On the road south to a meeting in Perth this morning I noted that the upper Spey was very dirty, the colour of milky tea. I like to find out the cause of these events so on the road back home this afternoon I peeled of the A9 at Newtonmore. A glance whilst crossing the Newtonmore Bridge showed that the Spey was still dirty whilst the Calder was running clear.There was nothing for it but to head upstream to find the source.
It was the same at Laggan Bridge.
Onwards to Spey Dam where the water was found to be very dirty coming from the reservoir. Something significant must have occurred to discolour the loch.
The Markie, one of the major tributaries entering the Dam was a potential source so I drove up to have a look. At the gate (site for the Beauly-Denny powerline upgrade) I met the environmental clerk for the works. He told me that the Markie was responsible and that it had been discoloured on Monday. When they saw the dirty water he had been concerned that it was related to their works but they hadn’t yet reached that area.
There is a large metal barrier across the Markie which was installed in the 1960s as a sediment barrier to prevent the build up of sediment at the dam. There was plenty evidence on the barrier of a recent large spate, there was a build up of fine sediment upstream and a lot of debris on the heck.
There had been very heavy showers and thunder in the upper Spey on Sunday so I can only assume there must have been a localised spate in the Markie area resulting in a landslip or bank erosion. Spey Dam was discoloured from one end to the other, there must have been a huge amount of sediment washed down. The Markie Burn was running clearer by today but it will take a while for the loch to clear.
So it was a natural event that was responsible. The bottom of the burn was coated in sand/silt and there must be a layer of fine silt across the bottom of the loch. A localised minor incident in geological time scales but the evidence will remain in the loch sediment for ever more.
The Markie heck was constructed many years after Spey Dam in response to problems with gravel coming down the Markie. The heck was designed to trap the sediment before it reached the reservoir. The hideous thing also looks like a very effective barrier to fish migration, but then by the 1960s the salmon population above the dam had all but died out, maybe they thought there was no need to include a fish pass.
It is always good to find the cause of dirty water in the river. High silt loading like that is not good for a salmon river, especially as it will persist for a while in the low water released from the dam, would a freshet release help?
It is unlikely that the dirty water in the upper river will reach as far downstream as the lower river, the sediment will settle out in Loch Insh. The dirty water that had been present in the lower river in the last couple days was probably due to landslips in the Grantown/Aviemore area.