1st May, 2020
Earlier in April the Foundation’s monitoring and data team made the most of the good spring weather to complete our annual surveys on the river Elan.
This was the latest phase of our so far successful project to restore lost gravels, restore the ecology and boost salmon numbers in this section of the upper Wye tributary.
Following closely the Government guidelines on social distancing, two members of staff recorded both the invertebrates and the amount of gravel at various sites in the 4km of the river downstream of Caban Coch dam to Dolfallen Bridge (with a control invertebrate site at the confluence). The results were mixed but probably in line with expectations following the high flows of the winter and our unsuccessful attempts in 2019 to find source sites where owners were willing to allow us to operate.
- There was an average increase in gravel depth of 2.12cm compared to 2019. The average increase since the project started is 7.49cm.
- It appears that most of the gravel in the close vicinity of the introduction site has moved 300m downstream or more.
The invertebrate surveys seemed to reflect this. Their abundance and diversity only increased on 2019 from Site 4 down (1km from introduction site) and decreased at the upper two sites.
- The diversity of invertebrates found was remains much higher than that recorded during the 2018 surveys.
- 8 salmonid fry were found during the invertebrate surveys. We hope to be able to conduct electrofishing surveys in the summer and that they will confirm that the Elan is now one of the most important spawning tributaries for Wye salmon.
- Despite the quick recovery of donor sites, the project continues to be held back by a lack of suitable sites in the upper Wye and Elan where owners are willing to allow us to abstract gravel.
2020 Gravel Survey
Last year was the first since 2015 in which the Elan had received no new gravel. Our problems finding a suitable source site meant that all we could do last autumn was loosen what remained on the bank from previous introductions ready for the 2019/2020 winter flows to disperse. This area has now been scoured and all the gravel has moved.
This was not an insignificant amount, however. Our survey showed that on average, the depth of gravel across the 42 sites had increased by 2.12cm from last year but that the 6 of the 7 sites in the first 130m below the introduction site had lost almost all their gravel. This shows the need for repeated introductions.
|Average increase in gravel depth in the Elan over 42 sites since project start
2020 Invertebrate Survey
Kick sampling surveys were once again used to record the number and diversity of invertebrates at six sites within the previously impacted area and a further 7th site just above the confluence of the Elan and The Wye. The Biological Monitoring Working Party (BMWP) scoring system was used to determine the biological quality of the survey sites, and abundance determines the ecological quality.
The results suggest that BMWP scores improved at 3 of the 7 sample sites between 2019 and 2020. The 3 sites where BMWP scores improved (sites 4, 5 and 7) represent 3 of the 4 furthest downstream sites. The improvement of the BMWP score of site 7 raised its biological quality status from “fair” to “good”. The biological status of site 6 was lowered from “good” to “fair”. Site 1 to 3 and site 5 all maintained “poor” biological status, whereas site 4 maintained “fair” biological status.
Between 2019 and 2020 the number of different macroinvertebrate taxa found decreased at every site, with the exception of site 7 where it increased.
Additionally, the abundance of invertebrates decreased at 4 of the 7 sites, with the middle 3 sites (3–5) representing the only increases in abundances. The largest decreases were recorded at the lower 2 sites, with the abundance at site 6 decreasing from 126 individuals in 2019 to 75 in 2020. However, an abundance of 75 is still significantly higher than the 41 recorded at the same site in 2018.
This overall decrease in BMWP score and abundance at the upper sites is unsurprising as the gravel was scoured from these sites. The sustained high water will have also promoted invertebrate drift downstream. The dams stop invertebrates moving into the section from upstream. The increased BMWP score of the lower sites indicate that the continued movement of gravel introduced in previous years has improved macroinvertebrate habitat further downstream.
2020 Gravelling the Elan Project
The search for new gravel donor sites will continue. With funding in place for the next few years, the lack of source sites with willing owners is the limiting factor in maintaining and extending the successes of the project so far. Failure to secure new sites will lead to a deterioration in Elan’s ecology and salmonid populations. The river will return to its previously poor ecological state.
The next planned activity for this project is the electrofishing surveys later in the summer. We hope to be able to carry these out and report on the results but it will depend on any restrictions still in place to counter the COVID-19 outbreak.
Our thanks to project partners Natural Resources Wales, Dwr Cymru Welsh Water and Elan Valley Trust.