Macroinvertebrates

Aquatic macroinvertebrates, such as stonefly, caddisfly and mayfly larvae, live in riverbed sediments and play a crucial role in the riverine food web. Many macroinvertebrates are sensitive to sudden changes in flow and water quality, especially when that is outside the normal range of their habitat. For example, during extreme flood events, fast and turbulent water can scour the riverbed essentially "washing" macroinvertebrates downstream, while shifting cobbles and boulders crush vulnerable individuals. This is known to dramatically impact the abundance and diversity of macroinvertebrates.

The timing of flooding is crucial for understanding the rate of macroinvertebrate recovery, as many species have highly seasonal mating pattern, with juvenile larvae developing over winter and emerging to mate and lay eggs in the spring and summer. As a result, the timing of the recent November flooding suggests we may see a slow recovery rate of macroinvertebrate abundances and fewer emerging macroinvertebrates in 2026 and possibly 2027. Reduced macroinvertebrate abundance and diversity will have ripple effects throughout the food web, affecting the availability of prey for fish (e.g. trout, salmon, grayling), birds (e.g. dippers, kingfishers) and mammals (e.g. Otters and Bats).

Fish spawning (trout and salmon)

The Monnow is well known for its brown trout and recovering Atlantic salmon populations. Between October and January, both species create nests called redds in the gravel of the riverbed, where eggs are laid, fertilised and left to develop. Extreme flooding during this crucial period can wash out or bury redds, break eggs as gravel is rolled around in turbulent water, or expose them to predators. Salmonid eggs require a constant flow of oxygenated water for the embryos to develop. However, post-flooding sediments are deposited when the river level drops and can suffocate redds that might have survived the turbulent scouring. These combined effects may reduce the number of juvenile fish and could be reflected in the number of returning adults over the next few years.

Salmon or Trout Redd

Effects on other mammals

The effects of flooding on wildlife are not just limited to aquatic species; the flash-flood nature of the Monmouth flood could have caught animals off guard and swept them up in the flood water, including badgers, foxes and dormice. In addition to this, hedgehogs start to hibernate in October-November in low-lying ground, such as in tree roots and hedgerows, leaving them vulnerable to drowning if their hibernaculum floods. Finally, otters are known to occupy the Monnow, and their holts may have been washed away by the flood, meaning they must spend more energy and resources building new holts instead of foraging for food.

Illustration showing NFM techniques working together on a catchment scale. Emma Wren, Megan Barnes, and Amanda Kitchen. The natural flood management manual. CIRIA, 2022.

What can we do

Climate change in Wales is likely to increase the frequency and intensity of extreme weather events, including rainfall and flooding. It is important that we build an environment that is resilient to the effects of climate change by centring nature and wildlife and working with natural processes rather than fighting against them.

On an individual level, we can help to hold more water in our environment by using water butts to capture rainwater and emptying them before heavy rainfall events. By removing impermeable ground from gardens, like concrete and patios, and replacing it with natural vegetation and ponds. Or by volunteering with local groups to do litter picks, habitat work parties or join citizen science projects such as The Riverfly Monitoring Initiative. For those who own land (especially in catchments that are susceptible to flooding), installing Natural Flood Management (NFM) techniques such as leaky dams in streams or ditches, across-slope planting to slow water runoff and excavating ponds and scrapes can help reduce the severity of flooding.  

For NFM to work, it must be deployed across the catchment, not in scattered pockets. For example, a few ponds and leaky dams will make only a small difference, but many working together can slow the movement of water through the catchment and increase resilience to extreme weather events. Implementing NFM on catchment scales requires collaboration between government bodies such as NRW, water companies, landowners and managers to coordinate planning, funding and long-term maintenance. It is equally important to share objectives and techniques so that measures are placed strategically rather than in isolation. Over time, this collective approach will improve climate resilience across the entire catchment, while centring wildlife and working with natural processes.

This appeal has been set up by the Monmouth community to help those affected:

Monmouth Aid | Storm Claudia Flood Relief & Community Support