It’s not the length that matters….

Does river habitat restoration have to be a certain scale before it can be considered beneficial to the wider ecology of a river? It’s a question in one form or other that our WTT Conservation Officers often get asked. Is it really worth putting that one log deflector or hinged willow etc into that reach? Without the time or resource to conduct a robust scientific study, we’re often simply basing our opinions upon experience of what has seemed to work before. Despite the increasing number of river restoration projects being initiated across the world, scientific evidence on the long-term impacts of such projects and what makes them a success or a failure is still quite thin on the ground.

Well, apparently it is worth doing at the small scale according to some new research, provided that quality and diversity of habitat are accounted for.

As part of a large project involving sites across 10 different regions of Northern Europe, a research team has studied the effects of two restoration projects in each region: one on a short section of river and one on a longer section. At each restored section, the team not only sampled habitat composition in the river, but interestingly also its floodplain. To complement this, they sampled aquatic plants, insects and fish, as well as floodplain vegetation and ground beetles, and the food web composition and any land–water interactions across the boundary between the two.  These findings were then compared to samples taken upstream at non-restored sections of river of roughly equal length.

After accounting for potential variations in river size and restoration approaches across the different regions, the team found that the length of the actual restoration made no significant difference to its apparent ecological health and the diversity of life within.  The most important driver was the substrate composition of the river bed, i.e. the aquatic habitat it provides for organisms. So, exactly the sort of measures that WTT advise on and instigate to rehabilitate river habitat such as increasing the presence of wood (e.g. tree kickers or hinged willows) to diversify water flows and depths and sort gravels, were found to be responsible for increased populations of fish, aquatic insects, aquatic plants and floodplain vegetation.

Perhaps one of the most interesting findings was the marked effects that were felt beyond the boundary of the river and into the actual floodplain. In-river restorations tend to improve the linkage between the water and the land by creating habitat types close to the river bank, such as bars of deposited gravel and sand, which are often absent in degraded sites. These are readily colonised by floodplain plants and animals which have good dispersal ability.

The study (available here) provides further scientific evidence underpinning some of the holistic restoration work that WTT does to improve habitats, not only for trout, but for all flora and fauna living in and along our river corridors.