Assessing river condition: A multiscale approach designed for operational application in the context of biodiversity net gain

Abstract

Assessments of river condition are needed to guide all aspects of river management. Such assessments have evolved over three decades from simply capturing the mosaic of river physical habitats to recognizing that habitat mosaics are dynamic, driven mainly by physical processes and modified by human (indirect) pressures and (direct) interventions. To embrace these broader aspects, riparian as well as in-stream environments need to be evaluated, going beyond subjective assessments to incorporate observations that support understanding of both physical habitat structure and cause-effect relationships. This paper reports on an operational approach to assessing the physical condition of rivers, which attempts to bridge the gap between a physical habitat and a geomorphic condition assessment. The approach forms part of Biodiversity Metric 2.0, a habitat-based methodology for measuring and accounting for biodiversity losses and gains resulting from development or land management change at individual project sites across England. The river condition assessment component adopts a bottom-up multi-scale approach that integrates field observations of physical habitats and of features indicative of geomorphic processes to deliver assessments of longer subreaches, whose condition is then evaluated within the context of the reach-scale geomorphological type of river. By applying the assessment before, immediately after, and following recovery from project implementation, changes in condition and their causes can be evaluated. The assessment method is presented to an international audience, outlining its structure, application and testing, and critically discussing its strengths and weaknesses, because the methodological approach could be helpful for devising methods for application in other environmental contexts.