A Geomorphic Analysis of River Change for Five Rivers in the Wellington Region: Implications for ‘Space to Move’ River Management Interventions

Abstract

River management has progressively shifted from hard engineering methods towards processbased restoration initiatives that focus on physical and ecological processes. This hydrogeomorphic approach entails a more hands-off approach to flood protection and ecosystem management through endeavours such as ‘space to move’ or ‘freedom space’ initiatives where channels self-adjust on the valley floor, thereby consuming their own energy and retaining their natural diversity. Geomorphic analysis of five rivers in the Wellington region shows they have undergone long histories of geomorphic change where their active channel zone have reduced in area in response to external pressures within their catchments. Channel migration across the valley floor has been severely restricted as a result of set channel boundaries. Reduction of active channel areas varies between the studied river reaches as follows: Otaki 49.0%, Waikanae 42.2%, Hutt (lower) 52.5%, Hutt (upper) 56.1%, Waiohine 24.9% and Waingawa 39.0%. Channel confinement as a result of land-use and flood management practices (stop banks, rock walls and riparian vegetation buffer zones) has induced transitions to less sinuous, single channelled, gravel bed rivers. Changes to the pattern and assemblages of instream geomorphic unit assemblages has reduced the geomorphic diversity of these rivers. Features such as point bars, mid-channel, bars and localized braiding are less common, as the rivers are now dominated by sequences of alternating bars. Analysis of changes in river behaviour and the resulting morphologies, framed in context of catchmentscale considerations, is used to outline possible future trajectories of river adjustment and allow an evaluation of ‘how much space’ is required for these river types to be managed within the ‘space to move’ framework.