Macquarie Rivulet and Mullet Creek are the two main rivers draining the Illawarra escarpment west of
Lake Illawarra, NSW. Both rivers have low run-of-river salinity-control weirs to prevent brackish water
from Lake Illawarra penetrating upstream into the main agricultural areas.
The weir on Mullet Creek has been in place since the late 1800s associated with a river crossing
whereas the Macquarie Rivulet weir was only emplaced in 1963. Detailed GIS analysis of aerial
photographs shows no signs of sediment build-up behind the weirs and no significant scouring on the
downstream side of the weirs. This is especially apparent on Macquarie Rivulet when pre- and postweir
construction could be analysed. The results show that channel surface areas and rates of
change through time occur independently of the weirs with most change occurring after major floods,
e.g. the 1984 flood on Mullet Creek.
Surface sediment samples and vibracores were collected both upstream and downstream from the
weirs in both rivers. Upstream from both weirs showed significant scouring with almost no sediment
accumulation in the middle of the rivers and only minor accumulations of both coarse and fine
material near the banks. During flood conditions when the river level is 2-3 m above the weir both
coarse and fine material are entrained by the turbulence upstream of the weir and transported over
the weir. Immediately downstream of the weir a scour pool is present with sediment deposition
occurring 30-50 m below the weir. Coarse layers, up to 50 cm thick in bank-attached bars, represent
flood deposits that may be separated by finer grained low flow deposits. The progressive fining of
sediment downstream towards the mouth of the river is a function of the backwater effect caused by
the lake. No anthropogenic pollution is being stored in fine material upstream of the weirs.