As if the situation with hydro abstraction on the Spey wasn’t bad enough Envirocentre Spey Abstractions 2021 Report a visit to Spey Dam yesterday to check how the sediment abstraction above the dam was progressing highlighted yet another impact of hydro dams.
Sediment builds up behind every dam, this being one of the main reasons for abandonment or removal, but it may be particularly bad in the case of Spey Dam as it was located to capture the flows coming down the Markie Burn. The Markie is a steep and torrential Spey tributary which drains from a big corrie to the north. The banks of the Markie are highly erosive and there have been several incidents of landslips in recent years, one of which was large enough to discoluor the whole Spey, as far down as Loch Insh https://www.speyfisheryboard.com/dirty-spey/ . There is nothing new in these slips the hydro operators retrofitted a “heck” in the lower Markie in 1958 to catch the sediment before it entered Spey Dam. The heck is effective in catching the larger cobbles and boulders but not the fine sediment, most of which settles in the reservoir, just above Spey Dam.
This week the Spey Dam operators were removing some of this sediment using excavators and dump trucks whilst the reservoir was low. Unfortunately when the reservoir level wass reduced low enough to allow excavation to proceed, the river cut its own course through the fine sediments, liquidising it and carrying them through the dam into the river below.
This smothering of the river bed extends downstream, at least as far as the road bridge, a distance of almost 300m. The quantity of sand and silt deposited downstream of the dam must run into the thousands of tonnes. The impact of this on the ecology of the affected stretch will be catastrophic, imagine a freshwater pearl mussel under that lot. Further downstream the river bed was covered in fine silt but during freshets or when the dam spills the deposits downstream of the dam will be washed downriver, affecting other areas.
In order to assess the impact of the works on the fish population we repeated the annual timed mainstem survey at the Blargie corner. Thankfully the fry and parr counts were similar to the previous survey 2 weeks ago. The river here was silted but, as with the spawning gravels upstream, the slug of sediment hadn’t yet reached this area.
The impacts of hydro are often cited as abstraction, flow regulation, sediment starvation, fish passage (upstream & downstream) and unnatural temperature profiles, but here is another. The movement of fine sediments downstream is a natural feature of rivers, but they usually occur during high flows with the fine particles deposited on the banks or floodplain. The only times I have seen a river smothered in fine sediment like this was during the last time the sediments above the dam were excavated.
I personally don’t blame the current Spey Dam operators for this latest ecological impact on the Spey, removing the fine sediment from above the dam (an operational requirement) without impacting the river below is almost impossible, although with hindsight better control of reservoir levels as it was drawn down may have helped avoid the worst impacts. Nor do I blame the contractors, they were doing the best they could. If anyone is to shoulder the blame it should be the original designers. Siting the dam immediately downstream of such a dynamic and volatile tributary was not a good long-term decision. This is just another adverse hydro impact.
The Spey below Spey Dam is becoming starved of spawning gravels, large grade sediment suitable for salmon spawning simply cannot bypass the dam. So far we have been unable to agree a proper sediment re-introduction protocol for below Spey Dam with the operators, and SEPA, the regulators, yet thousands of tonnes of harmful sediment are released during maintenance works. There is something not right somewhere.