Restoring our rivers - how wood can be part of the solution

The Four Rivers for LIFE project aims to improve the habitat structure and function of four SAC Rivers; the Teifi, Tywi, Cleddau and Usk.

On the Western Cleddau river in Pembrokeshire, project officers have completed work to introduce wood into the river as part of efforts to restore the river’s natural structure.

In this blog, Four Rivers for LIFE River Restoration Officer, Duncan Dumbreck, explains the importance of this work.

The history of the Cleddau River at Llangloffan Fen

The Western Cleddau SAC flows through a lowland wetland called Llangloffan Fen which is located upstream of Letterston village in Pembrokeshire.

The fen is a National Nature Reserve (NNR) and a Site of Special Scientific Interest (SSSI), and forms part of the Western Cleddau Special Area of Conservation (SAC).

The SSSI is managed in parts by the Wildlife Trust of South and West Wales (WTSWW), Natural Resources Wales (NRW) and some private landowners.

The Cleddau River SAC is protected as it is home to important fish populations such as bullhead and lamprey, as well as otter and rare plants like water crowfoot.

    

Images left to right - Bullhead and Lamprey, otter and water crowfoot (credit Jack Perks).

The river has a long history of modifications dating back to the 1800s. Practices such as channelisation (straightening) and dredging (removing boulders and gravel) has led to a relatively featureless riverbed, with parts resembling more of a ditch than a natural meandering river.

By the late 1970s it was recognised that this was the last of a valuable habitat, and efforts to modify it were stopped.

Why are we introducing wood into the river here?

Since the 1980s research has highlighted the importance and value of wood in our rivers, and evidence now shows us the ecosystem benefits of wood either left or introduced back into our rivers.

Naturally fallen trees are a vital component of a healthy river as they help to diversify the river structure and provide important habitat for invertebrates and fish. The existence of a range of species of insects, fungi and other microbes depends on wood.

Leaving fallen trees (where it doesn’t pose a flood risk to property) in the river channel can help rivers recover as woody debris acts as a catalyst to trap gravels and silts, raise bed levels, and help create different flow patterns.

To step-up the recovery of the Western Cleddau river the Four Rivers for LIFE project is using a method called process-based restoration.

By mimicking the river’s natural processes and amplifying beneficial processes, we enable the river to take control and to restore itself over time.

In this case, large wood was introduced to the channel and along the banks to mimic what happens when a tree falls into the river naturally.

The river now flows around and through the wood, which gives the water less space in the channel, causing it to ‘pinch’ the flow and force the water against the opposite banks.

In high flows the water level upstream increases slightly, increasing the pressure and so water pushing past the wood must flow faster. This faster flow causes some localised erosion of the riverbanks. These points of erosion will form the start of new meanders in the otherwise straight channel.

 

Images above: wood introduced this year aimed at kick-starting the re-naturalising process (credit NRW).

As material such as gravel from this erosion deposits in the flow shadow (a subtle area of slack water) downstream of the wood, new gravel beds are created. The results of that process can be seen in the image below, only four months after adding wood to the channel (credit NRW).

Wood is a very dynamic habitat. As the wood rots and parts snap off the size and shape of the wood deposit will change. Also, other bits of wood from upstream will float down and lodge on more stable pieces. This dynamism means that new habitats are constantly being created and altered.

The wood provides shelter for fish from fast flows, shade (and cooler water), cover from predators, territory markers, feeding opportunities and breeding areas.

As time goes on we hope to see more fish and wildlife in this part of Llangloffan Fen, and will continue to monitor the site as the project progresses.

More about this method of restoration can be found in The Wildlife Trusts booklet on Managing Woody Debris

To keep up to date with our work you can follow us on as Facebook, X (formerly Twitter) and Instagram   or subscribe to our newsletter here