Environmental flows drive sediment and seed supply in regulated rivers

Abstract

Environmental flows are commonly provided in regulated rivers to maintain and restore riverine environments, including riverbank vegetation. Given the expense and political sensitivity of providing water for the environment, greater certainty regarding the expected benefits of environmental flows is required. We assessed the role of managed flows (for both environmental and consumptive purposes) in delivering sediment and plant propagules to facilitate riverbank recovery. Using artificial turf mats, we determined sediment and seed deposition from 11 flow events over 3 years of varying magnitudes, durations and timing in a heavily regulated lowland river in south-east Australia. At three sites on the Goulburn River, Victoria, mats were placed on different geomorphic features along an elevation gradient (bars, benches, banks and ledges) before a flow event, and the sediment deposited during the event was analysed and assessed for seedling emergence. A total of 401 kg of sediment and 110 980 seeds were sampled. Mats that were inundated via managed flow releases had an order of magnitude more sediment, and around twice the richness of seed taxa, than mats that were not inundated. More sediment and seed taxa were deposited on low-lying bars than on more elevated features, while seed composition but not abundance varied across features. A longer duration of inundation was associated with the deposition of more sediment, but not seed taxa or abundance. Unexpectedly, there was no evidence that managed flows delivered less sediment and seeds compared with unregulated tributary inflows. Our findings suggest that in regulated rivers, environmental flows are likely to be critical for providing the sediment and seed necessary to restore geomorphic conditions and promote plant recruitment. Longer flow durations are likely to provide more sediment, while high-flow events may be particularly important for promoting diverse native riparian plant communities across the full range of in-channel geomorphic features.