Re-blogged from the TrUck – Trees on the River Uck – project’s blog “Wood in rivers, in or out?”
Historically woody debris that occurs naturally in our rivers has been removed, in the belief that it reduces the movement of fish upstream, and exacerbates flood issues. Recent research shows however that in-river wood is of great importance in river health and the provision of spawning grounds and shelter for fish.Wood in the river channel can accumulate forming complex dams of branches, twigs or whole trees. In some places they are caused by living trees that have fallen but continue to grow.
These dams help to trap sediments which would otherwise be carried downstream and settle on lower lying areas increasing flood risk. The addition of wood to the channel greatly increases the diversity of flow in the river, and provides shade and shelter – all of which of important to a wide range of fish species. They can also help filter out pollutants and nutrients that can reduce water quality for fish and other river speices.
In summary, river channels without woody debris often lack the range of different habitats that are vital for species diversity and river health.
We hope to encourage landowners and land managers to leave woody debris in the River Uck where possible. Some areas of the Uck, such as between Hempstead Mill and Uckfield town centre, the channel must be kept clear of large debris, as this could exacerbate flooding here. This must be a key consideration in management of in-river wood.
There are a number of other national projects that are using woody dams to encourage water onto the floodplain in strategic places, in order to increase upstream flood storage, and also to help restore degraded watercourses. These dams are designed to allow fish passage, and pinned in place to avoid movement. They are placed in areas where increased flooding is possible, and avoid areas where they could have a negative impact, for example in close proximity to bridges or culverts. See the work of the Slowing the Flow at Pickering project for an example of woody debris in action.
We are using digital mapping software to help identify areas for work, and areas to avoid. We are also looking at ways we can increase water storage in woodlands through created of ditch-top woody dams that help to channel water into lower lying areas of the woodland under high water flow.
If you would like to know more about the role of woody debris in rivers, listen to this BBC radio programme, or read Staffordshire Wildlife Trust’s woody debris document ‘Fish Live in Trees too‘. If you would like some advice on management, addition or removal of wood in your local stream or river then get in touch via our contacts page.