Hitting rock bottom: morphological responses of bedrock-confined streams to a catastrophic flood

Abstract

Abstract. The role of extreme events in shaping the earth's surface is one that has held the interests of Earth scientists for centuries. A catastrophic flood in a tectonically quiescent setting in eastern Australia in 2011 provides valuable insight into how bedrock channels respond to such events. Field survey data (3 reaches) and desktop analyses (10 reaches) with catchment areas ranging from 0.5 to 169 km2 show that the predicted discharge for the 2011 event ranged from 400 to 900 m3 s−1, with unit stream power estimates of up to 1000 W m−2. Estimated entrainment relationships predict the mobility of the entire grain size population and field data suggests the localised mobility of boulders up to 4.8 m in diameter. Analysis of repeat LiDAR data demonstrates that all reaches (field and desktop) were areas of net degradation via extensive scouring of mantled alluvium with a strong positive relationship between catchment area and normalised erosion (R2 = 0.8). The extensive scouring in the 2011 flood decreased thalweg variance significantly with the exposure of planar bedrock surfaces, marginal bedrock straths and bedrock steps, along with the formation of a plane-bed cobble morphology. Post-flood field data suggests a slight increase in thalweg variance as a result of the smaller 2013 flood, however the current nature and distribution of channel morphological units does not conform to previous classifications of upland river systems. This suggests that extreme events are significant for re-setting the morphology of in-channel units in such bedrock systems. As important, is the exposure of the underlying lithology to ongoing erosion.