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

Abstract

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. Figure 4. Channel cross sections and bench sediments of (A) upstream right bank bench, (B) upstream unvegetated section (sand splay) of downstream right bank bench, and (C) downstream section of downstream right bank bench.Figure 5. Bivariate plots of graphic mean size versus (A) inclusive graphic standard deviation and (B) inclusive graphic skewness, showing that the bench nucleus sediments are similar to the adjacent bed and bar sediments.Figure 6. Size-class (DBH) distributions of live (white) and dead (black) stems of C. cunninghamiana on (A) upstream right bank bench, n = 510; (B) downstream right bank bench, n = 275; (C) left bank bench, n = 139.Figure 7. Model of bench development and growth from a point bar, and bench destruction by catastrophic floods or a sequence of large floods in rapid succession.

Potential controls of alluvial bench deposition and erosion in southern Piedmont streams, Alabama (USA)

“…The greater drag introduced by the vegetation resulted in greater deposition over all benches at all sites. This agrees with the findings of many researchers (Steiger et al 2001;Corenbilt et al, 2007;Erskine et al, 2008). Unexpectedly, the simulations accounting for vegetation also predicted greater erosion along the bench face of all benches at all sites.…”

“…The model simulations seem to confirm findings by others (Steiger et al, 2001;Corenbilt et al, 2007;Erskine et al, 2008) where vegetated areas received significantly more deposition than unvegetated areas. An unexpected result, and one previously undocumented, was erosional processes predicted by the model being affected by vegetation.…”

Can the sedimentological and morphological structure of rivers be used to predict characteristics of riparian seed banks?

“…Parts of the system have been subject to historical channel incision and widening induced by widespread clearance of floodplain vegetation that occurred postEuropean settlement and a series of floods that occurred in the 1940s and were the largest on record (Erskine, 1996(Erskine, , 2008. Since this time, many of the severely widened channels have undergone extensive channel contraction through the formation of depositional benches (Erskine et al, 2009;Fryirs et al, 2012). Since the 1980s, an increase in vegetation coverage within the channel and on the floodplain has greatly increased the stability of streams within the catchment (Erskine et al, 2009).…”

Riparian proper functioning condition assessment to improve watershed management for water quality

“…Meanders create a low velocity zone adjacent to point bars that form through deposition of bedload and later suspended sediment. Vegetation growing on deposited coarse bedload material increases stability and roughness, decreases flow velocities, increases deposition of finer suspended sediment (figures 2e and 3b to 3g) (Robertson and Augspurger 1999;Steiger and Gurnell 2003;Rood et al 2003;Robertson 2006;Polzin and Rood 2006), and encourages meander development (Erskine et al 2009(Erskine et al , 2012Rominger et al 2010;Tal and Paola 2010), which decreases stream gradient and power (Bagnold 1966). Deposition of fine alluvial soil facilitates revegetation while it removes water pollution.…”