson dam

Abstract

AbstractThe particle‐composition (0.15 to >64 mm) of the stream‐bed was monitored by Surber sampling at sites above and below the site of the Thomson Dam during the two years of most active construction. Levels of particles between 0.5−2 mm were highest 9.5 km below the dam site, and increased throughout the study. Levels of these grades at sites above and well below the dam site were variable, but showed no consistent trends. Material greater than 2 mm showed no substantial variation either over time or between sites spanning approximately 70 km of river length and 250 m of elevation.Artificial substratum samplers were used to estimate the rate of sediment input (0‐2 mm) between sampling dates. Sediment input was clearly highest 1 km and 9.5 km below the dam site. Input was seasonal, with maxima in winter and spring of each year. Size classes 1‐2 mm and <0.063 mm contributed over 60 per cent of the total deposited sediment. Over the second year of the study, high levels of sediment <0.063 mm were deposited at a site 33 km below the dam site, indicating the progressive movement of material down the river.Some fine sediment was found to penetrate unconsolidated bed material to depths down to 60 cm. It is suggested that such material, resulting from land disturbance, is capable of filling interstitial spaces in the stream‐bed, and that major disturbance of the bed may be necessary to dislodge it.