Spawning gravels

One of the less well known impacts of dams and weirs is the interruption of the downstream transport of gravel that occurs in every river naturally inhabited by salmon. This is very apparent in the Allt Bhran, a tributary of the River Tromie, which was dammed and dried when the hydro scheme was built in the 1930/40s. There is a large pool upstream of the abstraction weir which acts as a settling pond for all the gravel that moves downstream during spate conditions. Every now and again the hydro company excavate the gravels and deposit them on the downstream side of the weir. This was done last year but there is a now a large bund of river gravel on the banks of the river downstream of the weir.

Duncan and myself went up there today to try and quantify the amount involved. Neither of us are quantity surveyors but we took some rough measurements of the bund and came up with a figure of about 1800 tonnes. The bund was much larger last year but the part nearest the weir has been washed downstream during flood events. The remainder is likely to remain there as it is too far from the channel. This gravel is an important part of the Bhran and Tromie ecosystem and we are currently figuring out the best strategy to return it back to the river.

Erodd bank of the gravel bund. There are a mixture of particle sizes including grades suitable for spawnign fish.

Eroded bank of the gravel bund. There are a mixture of particle sizes including grades suitable for spawning fish.

Allt Bhran gravel bund from downstream

Allt Bhran gravel bund from downstream. Last year it extended up to the weir face, the portion nearest the weir  has already been washed downstream but the rest is likely to remain in situ for many years.

The Bhran upstream of the weir looked very good today. Every time I see it I am more certain that it will be an excellent spawning burn if sufficient flows are restored.

Allt Bhran above the intake dam. Lovely natural braided channel with a variety of flows - what more could a salmon ask for?

Allt Bhran above the intake dam. Natural braided channel with a variety of flows – what more could a salmon ask for? Note that the snow is now largely restricted to above 2500′ altitude where there are large and deep banks. This snow should last a long time, and supply a nice steady flow in the river, providing there are no warm winds combined with heavy rain.

All the flow was abstracted today. If salmon access is restored upstream of the weir smolt screening will be required to prevent smolts being drawn down the tunnel

All the flow was abstracted today. If salmon access is restored upstream of the weir smolt screening will be required to prevent smolts being drawn down the tunnel

The impact of the abstraction is clear from this photo

The impact of the abstraction is clear from this photo

On the way down the glen we stopped for a blether with the Glen Tromie gamekeeper. He reported that he had seen several salmon redds well up the Allt na Feinnich, which is the furthest upstream unabstracted tributary of the Tromie. I had walked a short distance up the Feinnich last spring after having mistaken it for the Allt Fearna, a heavily abstracted tributary which joins on the same side a short distance upstream. These snippets of info from the keepers on the ground are very useful. We will be surveying the Tromie in detail this summer and will include sites on the Feinnich to establish have far upstream salmon are present.

Allt na Feinnich, a very nice little tributary of the Tromie.

Allt na Feinnich, a very nice unregulated tributary of the Tromie.

Hearing that salmon spawn in the Feinnich highlights the potential significance of the much larger Allt Bhran as a spawning and nursery tributary. Surviving here long enough to witness salmon spawning in the upper Bhran is now my interim career target!

There are 2 comments for this article
  1. Bryan Herbert at 12:28 pm

    Brian it’s just criminal that all are above has been lost to the spey catchment spawning areas not to mention the loss below and the damage done with the loss of gravel. This lack of gravel movement I presume will result in more fish having to use less sutible areas to spawn and more redds being cut on top of each other in the best areas resulting in reduced production of that tributary.

    • Brian Shaw Author at 12:44 pm

      Hi Bryan,

      Yes you are absolutely right, although hopefully it will be restored to some degree soon. The gravels issue is an important one and we hope to resolve that too. When the Truim and Tromie electrofishing results are compared the Truim supports a much higher density of fry than the Tromie, although its parr density is closer to thant typicaly found in the Truim. The Tromie is a higher gradient river and would natually be more bouldery but part of the problem will be spawning gravel starvation such as occurs at the Bhran intake but also behind Tromie Dam. Sediment would naturally have moved along the shores of Loch an’t Seilich then into the River Tromie before the dam was built but that has stopped as well. There is a greater understanding of these issues now and SEPA are requiring dam operators to produce sediment managment plans which should help.
      Best regards
      Brian

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