How to Build an Artificial Fishing Lake
Les Rouvets "Man Made" Coarse Fishing Lake in Guernsey.
Ideally, when you decide to build a fishing lake you need first to find a suitable piece of land, and then have the "vision" to imagine how that piece of land will look once flooded with water.
The most suitable locations are to be found on low lying land, so ensuring that water will naturally drain into the lake frequently and avoid it drying up. An added bonus is if a stream flows alongside the lake, as this can naturally provide a supply of water that will drain through to the newly built lake if you build a small ditch from the stream to the lake at the lowest point of your new lake. Do not be tempted to allow the stream to directly feed into the lake itself, as in a matter of a year or two the silt build up in your new lake will have reached levels that require urgent attention. This is not only expensive to resolve, but often hard to achieve once your surrounding trees have become established, (so making it difficult for machinery to access the silted up areas), and may even require you draining down your lake to enable the silt to be safely removed, (which leaves you with the new problem of what to do with your fish stocks whilst the work is completed).
Naturally it is very tempting to simply dam the stream and allow the man made lake to fill to the point it overflows the dam, (as was done at Les Rouvets Lake pictured above). The risks here are that once the wooden dam degrades or begins to rot, it could give way at any time, in which case where is all that water going to go? Also, the silt does gradually ruin the lake unless removed every few years. In the case of Les Rouvets Lake above, the problem has been that the end of the lake where the stream enters, is now only a couple of feet deep due to the large level of silt that has accumulated since it was first dug out in 1963. In the years since the lake was first built, the surrounding trees and shrubs have matured, and now, the only way to remove the silt would be to drain the lake to around half full, then allow a number of weeks for the silt to dry out before getting ramps built to enable a digger to access the silt and remove it, without itself sinking into the silt buildup. Even then, a number of the mature trees and shrubs would have to be removed to allow access to a JCB of suitable size to complete the work, and this in itself would prove a very costly exercise.
Make sure that if you do have a stream running next to your new lake, the bank between them should be high enough that if the stream is flooded after heavy rains it will not affect the lake and will follow it's own natural course as usual.
At the deepest part of the lake a "monk" should be built, which allows the water level to be controlled, but also allows the lake to be drained completely if required.
Les Rouvets "Man Made" Lake in Guernsey
Do Trout Need Running Water?
It used to be believed that for trout to thrive they required running water, but this has proven to be a false assumption. So long as the water is clean and deep, trout will flourish. It has been proven over and over again that trout have successfully survived in man made concrete reservoirs and artificial ponds with no flow of water running through them.
A Small Lake in the Early Stages of Being Built
Depths and Temperatures
The depths you choose for your lake will largely depend on the fish you intend to stock it with later. Temperature-wise any water that remains over 20 degrees Celsius over the Summer is ideal for coarse fish, a cooler water that does not go above 20 degrees Celsius in Summer is better for trout. Rainbow trout will cope better with higher temperatures than brown trout. Coarse fish such as the Cyprinids, e.g. carp and tench, are pretty tough, although ideally they should have deeper areas of over 1.5 - 2.0 metres where they can go to for the duration of the winter months and remain semi-dormant until the warmer weather arrives. Trout will require a minimum of 2 metres to ensure the water remains cool enough for them to thrive.
Naturally oxygen levels will be very important to your eventual fish stocks. In hot weather oxygen levels can drop to dangerous levels. To avoid this potential problem it is important to firstly ensure your lake is not too muddy, or full of underwater weed, and secondly that the surface area is sufficient to absorb as much oxygen as possible, because a deep lake with a small surface area will not absorb as much oxygen as a shallower lake with a large surface area.
It will be an important consideration as to the quality of water you will be able to provide in your new lake. Ideally the pH levels should be between 6.5-8 regardless of whether to intend to stock the newly built lake with trout or with coarse fish.
You will also need to determine that the water that feeds your lake is not polluted in anyway, so follow your feeder streams to their source to ensure cattle, pesticides, sewage etc are not inadvertently being fed into your new lake.
Suitable Species of fish for Clay or Gravel Bottomed Lakes
If your lake has a gravelly or hard clay bottom, then it will prove most suitable for trout, as the Cyprinid species of coarse fish such as carp and tench tend to stir up the muddy bottoms of lakes looking for food, which would be difficult for them if the bottom was gravel or hard clay.
Wildfowl such as ducks on your lake are not a problem if the numbers are minimal, as the droppings they produce will fertilise the water and the lake will therefore be the perfect habitat for the natural foods coarse fish consume.
Planning Your Lake
Before you go ahead and dig a big hole in the ground, consider that most anglers enjoy the feeling of being "the only person fishing". It is therefore important to create a lake that is not a perfect circle, and has bends and curves that can be separated by trees and shrubs to give the illusion of isolation for the anglers. In addition to this advantage, there will be a far greater fishing space around a contoured lake than a perfect circle.
You will obviously need large earth moving machines to commence building your lake, and even this only follows you obtaining the correct planning permissions if you don't want to be forced to fill your newly dug lake in again.
Make sure you get a number of quotes for the work involved, ideally from companies who have experience of digging lakes out previously. Quotes can vary greatly, and make sure they are based on the completed job, not on an hourly rate.
Damming Your Lake
To dam a lake the best course of action is to find an area of low lying ground such as a valley, then build a dam to trap the water and prevent it flowing away. This will be made far easier if the lake will be fed by a stream, although an an overflow will then obviously be necessary. Bear in mind what I said earlier though about the stream not ideally feeding immediately into the lake to avoid silt build up.
Firstly you will need to remove all trees and shrubs from around the proposed area, as the roots of these would, over time, act as a point for water to drain out of the lake. The same will apply to grass, reeds etc, as whilst they rot they will create routes where water can drain through, and water will always find a way!
Next, you will need to build a ditch approximately half a metre deep across the valley where you intend to place your dam, (see figure 1 below). This will ensure the dam is secured on the site and avoid water seeping through. Please do not avoid this step as a shortcut, or you may well live to regret it.
You will next need to lay a drainage pipe at the base of the dam, at least 30cm in diameter assuming you have only a very small stream feeding your lake. This pipe will only need to be used in the event you need to drain your lake using the "monk" which also needs to be built. Ensure your pipes have posts driven in each side of them to avoid the heavy action of the machinery from disconnecting them later on. These pipes must angle slightly downhill to make sure water flows in the right direction, (much like your crockery draining board at home).
At the top end of the pipe you need to dig a hole for the foundations of the "monk", the bigger the better. Depending on whether you have a clay soil or not determines on the thickness. Hard clay will require one metre thickness, but softer clay will need digging out to whatever level is necessary to reach firm ground. A monk of 3 metres in height in soft clay will require a concrete foundation of at least 2 metres cubed. Do not make the mistake of taking shortcuts on this, as you will only end up with a monk that tilts or is not functional.
Make sure reinforcing rods are used within the concrete foundations (which must not be too wet to avoid water seepage), and these rods should go to the full height of the monk.
Bear in mind your monk should always be built at the deepest point of the lake in case you ever need to drain the lake down completely.
Draining your lake every year or two enables removal of silt, debris, and diseased fish, and facilitates easy repairs.
Building your monk will largely depend on the depth of your lake. Any depth over 2 metres will require a concrete or brick monk, whilst under 2 metres a wooden one will suffice.
The advantages of having a monk are many, not least of which is the fact you can drain down your lake periodically to remove silt, debris, diseased fish etc.
It is important that your monk is made from material sufficiently strong to allow for the huge water pressures it will have to cope with in the event you need to drain the lake. Ideally a lake deeper than 2 metres will require a concrete monk reinforced with iron rods.
The three walls of your concrete monk will each need to be at least 30 cm thick, and in the event you plan to build a deep lake your monk may need to be four metres high or more.
The diagrams below illustrate the view of the side of the monk open towards the lake (figure 2), and the view from above the monk looking down (figure 3). These illustrate the grooves where the stop boards and gratings will be situated. U irons are ideal for using in these grooves as they allow the boards to slide up and down very easily when required. If the pipeline and the apertures of the monk measure 30cm the two irons will need to be welded together so they are running parallel 30cm apart. Holding the two U irons apart should be two cross bars, one at the top and one about 10cm from the bottom. These must not be on the lake side of the frame or they will make handling of the boards difficult. Ensure all iron is covered in anti-rust paint.
The two bottom ends of the U iron frame will need to be embedded in the concrete foundation, ideally when the concrete is being poured. The frame must be absolutely straight, and should be checked with a spirit level both horizontally and vertically and supported in their position with poles. When the concrete has set the frame can be used as a guide for building up the monk.
Depending on whether you need to simply let off surface water, or if you need the ability to let water away from differing levels will determine how many sets of boards you need. A single set will only allow drainage of surface water, whereas for differing levels two sets of boards must be allowed for. This means three sets of U iron grooves. The outer will be to keep back debris and fish, the third inmost, to hold the boards regulating the level of the lake. The second, middle one to hold boards, one of which is replaced by a similar sized grating. This grating must slide up and down easily and it can be put at the level at which you wish to draw off water, (see figure 4 below). If for example you wish to draw water off from the bottom of the lake, you will slide the grating down to the bottom of the second, or middle, set of grooves, and slide the boards above down to rest above the grating. The water will then flow through the grating and then up and over the third, inmost section, down through the monk and out through the pipeline, (see figure 5). The actual level of the lake is always governed by the height of the boards in the innermost set of grooves.
Naturally you will need to alter the draw off level when required, and obviously when the lake is full the grating and the boards will be covered in water so you cannot get to them. The way this is achieved is by placing an extra board temporarily in the third innermost set of boards so that all water stops flowing. Each board has eyelets attached to them, and using a long hook iron, (see figure 6 below), you will then draw up all the boards and the grating, then remove the temporary board from the third section so that the water flows again.
The dimensions of the grating should be square so that the bars can be aligned either horizontally or vertically as required. When the lake has to be emptied the grating can be placed so the bars are horizontal. As the water flows through the debris will only block up the top bars, allowing the water free flow through the lower bars. Naturally this will require less attention than if all the bars were vertical.
The boards used should not be made of oak, as if left in contact with the U iron for a long period the action of the acid in the wood can destroy the iron. For stop boards elm is best.
Once your Monk has been built it should be rendered inside and out to prevent leaks.
Ensure that the last three or four pipes at the outflow are bedded in with as much concrete as possible. This is because in the event of an emergency and you need to drain your lake down quickly, the force of the water could shift the last pipes with the result of washing your dam away completely.
The Dam Building Stage 2
Once your monk is completed and the concrete has hardened, you will need to dig a further ditch on the downstream side of the outlet pipes, and this will then need to be connected to the original stream. On the upstream side of the lake in front of the monk (which currently does not have any stop boards or grating fitted), you dig a further ditch towards the stream so that the stream is directed through the monk, into the original stream bed.
Next you need to block off the old stream bed so that it dries out. It will soon be filled in once the dam building commences.
The stream bed where the foundation of the dam will be must be completely cleared of all vegetation, after which you can bring in the heavy machinery.
Assuming you are using a bulldozer it is important that the dam is built up layer by layer completely. For example, if your dam is going to be 50 metres wide at the base, the bulldozer should from the start push over the whole of that 50 metres of width, and as the dam grows higher and narrows, he should continue to travel over the full width of the dam. The weight of the machines will then consolidate the earth below.
As the machine comes closer to your pipes you will need to be very careful that the pressure of the earth he is moving does not push your pipes apart and cause them to block up. This can happen even when the bulldozer is several metres away. To prevent this get the machine to stop 8 metres away from the pipeline. Then make the machine push it's bladeful of soil at a reduced speed, and gently lift the blade and let the soil fall down in front of the machine, before reversing away a good 20 metres or so at gentle speed. This will need to be repeated until there is sufficient earth for two or three men to shovel it further towards the pipeline, and eventually onto the pipes themselves where they will need to tread it down firmly. Once there is three or four metres of soil on top of the pipes the bulldozer can carefully drive over the soil to compress it down. For the first 20-30 journeys over the buried pipes the machine should be driven slowly until the earth has been fully compressed and the pipes have no tendency to move. It is worth periodically checking the pipe by looking down the end of it to ensure daylight is still visible at the other end. If not, the pipes have shifted and will need to be dug up and realigned.
The soil used to build your dam should be as waterproof as possible, therefore the obvious choice is a clay soil possibly topped with another type of soil so that the clay is not likely to simply slip away.
You can never overdo the width of your dam, and the wider it is, the more stable it is going to be. Generally speaking, the width at the top of the dam should be equal to the height of the dam. In other words, a dam that is 5 metres high should also be 5 metres wide at the top, (see figure 9 below).
The final height of your dam should be around 50-60cm above the highest water level of the lake. The larger the lake the greater this safety margin should be. The overflow will regulate the level of the lake, and this level should be a little below the top board of the monk. In other words, the surplus water should always exit through your overflow, not through your monk.
What is very important is that the depth your machines dig to should never be deeper than the bottom of the monk, otherwise you will be unable to drain the lake completely. Also the sides of the lake must slope towards the centre and the bottom of the lake must slope towards the monk and the dam.
An overflow is important to have in addition to a monk, (which in itself should not be relied on solely as a means of maintaining water levels). Even if the stream feeding your lake is very small, the overflow will need to be considerably larger to allow for unforeseen storms or floods that may produce a massive surge of water through your lake. If you look at figure 7 below you will see that the overflow has been located where the dam joins the edge of the valley. It is important that your overflow is built on virgin soil that has not been disturbed by machines to minimise the risk of water seeping through, and possibly eroding away your dam at high speed.
First dig a channel 30 cm below the future level of the lake. Commence this digging on the lake side, and then slowly downwards in an arch towards the stream where it emerges out of the pipeline under the dam. The width of the channel on the lake side should be about 4 metres. This channel can become narrower and deeper as it slopes down the offside of the dam. The intake, the lake side, of the overflow needs to be very wide to prevent sudden rising of the water levels within the lake. It is also wise to allow room for a fish barrier made of finely meshed plastic net, which will prevent your fish from escaping, and catch any debris. The wider this barrier is the less attention it will require to clear it. The fish barrier is usually semi-circular and staked firmly one or two metres in front of the overflow.
The 30cm channel you dug below the level of the lake is to form a floor for the overflow made of concrete. This floor will need to be extended a metre or more into the lake, (see figure 8) below to view this in profile.
The floor will need to be extended a metre or so into the lake to prevent water seeping underneath. The sides of the overflow to the back of the dam can be made of brickwork, or concrete, and the whole structure will need to be reinforced with iron rods.
It will also be necessary to have "ears" of concrete both sides of the overflow that extend into the soil, also to prevent seepage.
Do not forget when the overflow is being built that it controls the level of the lake, therefore in relation to the monk it must allow for a level of water slightly lower than the top of the board in the monk.
Bear in mind the overflow is being built before the lake has started to fill with water. Currently all water is running through the monk and the pipes under the dam. Problems could arise if you experience unforeseen heavy rains for a number of weeks, in which case the monk and pipes may not be able to take all the water. If the water rises to a point where it reaches the top of the dam and then runs over it, your dam could be washed away completely. If your stream is unpredictable it is a good idea to build the overflow before the dam.
The lake can be filled three to four weeks after the work has been completed. This delay is to allow all concrete to harden, and to check to see if the monk or the overflow have settled in any way. When you do commence filling the lake it should be done slowly, and the inflow hatch boards should be adjusted to ensure the lake does not fill by more than 5 to 8 cm every day. As the water rises the pressures on your structures will be considerable, so if the water only rises slowly it will enable you to correct any problems.
1) Find a piece of land that is suitable for the type of fish you intend to stock your lake with. i.e. 2-3 metres of depth for a trout fishery or at least 1.5 metres for a coarse fishery.
2) Determine your levels so that no neighbouring land is in danger of being flooded.
3) Divert the stream (if you have one), into a new stream bed around the lake.
4) Clear away all vegetation from the ground where you intend to site your dam.
5) Lay your pipes and build your monk.
6) Ensure that the earth that makes up the newly built dam is properly consolidated and that the correct angles have been achieved, i.e. towards the lake is 1 in 4 and on the offside the angle is 1 in 3, (see figure 9 below).
7) The overflow is built in it's correct place where the dam joins virgin soil on the side of the valley.
8) Dig the draining ditches on the bottom of the lake to make draining in future years easier.
All you need to do now is to consider what vegetation to plant around your lake and if you want to build decked fishing swims etc. It is wise to give the lake a month or two to settle down before attempting to stock it with fish, and then you will probably need to supplementary feed for a while until the natural food supplies establish themselves.
Apart from my own experience I found the 1984 book called "Sports Fisheries in the Making" written by the late Alex Behrendt a very useful source of research.
The diagrams used in this Hub were drawn by Katharine Behrendt and were also obtained from the same book as above.
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