Managing River Low Flows to Enhance Instream Vegetation Recruitment.

River regulation has reduced natural flow peaks in rivers globally, and in some cases has also reduced the occurrence of low-flows that expose the riverbed. Minimum low-flows are commonly mandated for temperate managed waterways in summer to maintain water quality and aquatic habitat for flora and fauna, at levels which prevent riverbed exposure. Very low flows that allow partial riverbed exposure may have many important roles in naturally impermanent waterways, including promoting plant recruitment. We conducted an in-situ field experiment in a regulated river by drawing down flows for two weeks in austral autumn to facilitate plant recruitment from the riverbed. We also conducted a concurrent ex-situ experiment in controlled conditions using sediment samples from field plots and subjecting them to exposure and inundation treatments. Two-week exposure of riverbed sediments was sufficient to trigger the germination of thousands of flood-tolerant riverine plants in both the in-situ and ex-situ experiments, but aquatic plants showed little response. Terrestrial plant seedlings were uncommon within the river-bed substrate. Seedlings were tolerant of early re-inundation but prolonged inundation resulted in senescence and mortality for non-aquatic plants. Very low flows in rivers for at least two weeks may facilitate recruitment of flood-tolerant riverine plants but the event timing and re-inundation regime will influence the likelihood of successful plant establishment. While there are potential risks for some aquatic taxa by implementing very low flows, this needs to be weighed against the potential benefits of riverbed exposure for promoting important biotic processes including plant recruitment.