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
Australian rivers frequently exhibit long periods of lowor no flow, trees and shrubs that growon the channel bed,and discharges that decline downstream. Four channels of the Gwydir distributive fluvial system (Gwydir andMehi 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 sectionsacross all four streams are assembled into standard exponential bivariate hydraulic geometry plots, with therelationships shownto exist outside the range of previously investigated downstreamchanges (i.e., in contrast tothe 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=00.4) and high values for velocity (m=0.260.42) reflect theimportance of slope in accommodating changes in discharge. In sharp contrastwith nearly all previous hydraulicgeometry investigationswhere discharge increases downstreamand slope decreases, in theGwydir systemslopedeclines in sympathywith discharge resulting in a marked downstreamdecline in streampower. The presence ofin-channel vegetation is also argued to be a highly significant influence on the downstreamhydraulic geometry ofthese streams. Because these streams are frequently dry, trees grow in abundance on the bed, appearing todisplace the flow laterally and causing the channels to widen and shallow downstream an adjustment thatcontrasts the high mud content of the boundary. The result is a very different hydraulic geometry in thisanabranching-distributary system to that commonly described for other types of river.