BEDROCK CHANNEL INCISION ALONG PICCANINNY CREEK, AUSTRALIA

Piccaninny Creek in northwestern Australia drains approximately 60 km(2) of high relief sandstone and conglomerate terrain. The drainage basin lies within the seasonal tropics and is characterized by ephemeral, high-magnitude flows. Flood flows of 50 to 100 m(3)s(-1) are probably primarily responsible for the spectacular erosional features along the bedrock channel segments of the creek. These erosional features occur in two basic forms: as parallel longitudinal grooves, and as a downstream sequence that begins with shallow linear depressions parallel to flow and ends in a deep, narrow, inner channel. The longitudinal grooves are assumed to result from longitudinal vortices, while the erosional sequences, which closely resemble von Karman vortex streets, are interpreted to result from turbulent vortices shed off irregularities of the channel bed. The longitudinal grooves and the inner-channel sequence alternate with each other and with gravel-floored depositional reaches downstream along the channel. The locations of these three types of channel beds do not correlate with channel-substrate characteristics like rock strength, lithology, or structural variability. The locations do appear to be related to channel gradient, however. The number and length of channel segments with inner channels are greatest at, and immediately upstream of, a 1 km length of steeper channel interpreted as a knickzone.