The importance of physical habitat assessment for evaluating river health

1. Physical habitat is the living space of instream biota; it is a spatially and temporally dynamic entity determined by the interaction of the structural features of the channel and the hydrological regime. 2. This paper reviews the need for physical habitat assessment and the range of physical habitat assessment methods that have been developed in recent years. These methods are needed for assessing improvements made by fishery enhancement and river restoration procedures, and as an intrinsic element of setting environmental flows using instream flow methods. Consequently, the assessment methods must be able to evaluate physical habitat over a range of scales varying from the broad river segment scale (up to hundreds of kilometres) down to the microhabitat level (a few centimetres). 3. Rapid assessment methods involve reconnaissance level surveys (such as the habitat mapping approach) identifying, mapping and, measuring key habitat features over long stretches of river in a relatively short space of time. More complex appraisals, such as the Physical Habitat Simulation System (PHABSIM), require more detailed information on microhabitat variations with flow. 4. Key research issues relating to physical habitat evaluation lie in deciding which levels of detail are appropriate for worthwhile yet cost-effective assessment, and in determining those features that are biologically important and hence can be considered habitat features rather than simple geomorphic features. 5. The development of new technologies particularly relating to survey methods should help improve the speed and level of detail attainable by physical habitat assessments. These methods will provide the necessary information required for the development of the two-and three-dimensional physical and hydraulic habitat models. 6. A better understanding of the ways in which the spatial and temporal dynamics of physical habitat determine stream health, and how these elements can be incorporated into assessment methods, remains a key research goal.