Volunteer river restoration at Morden Hall Park

In March 2022, our volunteers and members of the Morden Hall Park Nature Group spent three days in the glorious sunshine restoring a stretch of the River Wandle as it flows through Morden Hall Park.

Now owned by the National Trust, Morden Hall Park was once a deer park for a country estate. With the Wandle splitting into many meandering channels, the park remained as a green oasis throughout the river’s industrial heyday.

This was the latest stage of an on-going project, started in 2015 and due to run until 2024, giving volunteers the chance to improve the river channel at the park, writes Jess Mead.

 

Improving the chalk stream riverbed for invertebrates and spawning fish

The volunteer restoration days within Morden Hall Park focused on part of the river that was artificially straight and very over-wide for the volume of water flowing along it.

These historical modifications to the channel meant that the power of the water was insufficient to move small particles of sediment along the river and keep the gravel riverbed clean from silt.

In turn, this meant that the classic chalk stream gravels we would expect to see were choked with silt and fine sand, making a poor habitat for invertebrates and spawning fish.

The restoration work carried out in March built on the work previously delivered by the Trust in 2015 and 2020, which provided valuable refuge habitat for small fish fry and helped to kick start natural processes.

In February 2020, we had created a huge brash berm which narrowed the channel and added in pieces of large woody material to create a variety of riverbed features, including deeper pools and shallow gravel riffles.

The River Wandle at Morden Hall Park was over-wide, restricting habitats for wildlife © South East Rivers Trust

Planting iris and removing pennywort

By March 2022 our amazing berm had trapped plenty of silt, which allowed us to plant some wonderful native iris along it. When the iris grow and spread across the berm, their roots will stabilise the silt and encourage further vegetation of the berm by other plants; eventually it will look like a natural extension of the riverbank.

We also spent some time removing floating pennywort from along the section of the river we would be working on. This invasive non-native species causes issues along the River Wandle. This North American species was introduced to Europe as an ornamental plant in the 19th Century.

It can grow up to 20cm per day and can quickly cover a water body, smothering the wildlife underneath, blocking out sunlight and reducing oxygen levels.

Iris was planted on a berm previously installed at Morden Hall Park © South East Rivers Trust

Making great use of a fallen tree trunk

Our Assistant Project Officer, Harry Clark, helped the volunteers to secure a tree trunk that had naturally fallen into the channel several years ago.

This kind of woody material is fantastic for improving the health and biodiversity of a river.

As the water flows past and has to squeeze around the log, the extra power it gains helps to carve out deeper pools.

Then, as the water spreads out again, it loses power and deposits the clean gravel to create shallow areas called riffles.

These natural processes in action are what helps to create a nice variety of different habitats to support a higher diversity of species.

Securing a fallen tree at Morden Hall Park, March 2022 © South East Rivers Trust

Taking out oily toe boarding

The team also managed to remove toe boarding from both banks along a 200 metre stretch of the river. This was enough toxic wood to fill a whole skip! Toe boarding is preserved wood that is installed along the riverbank to prevent natural processes of erosion and deposition from acting as they naturally would.

When the toe boarding was removed from the riverbank, the volunteers were shocked at how strongly it smelled of oil. These pieces of wood have been leaching their toxic chemicals into the river for decades, so removing them felt like a fantastic achievement.

Toe boarding removed at Morden Hall Park smelled strongly of oil © South East Rivers Trust

Berms for a meandering channel

To extend the section of narrower channel that we had created in 2020, the volunteers installed another brash berm on the opposite bank just downstream of our original berm.

By installing berms on alternate banks, we can create a meandering channel that is much closer to the width you would naturally expect. Creating the brash berm takes a lot of people power – to bang in the posts, move all the twiggy material from fallen trees in the park to the berm’s location and tie it all in place to prevent it being washed away in high flows.

We would like to say a big thank you to all the volunteers who joined us over the three days, to the Natural Trust for facilitating the work and to the Environment Agency for funding the project. There will be more river restoration days in Morden Hall Park later this year.

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Volunteers installed another berm at Morden Hall Park in March 2022 © South East Rivers Trust

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