Page:Popular Science Monthly Volume 54.djvu/59

Rh the water of a stream is turned from its old channel into the new one that has been prepared for it, the operation is generally very gradually performed, so as to enable the water to fill up the old bed as much as possible by depositing its silt.

A plan pursued with much success is that of building out from the high bank of a stream (which it is desired to confine into a narrower channel) insubmersible spurs, stopping at the points where the new bank is to be situated. The water flowing in between these spurs deposits its dirt and gravel, and gradually builds up the new bank. In many cases the ends of the spurs are connected by low artificial banks of masonry over which the water flows. These banks retain and protect the deposits, and, when the latter have attained a sufficient depth, the artificial banks are raised to their permanent height. Still another method pursued with the same object in view is that of starting up stream and building the banks to their permanent height until a point is reached where it is desired to "fill."

Here the artificial banks are left temporarily very low. The water overflows them, and the reduction of its velocity entails the deposit of its silt. When this has continued as long as necessary or practicable, the walls are raised to their permanent height along the section and the same process is repeated below. This gradual process is also very advantageous from a financial point of view. When the engineer finally reaches the mouth of the stream at one of the lakes, we should expect to find his difficulties at an end, as the lakes are usually so deep that the alluvion makes little impression on them, and their areas are such that floods are not much to be feared. But he is confronted here with a new difficulty, that of anchoring or securing the foundation of his artificial river bank. The soil is generally alluvial over a large area, and is very damp. He generally has to terminate the masonry before he reaches the less stable alluvial soil and continue the structure by means of wooden material, which retains its position much better under such circumstances and is more cheaply replaced. It would be natural to imagine that man's control of the water problem stops at this point. But not so with the Swiss; he even controls its exit from some of the lakes. This is notably the case at Geneva, where by means of ingenious dams the lake is maintained at what is deemed a proper level. When it is remembered that this lake is fifty miles long by ten broad, an idea is gained of the amount of water controlled. Every few years the level is lowered for a given period, so that repairs may be made to the walls and structures along the shores.