Natural versus anthropogenic sources of channel sand and fine gravel following integrated logging in the Letts Creek Catchment, NSW

Integrated logging immediately before a large storm in February 1992 caused extensive erosion of a feeder road in compartment 588/2 of Yambulla State Forest that allegedly supplied large amounts of sand and fine gravel to the downstream river channel where pools were supposedly infilled. However, most sediment eroded on the feeder road was actually stored on slopes and in filter strips before reaching channels. Subsequent soil conservation works rehabilitated the worst-eroded areas and they are still functioning effectively today. There is no direct connection of sand and fine gravel between the general harvest area and downstream channels because of many intervening sediment discontinuities, such as ponds, drainage lines, filter strips and floodouts. The catastrophic flood of February 1971 was the largest for at least the last 50 y in the Towamba catchment and caused substantial widening of lower Letts Creek and Wog Wog River. It also reorganised channel-spanning boulder steps on upper Letts Creek and a tributary in and immediately downstream of compartment 588/2, and may have caused gully erosion on the tributary of Letts Creek. Bank erosion supplied the sand and fine gravel in the river bed of these headwater channels. A cluster of large floods between 1988 and 1992 exacerbated bank erosion in the two channels draining the logged compartment and infilled pools.