Vegetation responses to managed river flow events and regimes

Christopher Jones , Lyndsey Vivian, Henry Wootton, Elliot Gould , Jian D.L. Yen, Bryan Mole, Freya M Thomas, Fiona Sutton, Diane Crowther, Ana Backstrom, Karl Just

Vegetation communities are an important part of riverine ecosystems and can be severely impacted by changed flow conditions in regulated waterways, making them a priority for waterway management. Environmental flows are a commonly used tool to mitigate some of the impacts from regulated flow regimes and provide benefits to vegetation communities. Ongoing and effective use of environmental flows requires evidence of vegetation responses to managed flow events and regimes, yet few studies have demonstrated these responses in managed waterways.

We conducted vegetation surveys at 44 sites within seven major river systems across Victoria (Aus.) between 2016 and 2023. Vegetation cover and richness were sampled at a range of bank elevations impacted by different flows within each waterway. Surveys before and after flow events over multiple years enabled us to evaluate vegetation responses to flow events and regimes. Generalised linear mixed models were used to evaluate responses of richness and cover for four different plant functional groups (aquatic, emergent, riparian and terrestrial) within delimited flow elevation zones on the bank.

Our findings demonstrate that while individual managed and natural flow events may produce modest immediate vegetation responses, their cumulative impact over time can be substantial, increasing plant cover and richness that may support the resilience and stability of riverine systems. The effectiveness of environmental flows is highly influenced by context and a range of flow and non-flow variables. Short duration environmental flows with appropriate timing and duration benefit native aquatic, emergent and riparian species. Terrestrial species can benefit from flows but are the most sensitive to prolonged flow. Managers will also need to apply on-ground actions to enable ecological outcomes from flows in some cases.