Processes, rates and pathways to recovering river channel functions through geomorphically effective floods: a case study of the highly regulated Snowy River downstream of Jindabyne Dam, Australia

The main purpose of this research was to determine how river channel functions in the gorge and upland reaches of the highly regulated Snowy River might recover after experimental floods and natural floods from the downstream residual catchment, since Jindabyne Dam closed in 1967. By quantifying the geomorphic effectiveness of various floods at the site, channel unit and cross-section scales, the processes and rates of adjustment with time were determined for channel width, depth, conveyance capacity and surface bed sediment size. This enabled the rates of adjustment and pathways to suitable recovery target endpoints to be quantitatively modelled. Current recovery models are based on few observations and measurements and are lacking well-articulated endpoints. This lack of understanding of channel recovery processes, rates and pathways, limits predictions that could assist in the management of this river. Channel adjustment was spatially and temporally complex. Adjustments were through different processes, at different rates and on different pathways in response to floods with different flood power characteristics. The ‘most geomorphically effective’ floods had high peak stream power (69-77 W/m²), long duration (49-280 days), moderate to large total energy expenditure (78610-128441 Joules x 10³), high peak discharges (126-447 cumecs), and multiple high discharge peaks on the rising and falling limbs of the hydrograph. These floods have disrupted the dynamic equilibrium after nearly 50 years of de-watering. By world standards these floods may seem small, but for the Snowy River, this disequilibrium has returned some river channel functions. Consistent and statistically significant geomorphic adjustment trends occurred in runs, with time but not in riffles, pools and pool-tails. Runs offer the most accurate indicators of channel widening, deepening, increasing conveyance capacity and sediment coarsening. Most run cross-sections selected were predicted to recover to average target endpoints by 2038 ±10 years (allowing for bushfire and flood) if up to 23 ±1 more, ‘most geomorphically effective’ floods are released annually from Jindabyne Dam. A new dynamic equilibrium for most runs might then be reached if adjustment trends in the four geomorphic variables are consistent. Once most run average target endpoints are reached, other ‘moderately’, or ‘less’ geomorphically effective floods can be released for channel maintenance and bank building. Phragmites had the right combination of root-sediment properties to best stabilise the overbank flood deposits that form the new banks of the Snowy River.