Role of a rheophyte in bench development on a sand-bed river in southeast Australia

摘要

Casuarina cunninghamiana Miq. is an important rheophytic tree in New South Wales, Australia because it is fast growing and can tolerate flood disturbance. Widden Brook is an active sand-bed stream that has widened substantially since initial European settlement in the early 1800s and is characterized by high flood variability and multi-decadal periods of alternating high and low flood frequency, called flood- and drought-dominated regimes. Channel contraction by bench formation is currently occurring. Conversion of coarse-grained point bars to benches is an important process of channel contraction. When point bars grow to a height where suspended sediment is first deposited to thicknesses of at least 50 mm by sub-bankfull floods, rapid establishment of C. cunninghamiana occurs. As the trees grow they partially block bankside flows, thereby locally reducing flow velocity and inducing further deposition on the benches. Such synergistic relationships between bar height and inundation, fine-grained sediment deposition, tree establishment and the development of a bankside low current velocity zone are fundamental for bench development. Size-class frequency data demonstrate that C. cunninghamiana on the benches consists of pure even-aged stands with most trees clustering near the average diameter. Two benches have similar size class frequency distributions but a third has significantly smaller trees. Recruitment on benches is episodic, may occur in areas open to grazing and is dependent on favourable conditions that allow tree survival. These favourable conditions include high seed availability, low levels of competition, deposition of fine sediments and adequate moisture for tree growth. Although C. cunninghamiana germinates on bars, seedlings are eliminated by prolonged inundation or flood scour and do not reach maturity. Recurring catastrophic floods or a sequence of large floods in rapid succession episodically destroy benches by substantial channel widening and initiate a new phase of bar and bench development. A conceptual model of the conversion of point bars to benches by thick mud deposition and C. cunninghamiana recruitment has been developed for sand-bed streams draining similar sandstone catchments.