The construction of dams significantly alters flow and sediment regimes with subsequent deleterious effects on the morphological and ecological character of rivers.Effective experimental floods can ameliorate the downstream geomorphic impacts of dams. The traditional view is that large floods are required to perform effective geomorphic work, and the geomorphic outcomes of small floods are often overlooked. Many river restoration frameworks do not consider small floods. Yet, there is evidence that the hydrological characteristics that ameliorate specific geomorphic impacts in a river are unique to each river, and a customised approach to setting the right mix of floods (including small experimental floods) is needed. In this study, we modify an existing flood effectiveness model developed for large floods, for determining the geomorphic effectiveness of small floods in a highly regulated Australian river. Two flood classes were added to the model (medium peak stream power and moderate total energy expenditure), and the flood power characteristics were rescaled to reflect the relative difference in the magnitude of the small floods and the magnitude of the geomorphic work performed. Using a step-wise approach, this customised model determined the geomorphic effectiveness of small floods.The best flood for ameliorating the geomorphic impacts of flow regulation had medium to long duration (10 to 51 days), high peak unit stream power (77 to 123 Wm −2 ) and moderate to large total energy expenditure (78,600 to 342,320 × 10 3 J). This approach to determining flood effectiveness for small floods is applicable to other geomorphically impacted river channels downstream of dams and can be used to inform experimental flood releases for geomorphic outcomes.

Experimental floods, generated downstream of dams, are used to recover specific bio-geomorphic functions in regulated rivers. Studies of the effects of experimental floods vary in their objective, location, and the hydrological and bio-geomorphic variables used to quantify recovery. Measurements of geomorphic change are required to guide future release strategies. The focus of this study was to determine if a large experimental flood in the Snowy River Australia, could promote geomorphic recovery of the river channel downstream of Jindabyne Dam following 35 years of flow regulation. The objectives of the release were to deepen, widen, and increase channel capacity and coarsen the riverbed substratum in the Jindabyne Gorge and Dalgety Uplands sections of the Snowy River. Data from the release were compared with that of a natural flood event that occurred after the experimental flow event. Both events showed channel adjustments and a degree of geomorphic recovery, but this varied between the two river sections. Marked channel adjustments occurred in the Dalgety Uplands reach following both the experimental and natural flood event and in the Jindabyne Gorge section following the natural flood event. Geomorphic changes were related to the hydrological character of each flood event. The number of flood peaks, the sequence of peaks, the flood duration, and the total energy expenditure differed markedly between the two events, and these four flood hydrological characteristics explained the greater geomorphic recovery associated with the natural flood event in the Jindabyne Gorge. No clear hydro-geomorphic relationship was derived for channel change in the Dalgety Uplands where existing morphological constraints limit flood effectiveness. KEYWORDS channel maintenance flows, conceptual model of channel adjustment, environmental flows, flushing flows, geomorphic recovery