Droughts are natural events, and there are lots of streams or sections of stream that we would expect to naturally flow intermittently due to climate, weather patterns and fluctuations in underground stores of water. Such streams are inhabited by organisms with adaptations that allow them to subsist in these habitats. For example, many aquatic invertebrates lay eggs that are covered in a gelatinous membrane, preventing the embryos from drying out, ensuring that they can survive without water during dry spells. Other aquatic animals actively seek refuge during droughts, burrowing in wet muds or sheltering in damp areas.
However, not all aquatic organisms are well-adapted to cope without water, which is one reason why droughts, particularly those brought on by, or exacerbated by human activities, and in streams for which drying up is uncharacteristic, can be problematic for wildlife (not to mention us). In recent weeks you may have seen images of the Environment Agency working hard to rescue fish left without enough water to survive. In the UK, our native fish have few adaptations to these conditions, other than the ability to move and seek pools for refuge. In other more arid countries, some fish have evolved adaptations that allow them to travel over land to find suitable habitat, or burrow into mud and produce a mucus cocoon.
There are lots of human activities that increase the likelihood of rivers and streams drying up or experiencing low flows. One of the most significant is our conventional approaches to land and water management. Historically, land management has considered water somewhat of a nuisance, and every effort was made to encourage water off of the land and out to sea as quickly as possible. After removing trees and hedgerows, digging ditches, installing land drains under fields, constructing river embankments, and dredging, channelising and clearing debris from our rivers, water now travels quickly downstream.
In most cases, Natural Flood Management is focused on reversing these activities and restoring the ability of the land to slow and store water. In doing so, water is once again allowed to infiltrate into soils and slowly drain into surface waters, or percolate deeper into soils and replenish groundwater stores, resulting in a more steady supply of water to rivers and streams. By reintroducing woody debris to streams, not only does it help to ‘slow the flow’ but it also encourages a more diverse flow pattern (which is often lacking due to channelisation and lack of debris) and influences geomorphological processes, including the formation of scour pools. These pools can be an important refuge for aquatic organisms during droughts, and if the stream does completely dry up, woody debris is likely to benefit burrowing invertebrates by providing heavily shaded areas where sediment remains moist for a longer period of time.
These benefits for wildlife were visible when we visited some of our woody debris dams earlier this week. It was easy to see the changes in streambed level, and the scour pools that have been formed since we installed the features. The stream was not flowing and had largely dried up, but there were numerous scour pools downstream of woody debris that were visibly sheltering a (crowded) community of aquatic invertebrates. It was great to see that our NFM work is bringing benefits year-round. The multiple benefits provided by NFM, are why we at the Sussex Flow Initiative are so passionate about this approach to reducing flood risk, by restoring and working with natural processes.