In order to facilitate the taking of water from the river into the canal, it is usual to provide a low overflow dam or weir which extends from the head gate of the canal across or diagonally into the channel of the stream. If the topographical conditions are favorable, this weir may be omitted, or in case of small irrigation canals where the owners are unable to provide a permanent dam, it is customary on the approach of the low-water season to build a temporary obstruction of stone and brush, turning the water toward the head gate. As the water continues to fall this is made more nearly impervious by adding straw, earth or sand bags. It is necessary to provide some form of head-gate to control the amount of water which enters the canal. Otherwise in time of flood the excess might overtop the canal banks and wash them away. Head-gates are also needed to regulate the quantity in accordance with the needs of the irrigators. These usually consist of stout walls and frame built of timber, masonry or concrete with sliding gates of wood or steel. The water enters usually under the . gates, the quantity being controlled by raising or lowering these.
The canals leading from the head-gates usually pass through a rocky or rough country, involving large expense in construction before the more nearly level open land is reached. In this upper part of the course it is sometimes necessary to carry the water in tunnels through projecting cliffs or to provide suitable timber, metal or masonry flumes to take it across rough country. When once the canal is out upon the agricultural land it is usually excavated with broad, shallow sections keeping the water sur face as high as possible consistent with safety, so that water may be diverted to the adjacent fields on the lower side of the canal. The fall or slope of the canal, taken in connection with the' cross-section, is so proportioned as to give a velocity in ordinary earth of a little over two feet a second — not enough to erode the sides and liottom nor so stagnant as to deposit silt usually carried by mountain streams. Con siderable skill and experience is required on the part of the designing engineer to lay out the canal system and its laterals or distributing branches so as to avoid erosion and sedi mentation.
When the fall of the canal is so great that it is impracticable to allow the water to flow freely down the slope, devices known as drops are introduced. These consist of timber, concrete or masonry foundations and walls within which the water can drop to a lower level without injury to the canal. Drops are usually built with a sharp overfall edge and a low dam or obstruction below the fall in order to maintain the pool or water cushion. Occasionally they are made in the form of an incline, with a pocket at the bottom to break the force of the falling water.
It is necessary in the construction of nearly every irrigation canal to take water across a depression at some point in its course. This is usually done by means of a wooden or metal lic flume or long box, usually rectangular or semi-circular in cross-section and supported above the ground by a frame or trestle of timber or metal. Such flumes are often used across rocky or sliding ground where it is impracticable to maintain a canal. This is
particularly the case near the head, where the water, after being taken from the river, is often carried along a narrow, steep-walled canon. Here the foundation for a flume is prepared along the rocky cliffs, supports being devised to suit the inequalities of the ground. One type of flume is that having a semi circular section and built of narrow planks or staves laid side by side and held in place by iron bands run around the flume, joined by nuts and threads by which the bands can be drawn up and the staves brought together. In crossing very deep depressions it is necessary to have a correspondingly high trestle in order to carry the flume across the grade. Such high trestles are expensive and liable to destruction from storms. In their place there have been built inverted siphons, made of wooden stave pipe, or aqueducts of other form. The stave pipes are similar in construction to the semi circular flumes of narrow plank, beirm carefully wrought to a given dimension and held in place by circular iron bands or hoops.
In some portions of the and region when an abundant supply cannot be had from surface streams or reservoirs, it may be practicable to obtain some of the needed water by digging or boring wells, especially at points near stream-channels or along the foot-hills. Out on the broad valleys it may be necessary to go to a depth of from 100 to 300 feet or more before reaching moisture. Where the supply of water from wells is ample, various devices have been employed, such as windmills, gasoline and steam engines and electric power for bringing it to the surface. It is very important to continue the well borings through the water bearing sands or gravel, so as to take advantage of the full thickness of the pervious deposits. Perforated pipe is often driven into the layers of coarse gravel and adds greatly to the capacity of the well. Artesian or flowing wells may be found wherever water is held under pressure in pervious material overlaid by day or tight shale. In such a well the water will rise to a height equal to the line of saturation of the gravel or pervious stratum in its outcrop in the surrounding country. Artesian conditions occur in nearly every State, but they do not extend over any considerable portion of the country, excepting on the Great Plains and in the broad valley of central California. Wherever they occur the water has especial value on account of the convenience incident to its rising above the surface. In some places, as the James River Valley of South Dakota, the pressure is 100 pounds or more to the square inch, throwing the water to a consider able height and enabling the wells to be used as sources of power. The quantity of water to be had from deep wells is governed by the diameter of the well, the structure and thick ness of the water-bearing rocks and the pres sure sustained by the water. With relatively dense rocks a slight head of water will throw only a feeble stream, but from thick layers of open gravel or sand rock large volumes are delivered. It frequently occurs that a four inch pipe will deliver all of the water which can reach this point, and increasing the diam eter of the well will not alter the flow.