Research and analysis

Improving river habitats to support wildlife during high and low flows: what works in which rivers: summary

Published 22 March 2024

Applies to England

1. Chief Scientist’s Group report summary

This project examined which river restoration measures can improve river habitats during high and low flows in different river types. Its findings will help the Environment Agency and others select appropriate actions to build geomorphological and ecological resilience to droughts and floods.

1.1 Background

Many English water courses have been physically modified by structures such as weirs and by actions like dredging and straightening channels. These actions can lead to reduced variation in river habitats, potentially making ecosystems more vulnerable during high and low flows. Anticipated changes in flow patterns due to climate change are driving action to create more diverse river habitats that retain function and structure in the face of such disturbances.

1.2 Approach

We undertook a literature review to identify evidence of the effectiveness of restoration measures in different situations. We plotted restoration measures, and a measure of the confidence we have that they are likely to improve geomorphological and ecological resilience, in different river types. This kind of ‘bounce-back’ resilience reflects the ability to recover from future disturbances. The information is provided in a ‘look up’ matrix. The matrix also shows those situations where little or no information on the effectiveness of measures is available. We used the matrix to select and then run hydraulic models to see what effect different measures would have in different river types under different flow conditions. We looked at both faster flowing streams with larger sediments and slow flowing chalk streams, as well as schemes using natural processes to create habitats.

Ecological and geomorphological resilience was assessed in the case studies by noting the variety and type of available habitats at different flows, including the presence of flow refuges (slow flow areas at high flows and flowing water during low flows).

1.3 Results

The literature review found that the most reported restoration measures are localised semi-engineering approaches such as flow deflectors, with nature-based approaches increasing in recent years. Papers generally provided limited information about the geomorphological setting of schemes. This made the river type and/or restoration measure hard to identify and restricted the amount of information that could be transferred to the matrix. Single channel, slow flowing river types were more studied than fast flowing rivers with coarser substrates.

The modelled case studies that looked at fast flowing rivers revealed that reconnecting channel and floodplain increases the number of slower flowing areas. During very low flows, habitat diversity was maintained. In more constrained sites (either because of land-use or urbanisation) improvements in geomorphological and ecological resilience were also observed from using a different range of restoration techniques to restore natural processes.  Evidence gaps remain, particularly in faster flowing rivers and with schemes encouraging natural recovery.

1.4 Conclusions

Restoration measures that consider catchment context and river type are more likely to result in improved geomorphological and ecological resilience. To provide resilience in years to come, it is worth considering whether the measures work in the range of flows we might expect in future. Larger scale benefits may be achieved by taking a wider system view in making space for channel migration, floodplain reconnection and allowing natural recovery over a larger area.  Our restoration matrix will help guide practitioners to select appropriate measures.

1.5 Publication details

This summary relates to information from project SC230009, reported in detail in the following output(s):

• Report: SC230009/R1
• Title: Improving river habitats to support wildlife during high and low flows – what works in which rivers
• March 2024
• Project managers: Judy England, Chief Scientist’s Group and Duncan Wheeler, Sustainable Abstraction.
• Research contractors:  1. Dave Mould, Luke Hussey, Andrew Griffiths, Matt Hemsworth - JBA Consulting. 2. Seb Bentley, George Heritage, Rob Williamson - Dynamic Rivers.

This project was commissioned by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency