Bankfull hydraulic geometry; the role of in-channel vegetation and downstream declining discharges in the anabranching and distributary channels of the Gwydir distributive fluvial system, southeastern Australia

Abstract

Australian rivers frequently exhibit long periods of low or no flow, trees and shrubs that grow on the channel bed, and discharges that decline downstream. Four channels of the Gwydir distributive fluvial system (Gwydir and Mehi Rivers; Carole and Moomin Creeks) greatly contrast the hydraulic geometry of most other rivers, particularly in the way they respond to changes in discharge downstream. Data describing 167 cross sections across all four streams are assembled into standard exponential bivariate hydraulic geometry plots, with the relationships shown to exist outside the range of previously investigated downstream changes (i.e., in contrast to the commonly obtained exponents for width [b], depth [f], and velocity [m] of ~ 0.5, ~ 0.4, and ~ 0.1, respectively). Comparatively low exponents for width (b = 0-0.4) and high values for velocity (m = 0.26-0.42) reflect the importance of slope in accommodating changes in discharge. In sharp contrast with nearly all previous hydraulic geometry investigations where discharge increases downstream and slope decreases, in the Gwydir system slope declines in sympathy with discharge resulting in a marked downstream decline in stream power. The presence of in-channel vegetation is also argued to be a highly significant influence on the downstream hydraulic geometry of these streams. Because these streams are frequently dry, trees grow in abundance on the bed, appearing to displace the flow laterally and causing the channels to widen and shallow downstream - an adjustment that contrasts the high mud content of the boundary. The result is a very different hydraulic geometry in this anabranching-distributary system to that commonly described for other types of river.