An assessment of river environment by physical characters: Evaluation by the degree of departure from reference conditions

The purpose of this study is to conduct a simple and comprehensive stream assessment based on a “Degree of Departure from Reference (DDR)” in physical environments. We define ‘departure’ as the distance between a reference site and each test site in multiple principal component dimensions of physical conditions. The assessment was conducted at the reach scale. The three categories were anthropogenic alteration, habitat diversity and stream & floodplain structure, with each consisting of specific indices representing physical conditions of the stream and floodplain. We established 68 test sites in a lowland, valley bottom segment of the Shibetsu River, Hokkaido, northern Japan, where relatively undisturbed reference sites were available. Physical conditions for each reach were measured, using a survey method modified from the River Habitat Survey, a quantitative assessment system that has been developed and practiced in the United Kingdom. The test sites were classified into principal components separately for three categories. A group of least disturbed sites was determined as a reference. The departure in physical conditions was smallest within the reference group and was large in other reaches which exhibited differences from the reference group in terms of multiple indices. These sites departing from the reference flow through urban areas and show a high level of alteration, low habitat divesity and sinuosity and less frequent occurrence of riffles, pools and bars. The Degree of Departure from Reference is an objective measurement of differences between reference conditions and a target site for assessment, which provides a comprehensive, accurate evaluation of stream physical environments. Our assessment method is applicable to other rivers. It can be also more easily used for spatially large-scale assessment relative to the ones with biotic indices, thereby providing an effective tool for ‘mass-screening test’ of river environments.