HYDRAULIC GEOMETRY OF NEW-ZEALAND RIVERS AND ITS USE AS A PRELIMINARYMETHOD OF HABITAT ASSESSMENT

'Downstream' hydraulic geometry relationships describe the variation o f water depth, velocity, and water surface width between rivers of dif ferent size at a characteristic discharge, whereas 'at-a-station' geom etry describes the variation of hydraulic geometry with discharge with in a reach. The instream flow incremental methodology (IFIM) also pred icts the variation in water depth and velocity with discharge at a rea ch scale, so that hydraulic geometry relationships can potentially be used as a preliminary method of habitat assessment. Hydraulic geometry relationships were calculated from instream habitat surveys of 73 New Zealand river reaches with mean flows varying from 0.6 to 204 m(3) s( -1) and an average gradient of 0.0047. The exponents of both at-a-stat ion and downstream hydraulic geometry relationships were within the ra nge of values reported in other international studies, although the ex ponents indicated that New Zealand rivers tended to experience greater changes in velocity and less in depth than the international average, probably because of high average gradient. The frequency distribution s of water depth and velocity were positively skewed in most rivers, a nd on average the modal velocity was 90% of the mean velocity and the modal depth was 80% of mean depth. The use of art-a-station hydraulic geometry relationships for instream habitat assessment was compared to depth and velocity predictions using habitat simulation techniques (I F IM) in two streams. Measurements of stream width and depth at five c ross-sections;at two calibration discharges were used to establish at- a-station hydraulic geometry relationships. These predicted Mean depth and velocity within 8% of the reach average values of the IFIM survey s within the range of calibration discharges and within 10-15% of the IFIM reach average when extrapolated beyond the calibration discharges . Hydraulic geometry can be used to indicate whether hydraulic conditi ons approach a 'threshold' such as a minimum acceptable depth or veloc ity, thus predicating the need for more extensive habitat survey and a nalysis. (C) 1998 John Wiley & Sons, Ltd.