Dartmoor Biodiversity Officer Morwenna discusses the work done in 2024 at Burrator Reservoir for river enhancement as part of the South West Water funded Green Recovery project and Environment Agency Water Environment Improvement Fund:

As a part of the South West Water funded Green Recovery project and Environment Agency Water Environment Improvement Fund, we have improved 7km of water course throughout the 3 rivers within the Burrator catchment. Interventions include large and course woody debris additions, floodplain leaky dams, embankment breaches and riparian woodland light management.

An ongoing project is monitoring changes over time. As more woody debris is placed in the river, the flow morphology is altered, allowing sediment to build up in more areas, and a more diverse morphology to form which provides breeding and spawning opportunities for fish, including brown trout. Additionally, floodplain habitats will benefit from more frequent flooding.

In order to measure the effectiveness of these interventions, a baseline river habitat survey was completed. This survey measures the usable habitats within and alongside rivers along a 500m stretch and creates a number of scores for the stretch. By repeating this survey in one or two years’ time, any new habitats and improved features will be noticed.

Results from river habitat surveys

The results show a mix of positive and negative factors for each river. Overall, the major impacts affecting the quality of these rivers are mixed, with some common issues occurring in each. For example, the diversity of the flow regime was an issue in every river. This measures the type of flow, over different substrates and compares to the natural state expected in similar rivers. The issue with these four rivers was that the flow type was fairly homogenous. This confirms the conclusions made in the geomorphology surveys and the need for more woody debris to be introduced, and rivers to return to more natural flow regimes.

Meavy

The river habitat survey was completed at the headwaters of the Meavy, and showed the river to have a few issues, related to riparian quality and agricultural sediment risk. These risks are heightened by poaching from animals alongside the river, and the headwaters of the Meavy being very incised, forcing it only to cut deeper into the valley floor and disconnecting it from the floodplain.

Two stretches of the river Meavy have been enhanced by the introduction of large woody debris. The first stretch was completed in August 2023. This stretch is 1.77km long, and a total of 32 debris dams were installed. The dams were all placed laterally, causing some of them to shift during flood events, and become lateral debris. This occurred because the pressure of the river during heavy flows was too high on the perpendicular logs. However, the lateral wood is effective in creating lateral habitats such as sidebars and point bars which are important features, diversifying the river. Other logs washed down the river and have gathered together to form clustered dams, which are more effective at slowing the heavy waters, and will create cross-channel features such as riffles, which are important for fish breeding. As well as in-river habitats, these dams have cause the river to flow onto the floodplain at certain points, causing regular inundation and improving lateral wetland habitats.

The second stretch was near the head of the Meavy at Devil’s bridge. This stretch was completed in June, 2024. Heavy horses were used to drag piles of brash and logs from locally sourced trees, down the river and placed in the river by volunteers. The brash and logs were strategically placed in sections along a 0.3km stretch of river in tight bundles which will slow the flow, reducing erosion on the bank sides and causing the river to flood more frequently. The use of heavy horses provided a great low impact, low Carbon alternative to diggers or quad bikes, which was received well by the public and garnered a lot of interest in the project. This stretch will be monitored to assess changes. 

Narrator Brook

The habitat survey found some good habitat quality scores on both the upstream and downstream section, but also a few negative scores. Realignment of the river, and embankments running along some sections have led to lack of diversity within the river, and historical over-deepening and management has led to lack of connectivity with the floodplain.

To address these issues, first, we hired a contractor to remove part of two key sections of the embankments, one in the upstream end and one in the downstream end. By removing these areas of embankment, the river is able to flood into adjacent floodplain areas more frequently, improving lateral boggy habitats and reducing the pressure of flow downstream.

After embankments had been removed, in August 2024, a team from Westcountry rivers trust was contracted to work on the Narrator brook. This work focussed efforts on key areas within the wooded part of the Narrator to introduce woody debris to the river course and floodplain, and manage light levels reaching the river. At 7 locations in the upstream section of the Narrator, the team have trimmed willows where growth was becoming tangled, dark and brittle. This allow more dappled light to reach the river which is preferred for many fish and invertebrate species. The cut trees were hinged and placed into the main channel, to increase roughness and slow the flow of water. This will help to create more discrete habitats within the river by trapping sediment to create small pools, riffles and accumulations of gravels within the river. Additionally, the tree’s structure within the river creates hiding places and nooks for fish to shelter in.

The next section, the downstream end, was completed in Septe,ber 2024. This section focussed on using larger, whole trees, including large willows, ash and sycamore to fell into the river while remaining attached to their trunks. The willow trees should grow back from the hinged point, creating fresh growth and improving shade management over time. Whilst sycamore and ash are unlikely to grow back, but will provide wider obstructions which can be more effective at pushing water out onto the floodplain. All of the woody debris placed in the rivers is passable for fish, as contractors ensure at least some space is left below or either side of the obstruction. When trees leaves and branches are still attached, this allows fish to move through the obstruction and find protection when flow is heavy.

Newleycombe

In the river survey, the Newleycombe section scored quite well in most sections, but poorly in the hydro-morphological impact ratio, meaning it had few river habitats where fish, insects or mammals could breed or rest. This is due to historic management of the river, possible over-deepening and likely straightening. This can be addressed with similar works to the other rivers.

In April 2023, a tributary into the Newleycombe was enhanced by introducing leaky debris dams to the small stream running through a deep gully, made for tin streaming. The dams were made of willow, and live stakes were planted to hold the leaky dams in place, some of these have started to sprout, with some browsed by livestock. During visits after this intervention, dragonflies and water scorpions have been observed using the boggy pools, and toad-spawn has been found in the pools.

In September 2024, the final section of river was completed, at the downstream end of the Newleycombe, where trees border the river, and small sections of wet woodland flank the banks. In this section, select trees were hinged into the river at key locations, where the floodplain was more connected, or river bed fairly homogenous. This will improve floodplain connectivity and create more in river habitats to diversify the river bed for species such as brown trout.