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Hertfordshire County Council

Why do we sometimes leave debris in the river, or add debris to the river?

Woody debris has lots of benefits, both to the river and the wildlife that lives in it. Leaving it where it falls, or even adding it artificially, is a great idea, as long as there’s no risk it could cause flooding or erosion.

Historically all wood that had fallen into watercourses was removed, because it might look unsightly, or because of the risk of it causing flooding or erosion. However, it’s much better to consider each piece individually – often the advantages of leaving it where it is outweigh the risks.

Woody debris provides habitat for fish and insects, can help reduce erosion of river banks and creates more variety in the flow and the depth of the river. It’s so valuable that we sometimes create it artificially in well-chosen locations, to improve the flow in rivers which have become too wide.

Improving the health of a river doesn’t just help things at the bottom of the food chain. The benefits are passed on to larger fish, and to the top predator in our rivers, the otter, which is making a comeback after decades of decline.

Will the debris left or installed in the river make it more likely to flood?

Whether woody debris has fallen into a river naturally or been installed there intentionally, it is assessed in the same way to ensure it doesn’t increase the risk of flooding.

In some places, there’s a risk that it will cause blockages beneath bridges or in culverts and increase the chance of flooding. If this risk is too high, the debris will be removed.

In other locations, woody debris can actually reduce the risk of flooding in built up areas, by helping to encourage the storage of flood water upstream. When flood plains work as they should, this helps reduce flood risk elsewhere.

Re-naturalising a channel

The course of many rivers has been artificially modified, making them very different to their natural state. This affects the way the water flows, and the amount of vegetation along the banks.

There are different ways to improve habitat in a river like this. Most simply, using pre-planted rolls of matting attached to the banks, vegetation can get established even next to steep artificial banks.

You can also restore the original channel by recreating the natural course. It’s a big job, but once successful the flow of the river and the shape of its banks will be much more natural. Everything about the new course can be designed for wildlife, and a river flowing naturally looks much better too. If well planned, there’s no reason the risk of flooding should increase.

Reducing shading vegetation

Just like a pond, a river is healthier when sunlight reaches the water and the river banks. Especially when the river is narrow, trees along the banks can create a dark tunnel which suppresses the life beneath.

Reducing those trees, maybe by traditional management like coppicing, lets extra light through. More plants will grow along the banks and in the river, and insects will increase thanks to the extra plants. The bankside vegetation is also good for special animals like water voles. Even the river benefits as its banks are stabilised and can recover their normal width.

Grazing animals by the river

If there are too many animals in fields along a river bank, it’s bad for the river. The bank gets eroded and the river becomes muddy and excessively wide.

On the other hand, having animals by the river is no bad thing. They control plant growth and create bare ground, which is good for wildlife. The important thing is to get the balance right – keep an eye on the site and move the livestock if needed.

Ditch management

Just like any wet place, ditches are really important for wildlife. They can also play a role in filtering out pollution before it reaches rivers.

The best way to manage ditches is by clearing plants and silt when they build up. This can be quite destructive for the wildlife living in the ditch. It’s best only to clear sections of the ditch each year. This gives animals a chance to find refuge, and means the ditch keeps performing its filtering role too.

When clearing ditches, there’s also an opportunity to do more. Deep ditches with steep banks have few attractions for the many creatures that thrive in shallow water. Changing the shape of the banks, by creating a broad, shallow slope on one side, makes the ditch much more attractive to wildlife.

Buffer strips along watercourses

Stopping pollution reaching our rivers is valuable for everything that lives in them. Strips of long grass along ditches or streams help to filter things out before they get to the water.

Problems include eroded soil and the chemicals found in fertilisers. They all do much less damage if they can be kept from our water courses.

Buffer strips are also great habitat for wildlife, creating cover for animals which like living along the water’s edge like water voles.

Control of invasive non-native species

Some plants which don’t originally come from this country have invaded our rivers and streams, damaging native wildlife. One of the biggest problems is Himalayan balsam.

Himalayan balsam is an attractive plant with pink flowers, but can do real damage to our river habitats. It’s found especially along river banks, and its seeds are spread along rivers by the flow. It grows so strongly and densely that it shades out other plants, killing them off and becoming dominant. It can be controlled by pulling or cutting before it sets seed, but it’s very hard to remove completely.

Contact the Countryside Management Service

01992 588433

Countryside Management Service Office (Car Park H)
Environment Department (CHG001)
County Hall
SG13 8DN




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