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.

The current state of the use of large wood in river restoration and management

“…It diverts water flows, increases local water levels and introduces turbulence, creating a mosaic of fast and slowing flowing areas. This hydraulic effect is essentially immediate, but varies with river discharge and level (Matheson et al ., ), providing essential shelter and refugia during high flow events for fish.…”

Floating matter: a neglected component of the ecological integrity of rivers

“…Individual pieces and accumulations of large wood obstruct and interact with water flowing in river channels to increase hydraulic heterogeneity, the complexity of flow pathways, and the variability of flow velocity for any given discharge. Typical local effects of in-channel wood accumulations include water ponding and sediment retention upstream; scour pools both below, laterally and downstream; bank erosion, avulsions and the scour of new channels in the adjacent floodplain; fluvial sorting and the deposition of the scoured sediment in the form of bars and benches (Montgomery et al 2003;Gurnell 2013;Wohl 2013;Elosegi et al 2017;Matheson et al 2017;Parker et al 2017). Furthermore, wood removal from river channels is rapidly followed by the disappearance of many of these scoured and depositional features (e.g.…”