Hydraulic assessment of environmental flow regimes to facilitate fish passage through natural riffles: Shoalhaven river below Tallowa Dam,New South Wales,Australia

Proposed environmental flow regime changes downstream of a major water supply dam have been assessed in terms of effects on depth, velocity and fish passage across natural, gravel‐bed riffles and rapids. This study focussed on passage requirements for Australian bass, Macquaria novemaculeata (Perciformes, Percichthyidae), a catadromous fish of considerable ecological and recreational fishing importance. Some 23 major riffles and rapids occur between the dam and the tidal limit over a river length of 25 km. Reconnaissance investigations of riffle slope, length, width, depth and morphologic characteristics indicated that wide‐shallow, steep‐turbulent and bifurcating riffle morphologies were most likely to cause problems for upstream bass passage under low flow conditions. Two approaches were used to investigate riffle depths and velocities over a range of flows. A rapid assessment approach directly measured thalweg depths and velocities under two controlled flow release rates in riffles identified as being potentially problematic to upstream bass passage. Detailed topographic surveys and two dimensional hydraulic modelling with River2D was undertaken for two riffles identified as ‘worst case’ examples of wide‐shallow and steep‐turbulent morphologies. Results from both approaches were consistent and complementary. Both approaches identified riffles where minimum depths and maximum velocities were likely to be problematic for upstream passage by Australian bass at a flow rate of 130 MLd^?1 (the current regulated flow release) but were mitigated at flow rates above 300 MLd^?1. Assessment of environmental flow regime transparency and translucency threshold options with regard to a 300 MLd^?1 target flow indicated that options where the transparency threshold was set at the 80th flow duration percentile (flows equalled or exceeded for 80% of time), and varied according to the monthly pattern of natural flows, improved hydraulic conditions for upstream bass migration. Copyright © 2009 John Wiley & Sons, Ltd.