Three types of valley morphology occur at the base of the escarpment in Bega catchment, on the South Coast of New South Wales, Australia. Each valley-setting contains a distinct river character and behaviour, namely cut and fill, bedrock-controlled discontinuous floodplain and fan. The character, behaviour and sedimentology of each base of escarpment river style are described and controls on their formation are assessed. Each river style operates under a certain set, or combination of boundary conditions that dictate the availability of accommodation space at the base of the escarpment. Subcatchments that do not drain above the escarpment (the uplands) have broad asymmetrical valleys at the base of the escarpment. The break in slope at the base of the escarpment is gentle and stream powers are low (around 45 W m-2 for the 1 in 5 year event). This aids sediment accumulation in these valleys. In addition, the downstream margin of these valleys is marked by a constriction which gives the valley a funnel shape. Large volumes of sediment accumulate behind this constriction, forming the cut and fill river style. In subcatchments which drain > 30 km2 above the escarpment, bedrock-controlled discontinuous floodplain and fan river styles are formed. The bedrock-controlled discontinuous floodplain river style is formed in a V-shaped, elongate valley at the base of the escarpment. These valleys do not have a downstream constriction, and have an abrupt break in slope at the base of the escarpment. This produces high stream powers (around 130 W m-2 for the 1 in 5 year event) with the potential to readily flush sediments downstream. Only narrow, shallow vertically accreted floodplains develop in sheltered sections of these valleys. In subcatchments draining into the fan river style, the valley at the base of the escarpment is concave and does not contain a downstream constriction. High slopes (around 0.025 m/m) and significant accommodation space allow coarse boulder alluvial fans to develop and be preserved. When placed in the context of long term landscape evolution, a model is developed that uses these valley-setting controls to explain the character, behaviour and evolution of the three river styles. Antecedent controls associated with relative rates of escarpment retreat versus valley formation at the base of the escarpment are proposed for each valley-setting.
|Number of pages||30|
|Journal||Zeitschrift fur Geomorphologie|
|Publication status||Published - Dec 2002|