Flow chutes in Fowlers Creek, arid western New South Wales, Australia: Evidence for diversity in the influence of trees on ephemeral channel form and process

This paper identifies a fluvial landform, the flow chute, from Fowlers Creek, an ephemeral stream in western New South Wales, Australia. On this stream there are contrasting configurations of channel-margin and channel-bed vegetation (channel-associated vegetation). In the most common type of channel reach, large Eucalyptus camaldulensis trees grow on the bed and banks, and contribute to high channel roughness. In some reaches of about 0.5 km length, however, no trees grow below mid-bank height. The bed remains completely unvegetated, and the channel narrows to one third or less of the width of adjoining reaches immediately upstream and downstream. The coarse end of the bedload grain size population is not represented in these narrow reaches, indicating that the coarsest grains are efficiently moved through them, overpassing finer sediments. These reaches evidently experience higher flow speeds than the remainder of the channel, are here termed flow chutes. The bed material within them is often moved as dunes that remain upon flood recession. Each flow chute ends with a reverse bed slope and just downstream of the terminus the channel widens, resulting in the deposition of coarse bed load in the form of gravel and cobble bars. The reverse bed slope detains water in the flow chutes, which remain as major pools or waterholes for months following flood events. The large and apparently old trees lining the flow chutes imply that these channel features are stable over periods of 102-103 years, in contrast with the behaviour of reaches with vegetated beds, where bank erosion and local lateral channel migration and/or widening are active. Flow chutes thus appear to be stable, equilibrium fluvial landforms.