Mechanisms of drainage basin denudation during the late quaternary, at Wangrah Creek, Southern Tablelands, New South Wales

Abstract

The mechanisms of denudation over the last 25,000 yrs are studied for Wangrah Creek, a headwater drainage basin of the Murrumbidgee River, N.S.W. The controls on denudation of climate, humans, and autonomous geomorphic change, are then resolved for different times in the past, by comparison with independent palaeoenvironmental data. The study also puts the history of the basin since settlement, 150 yrs ago, in it's longer term context. The stratigraphy of alluvial and colluvial deposits is investigated together with monitoring of streamflow, slope runoff and sediment yield. Historical changes to denudation are analysed from maps, air photographs and the most recent deposits. Extensive radiocarbon dating of deposits is included. The stratigraphy and morphology of Limekiln Creek, a tributary valley within the larger basin, allows modes of deposition to be attributed to certain lithologies. In particular, mildly organic, fine grained alluvium is associated with swampy meadows. These are valley floors with no continuous channel, a permanently saturated surface, and a dense vegetation of sedges and grasses. Planar bedded sands and gravels, deposited across the valley floor, are associated with discontinuous gullies. There was a period of rapid hillslope erosion in Wangrah Creek between 20,000 and 25,000 yrs BP. This left thick deposits in all valleys. A period of intense valley erosion removed the Pleistocene deposits from the main valleys and started incision of smaller valleys. The reworking of Pleistocene deposits became the dominant source of sediment in swampy meadows and small alluvial fans throughout the Holocene. Surface wash and gully extension were the mechanisms of erosion. Pleistocene deposits were the dominant source of sediment because of very low amounts of runoff from slopes. Impermeable sub-soils probably caused greater runoff from Pleistocene deposits. The Holocene was a period of low denudation rates, of the order of 1 mm of ground lowering per 1,000 years, averaged across the basin. Three times during the Holocene, as swampy meadows spread up valleys, intrinsically unstable conditions developed and a short period of gully erosion ensued. Monitoring of streamflow, and a simple hydrological model indicate that as swampy meadows expanded, runoff from moderate sized storms increased and small channels developed in the valley. The small channels are believed to promote gully initiation during large floods. Gullies started to infill once swampy meadows developed on the gully floor. Sediment delivery downstream then decreased to very low levels. Environmental changes during the Holocene had no effect on mechanisms of denudation and little effect on rates of denudation. With European settlement, gully erosion became more widespread than since the Pleistocene, although many gullies have now stabilised. In Wangrah Creek, gully erosion after settlement was initiated by different mechanisms than during the Holocene, and was probably the result of site-specific disturbances to the valley floor. Monitoring of erosion since a fire, coupled with the stratigraphic study, show that mild bushfires, whether anthropogenic or natural, will not accelerate erosion in Wangrah Creek. Aboriginal burning of the landscape in the late Holocene had little effect on denudation.