Does reintroducing large wood influence the hydraulic landscape of a lowland river at multiple discharges?

Abstract

Large wood is a key structural and functional component of rivers, and it is reintroduced to provide physical habitat, encourage channel stability, and influence in-channel hydraulic conditions at a range of scales. Typically, studies assessing the influence of reintroduced wood on in-channel hydraulic character have been undertaken at relatively small scales-at a site. Relatively little is known about how reintroduced wood influences in-channel hydraulic character at larger reach scales and over different discharges. In this study, the hydraulic character or hydraulic landscape of multiple reference (wooded), control (unwooded), and managed (wood reintroduced) reaches (average length = 430 m) at 10 different discharges in the Barwon-Darling River, Australia, were analysed. Landscape scale hydraulic character was compared using spatial metrics describing the size, shape, and relative location of hydraulic patches in each reach. It was hypothesised that reintroducing wood would significantly influence hydraulic landscapes within the managed reaches, with those reaches becoming more similar to reference reaches. The reintroduction of wood into the Barwon-Darling River did not significantly influence the hydraulic landscapes of the managed reaches; instead, they remained more similar to control reaches across discharges. Discharge did significantly influence the hydraulic landscape, with stepwise changes in the character of the hydraulic landscape in response to increasing discharge. The limited influence of reintroduced wood on the hydraulic landscapes is hypothesised to be because reintroduced wood did not match naturally occurring wood in terms of the character of individual pieces or their spatial distribution within the reach.