Regulation works at bends, besides arresting erosion, also reduce the differences in depth at the bends and the crossings. The excessive depth round the concave banks is diminished and the channel along the crossings deepened by giving a straighter course to the current and concentrating it by a reduction in width of the channel between the bends. Where there are deep pools at intervals in a river, shoals are always found above them, owing to the increased fall which occurs in the water line on approaching the pool, to compensate for its very slight inclination where it crosses the pool, whose ample cross-section serves for the discharge of the river through this part of the river bed. These variable depths can be regu lated to some extent by submerged rubble-dikes or f ascine-mat tress sills deposited across the bed of the pool, so as to reduce its excessive depth, but not raised high enough to interfere at all with the navigable depth. These obstructions in the pool raise the water-line towards its upper end, in order to provide the additional fall needed to effect the discharge through the pool with its diminished cross-section.
On large rivers, where vessels during high floods are exposed to injury from float ing debris and ice-floes, shelter can be provided for them in refuge ports, formed in a recess at the side under the protection of a solid jetty or embankment constructed in the river parallel to the bank. These ports are closed against floods at their upper end and have their entrance at the lower end facing down-stream. There are numerous examples of such river harbours on the Danube, the Rhine and other European rivers and in the rivers of North America. Many of them, made in the vicinity of towns, as at Dusseldorf, are inland ports of considerable size.
Many rivers have been improved, for the purpose of navi gation, by canalization; and in some cases lateral canals have been constructed alongside portions of rivers to avoid rapids, falls or other serious obstructions. (See CANALS AND CANALIZED RIVERS.)