STREAMS AND DAMS - 4
The pool
It is not enough to have good water and a strong dam for there is still the problem of mud. A small stream which runs crystal clear in ordinary weather will sometimes turn into a broad, silt-laden river after heavy rains. When water which is carrying silt and flowing rapidly is slowed down, as when it enters the pool behind a dam, it drops a considerable part of the silt. It is not unusual for pools which are as deep as 8 or 10 ft. to fill up completely within four or five years if they are located in a swift stream which flows through open country. Even in heavily wooded regions the silt problem is not entirely absent. On the other hand there are some streams which carry little silt even at high water. There is no perfect solution to the silt problem which is within the reach of individual property owners. A thorough program of reforestation and soil conservation in the watershed of the stream will gradually reduce the amount of silt, but such a program is a community project and, even at best, is not immediately effective. The construction of a settling basin upstream from the dam will get rid of a fair proportion of the silt during periods of normal stream flow, but at flood times the silt in the settling basin is likely to be picked up and deposited in the swimming pool. Settling basins of ample size require a considerable amount of land, and their ex- cavation involves a good deal of labor. Another and somewhat better method is to build into the base of the dam a large cast-iron or concrete pipe which can be used as a means of emptying the pool with fair speed. When the deposited silt becomes a nuisance, the water of the pool is allowed to run out of this bottom drain. Since it runs rapidly, it will carry a fair amount of the silt with it and what remains behind can usually be mixed with additional water and flushed out. One catch to this scheme is that is may lead to trouble with property owners downstream who don't want their own pools or even their portions of the stream bed suddenly swamped with silt. The best and also the most expensive way around the problem is to build a regular swimming pool at some distance from the stream and only to utilize the stream as a water supply for it. On some properties the pool can be located so that stream water can be run into it by gravity. In others a pump is necessary. Whatever the arrangement, water is moved from the stream to the pool only at times when it is clean. Unfortunately there will be a mud problem long before much silt is carried into the pool behind the dam. The bottom and sides are usually of soil, and will turn into mud under slight provocation. After the dam is constructed, but before the pool is filled with water, the bottom can be covered with several inches of clean, round gravel. To supplement this covering, the sides of the pool can be walled up with stone laid up without mortar. As much attention should be given to good workmanship in making such a stone lining as is given to the building of a good stone wall, for if the stone is not laid up well, it will slip into the pool and prove to be a considerable nui- sance to swimmers and divers.
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