THE REGULATION OF NON-TIDAL RIVERS Mitigation of Floods and Protection from Inundations.— As ithe size of the channel of a river is generally inadequate to carry down the discharge of floods, the river overflows its banks in flood-time and inundates adjacent low-lying lands. An enlarge ment of the river bed, principally by deepening it, in order to increase its discharging capacity, is precluded by the cost, and also, in rivers bringing down sediment, by the large deposit that would take place in the enlarged channel from the reduction in the velocity of the current when the flood begins to subside. Where, however, the depth of a tidal river has been considerably increased by dredging, the enlargement of its channel and the lowering of its low-water line facilitate the passage of the land water and consequently reduce the danger of flooding. The Glasgow quays, for instance, along the deepened Clyde are no longer subject to inundation, and the lands and quays bordering the Tyne have been relieved from flooding for Io m. above Newcastle by the deepening of the river from Newburn to the sea. (See fig. 6.) In certain cases it is important to restrict or to prevent the inundation of some riparian districts by embankments; and oc casionally low-lying lands are so unfavourably situated that pumping has to be employed.
The flow of water off the slopes of a valley can be retarded by planting trees on mountain slopes, which have too often been denuded by the reckless clearing of forests.
Proposals have sometimes been made to reduce the height of floods in rivers and restrict the resulting inundations by im pounding some of the flood discharge by the construction of one or more dams across the upper valley of a river, and letting it out when the flood has passed down. This arrangement, however, is open to the objection that in the event of a second flood follow ing rapidly on the first, there might not be time to empty the reservoir for its reception. When this provision against floods can be combined with the storage of water for economic use it be comes financially practicable. Two masonry dams erected across the narrow valley of the river Furens, a torrential tributary of the Loire, form reservoirs for the supply of the town of St. Etienne.
The most notable instance of the construction of a dam to form a regulating reservoir is the Gatun dam on the Panama canal (q.v.), one of the principal functions of which is the control of the flood waters of the river Chagres. The provision of flood control reservoirs in the Mississippi basin has often been advo cated and, since the 1927 inundation, it has received renewed consideration.
Methods of Increasing the Discharging Efficiency of River Channels.—The discharging efficiency of a river within the limits of its bed depends on the fall and the cross-section of the channel. The only way of increasing the fall is to reduce the length of the channel by substituting shorter cuts for a winding course. This involves some loss of capacity in the channel as a whole, and in the case of a large river with a considerable flow it is very difficult to maintain a straight cut, owing to the tend ency of the current to erode the banks and form again a sinuous channel. Cuts therefore should be in the form of one or more flat curves. Nevertheless, where the available fall is exceptionally small, as in lands originally reclaimed from the sea, such as the English fen districts, and where, in consequence, the drainage is in a great measure artificial, straight channels have been formed for the rivers and "drains." The removal of obstructions, whether natural or artificial, from the bed of a river furnishes a simple and efficient means of in creasing the discharging capacity of its channel, and, consequently, of lowering the height of floods. Every impediment to the flow, in proportion to its extent, raises the level of the river above it so as to produce the additional fall necessary to convey the flow through the restricted channel, thereby reducing the total avail able fall. In the absence of legal enactments for the conservancy of rivers, numerous obstructions have in many cases been placed in their channel, such as mining refuse, sluice-gates for mills, fish traps, unduly wide piers for bridges and solid weirs, which impede the flow and raise the flood-level.