What a week for weather! From Monday morning to Wednesday night the wind did not stop blowing. Five more big trees down, a Big Balsam Poplar in the paddock that can wait for a while. An Ivy coloured Ash tree across the top of the mill stream, a Horse Chestnut in Solids – the bottom of the beat, and two huge Aspen across the river by my employer’s garden gate, two more of this lanky shallow rooted Aspen Aspen will have to come down because they are dangerous. The Fishermen will welcome the loss of part of the Horse Chestnut on Solids; it was a real flytrap on the back cast and afforded safety to many a Brown Trout. The Balsam Poplar will have no affect on the fishing although its strong scent in the spring when its buds are breaking will be missed. A very brittle tree, its wood is used to make matches. This Forty foot tree, had roots extending barely eighteen inches down into the peaty ground, with the water table not far from its base it pushed its roots out extensively just under the surface of the soil. I also had to deal with several other trees around the village, one across the school path, a dangerous limb on the recreation ground and an ivy covered hawthorn across the Bransbury road.
While all this mayhem was going on, I could safely work on a weir that I had to replace without fear of any trees falling on me. The bridge that I have had to replace had a weir underneath for the purpose of holding up the water above the bridge to improve the fishing. Weirs can have several effects on the stretch of river in which they are placed. The most obvious feature is the lovely weir pool that is created by the falling water, it screams fish, and the crashing water mixing with the air, helps raise the dissolved oxygen levels in the river. This can be particularly beneficial in periods of hot weather when the water temperature rises, and the water is unable to hold as much oxygen as cooler water; couple this with a period of intense low pressure such as a thunderstorm and the waters ability to hold oxygen is reduced even further.
The biggest change to a stretch of river with a newly installed weir is to the stretch of water upstream, the flow is slowed and as a result the river is more likely to start depositing silt, this can affect the weed growth and subsequently fly life and fishing.
The weir I am replacing has been built to slow a particular section of the beat up. Without the weir the river falls considerably over a short distance, is very fast flowing and shallow and holds few Trout. Holding the water up with a weir made of telegraph poles has raised the level by six inches and steadied the flow sufficiently for Trout to find it a suitable place to lie and feed. The water still flows quickly enough for Ranunculus to grow, and deposition of silt is minimal. If the weir were any higher, the flow would be slowed even further and deposition may start to immediately upstream of the weir, which would affect the Ranunculus.
The construction of the weir is not just a matter of chucking a heap of bricks across the riverbed and hoping for the best. Whenever you place an obstacle in the river the water will try to find a way around it or under it. I have used telegraph poles, placed horizontally against oak posts driven into the riverbed. These must be made level, to ensure an even flow of water over the whole weir. To prevent the water forcing its way under the weir, sheets of corrugated tin are driven by hand down into the river bed and attached to the horizontal telegraph pole, these have the added bonus of preventing the undercurrent of the weir pool eroding the weir from the downstream side. Rubble topped off with gravel is then placed on the upstream side of the weir, for this I am using broken up staddle stones that were in place from the previous weir, raking the gravel on the river bed over the top.
It may be a small-scale construction using materials I have to hand, but the principles behind the siting and construction are the same as much bigger hatches and weirs further down the river.