Reducing silt inputs and restoring fish populations to the Mauchline Burn, River Ayr catchment

We recently secured funding for restoration work on the Mauchline Burn (a.k.a. locally as The Chalk River) with the ultimate aim of reducing silt and nutrient inputs and restoring a functional trout and salmon population within this small watercourse. Our surveys revealed that trout have more or less disappeared from this burn over the last 15 or so years and salmon have only ever been reported anecdotally and not for many many years. Carolyn will also correctly remind me that eels also appear to be absent and hopefully they too can be brought back soon.

In recent years, the burn has suffered from repeat pollution incidents (from more than one source) and it has been more than a few years since any salmonid fish have been found alive. A burn of this scale should be capable of supporting both trout and a few salmon particularly in the lower third but in reality, poor water quality has rendered it virtually fishless by 2024 (bar a few sticklebacks). This situation is not uncommon in rural Ayrshire where burns that flow through agricultural land are often impacted by the land use and inputs. The length of the burn accessible to fish is relatively short at around 3 miles, by which time is is more suitable for trout than salmon. Upstream of this point, the burn is heavily modified (straightened along field margins and culverted under farm steadings and roads etc.). However, this upper section is where much of the silt and nutrient inputs arise and the project includes measures that will hopefully address the worst of these issues. After looking at all the potential and known issues, we hatched a plan and sought funding.

Here is an image that clearly illustrates the problems we are trying to resolve with this burn. The River Ayr was clear and falling after a spate last week while overnight localised rainfall had brought the Mauchline Burn into a small spate carrying with it fine sediment.

Funding from the Wild Salmonid Fisheries Fund (circa £17,000), a contribution from the River Ayr DSFB (£1,500) and considerable resource inputs from Ayrshire Rivers Trust means that around £23,000+ is currently being invested into improving this habitat. Work includes bank stabilisation in areas where the burn is highly mobile and also erecting livestock fencing that will immediately reduce nutrient and silt inputs and allow trees to thrive away from grazing pressures. Silt is a major issue with this burn and there has been a constant movement of this from land to the burn, to the River Ayr for decades. Spates carry the silt downstream impacting spawning on the lower River Ayr but of course there are other dirty burns equally in need of restoration (Glenstang, Water of Fail and Water of Coyle all jump to mind). Livestock access is a main pressure manifesting as nutrient enrichment, silt inputs and erosion. Enrichment causes eutrophication, reducing oxygen levels particularly in summer months and this negatively impacts fish survival but also has other consequences for coastal bathing water quality and tourism for the area. Addressing problems such as these will only help the Ayr catchment improve.

A problem we discovered that had to be addressed rather than ignored. This stretch of the burn had recently been modified by a contractor. Unfortunately this type of alteration just increases rather than solves erosion and so too adds to the silt load. Compare this to the next image below after we completed the repairs

We used willow trunks cut on site to add protection to the toe of the modified bank and regraded the bank opposite before adding brash and coir biodegradable geotextile membrane. The brash will trap fine sediment and the willow branches should send down roots as it springs back to life. This approach offers a living green bank solution that provides stability, buffers and slows the force of water and all importantly, provides shade as the willows develop. Returning the burn back to its original width immediately moved the fine sediment deposits off of potentially good spawning gravels as the flow velocity increased. We had silt protection in just downstream throughout the works.

Apart form our planned work, the staff performed some unexpected restoration work after discovering some straightening had taken place just a few weeks ago. Modifications of this type exacerbate silt problems and leave banks prone to erosion for decades. River engineering should always follow the Controlled Activities Regulations guidance and SEPA’s General Binding Rules. Where in doubt, the best course of action for landowners or contractors is to speak to SEPA licensing team or independent consultants such as the Trust or farm advisers prior to undertaking any such work. Often more than following guidance may be required depending on the scale of the planned works so best seek advice before starting to determine if a license or registration is required. In this case, the Trust considered their options and decided to use green engineering techniques to repair the damage concurrent with their planned project works as leaving this to degrade further would render our aims somewhat futile. Ultimately the Trust bore the financial cost of these repairs and that was not insignificant.

Installing root wads and brash bundles to protect an undercut and eroding bend

The finished article should provide lasting protection as the brash traps mobile sediments and willows root binding everything together. In reality it may take a few years for this to become fully effective and mature but we know brash bundles can quickly and effectively rebuild banks within a matter of weeks

A bend that was in a state of collapse has been regraded and had brash staked into the toe of the bank and large root plates installed. Grass sed was spread the day after completion and should be through in a week or two given the current weather conditions

This work is both slow and skilled and requires vision of what a burn should be and how it should function. To ensure we could move the large roots and reach across the burn without damaging the banks, we hired a 15 tonne excavator on self drive. While the savings over hiring a machine and operator are small, it allows us to work methodically and precisely to achieve the desired results rather than trying to explain exactly what we are after to someone that may have their own thoughts on how to do this type of work. It is just easier and more successful when we have our own operator in the team.

While Stuart is qualified to drive these machines, others are keen to learn and it’s good that when we have time, they can have a go and start to learn new skills. Everyone has to start somewhere and Carolyn was quick to take the opportunity. Perhaps one day with a bit more experience behind her, she may be able to sit the test.

It is a team effort and all get stuck in. Here we manually placed large timbers between the root plates we had just installed to protect this eroding bank. Machinery helps but there’s still a fair amount of manual handling once the machine lowers the material to the work area. The bank was then refilled with soil and consolidated, covered with membrane and grass seed. Two weeks from now it should be turning green

The biologists have different opinions on whether to speed recovery by translocating a few juvenile trout and salmon back into the burn this year but after some pretty challenging discussions, it was decided to allow natural recovery to take place this winter and if it doesn’t, then we may agree to intervene next year; time will tell. The one thing that is certain is we all want the best thing for this burn after so many years without a functional fish population. After all this work, the future has to be better than the past.

With our phase 1 completed, our fencer moves onto the job to start phase 2 this week. Later we hope to ease a problem culvert that should help improve fish migration followed by phase 3 in the winter when we will plant trees behind the protection of the new fencing.

Updates will be provided as and when these phased works are completed.