This paper sets out an approach to habitat quantification by a simple assessment of stream geomorphology at a reach scale, based primarily on aerial photo interrogation, augmented by the use of LiDAR, and developed by targeted fieldwork. This provides a rapid, cost-effective means of assessing geomorphic diversity, vis-à-vis habitat quality, which is dependent on the physical structure of a river reach. We suggest the use of a natural character index (NCI) be incorporated as a tool to monitor baseline conditions and any subsequent deterioration of habitat in response to human activity in the river channel.

Interrogating archive aerial photography that pre-dates river management schemes in New Zealand provides a means of assessing the pre-intervention ‘natural character’ of a reach. Reach characteristics can then be compared with more recent imagery to assess the extent to which reach geomorphology has changed following intervention. This can then be used as a baseline from which to measure future reach adjustment. Airborne laser mapping (LiDAR) provides a means of assessing floodplain and channel geomorphology in greater detail, precisely defining floodplain and channel parameters and identifying previous channel courses. Targeted fieldwork to quantify key parameters likely to change in response to river engineering (e.g. substrate characteristics) can be incorporated to refine understanding of reach character. As such the NCI can be used as an ongoing tool to monitor river condition in modified reaches before, during and after river engineering, as well as at sites upstream and downstream of river works to provide context for the impacts of intervention and disentangle human-induced change from naturally-driven changes.

NCI is not a detailed descriptor of river condition, unlike the Tasmanian River Condition Index, for example. However, its value lies in the relative ease and rapidity afforded by such a desk-based and targeted field assessment. It should not be understood as an absolute measure of habitat diversity and quality, but as a means of quantifying relative changes in geomorphic characteristics and habitat quality between successive phases of human intervention in river channels. Its usefulness lies in providing a simple score, which can be re-measured as part of ongoing river monitoring. Deterioration of the score should be avoided by deploying appropriate river management strategies, and rehabilitation schemes may seek to establish improvements in NCIs of degraded reaches. As such, NCI provides a useful, simple approach to quantifying habitat quality, which has hitherto been overlooked, particularly in New Zealand.