Managing River Low Flows to Enhance Instream Vegetation Recruitment

Area of Science:

• Ecology
• Riverine Ecology
• Botany

Background:

• River regulation globally has reduced natural flow variability, including low-flow events that expose riverbeds.
• Managed low-flows in temperate rivers often aim to prevent riverbed exposure, potentially hindering natural ecological processes.

Purpose of the Study:

• To investigate the role of very low flows and riverbed exposure in promoting plant recruitment in regulated rivers.
• To assess the impact of exposure duration and re-inundation on plant establishment from riverbed sediments.

Main Methods:

• Conducted an in-situ field experiment involving a two-week flow drawdown in a regulated river during austral autumn.
• Performed a concurrent ex-situ experiment using riverbed sediment samples subjected to controlled exposure and inundation treatments.

Main Results:

• A two-week riverbed exposure successfully triggered germination of thousands of flood-tolerant riverine plants in both in-situ and ex-situ settings.
• Aquatic plants showed minimal response, and terrestrial seedlings were scarce in the riverbed substrate.
• Seedlings tolerated initial re-inundation, but prolonged inundation led to mortality in non-aquatic plants.

Conclusions:

• Very low flow events lasting at least two weeks can facilitate flood-tolerant riverine plant recruitment.
• The timing of these low-flow events and subsequent re-inundation regimes are critical for successful plant establishment.
• While potential risks to aquatic taxa exist, riverbed exposure benefits may outweigh them by promoting vital biotic processes like plant recruitment.