CANALIZED RIVERS Rivers whose discharge is liable to become quite small at their low stage, or which have a somewhat large fall, as is usual in the upper part of rivers, cannot be given an adequate depth for navi gation by regulation works alone; and their ordinary summer level has to be raised by impounding the flow with weirs at intervals across the channel (see WEIR and RIVER ENGINEERING), while a lock (q.v.) has to be constructed alongside the weir, or in a side channel, to provide for the passage of vessels (fig. 3). A river is thereby converted into a succession of fairly level reaches rising in steps up-stream, and providing a comparatively still-water navi gation like a canal ; but it differs from a canal in the introduction of weirs for keeping up the water-level and in the provision f or the regular discharge of the river at the weirs. Canalization secures a definite available depth for navigation ; and the discharge of the river is generally ample for maintaining the impounded water-level, as well as providing the necessary water for locking. The navigation, however, is liable to be stopped during the descent of high floods, which in many cases rise above the locks (fig. 4) ; and it is necessarily arrested in cold climates on all rivers by long, severe frosts, and especially on the break-up of the ice. (This also applies to canals, even in England; and the suspension of traffic during prolonged frosts is one of the disadvantages, com pared with railways, under which canals have to operate.) Many small rivers, like the Thames above its tidal limit, have been rendered navigable by canalization; and several fairly large rivers have thereby provided a good depth for vessels for con siderable distances inland. Thus the canalized Seine has secured a navigable depth of iolft. from its tidal limit up to Paris, a dis tance of 135 miles and a depth of 6-1-ft. up to Montereau, 62 miles higher up. Regulation works for improving the river Main, from its confluence with the Rhine opposite Mainz up to Frankfurt, failed to secure a minimum depth of 3f t. at the low stage of the river, and canalization works, carried out between 1883 and 1900 by means of six weirs in the 261 miles between the Rhine and Offenbach, above Frankfurt, provided a minimum depth of 83 feet. (See below for other references.) On ascending a river it becomes increasingly difficult to obtain a good depth by canalization owing to the progressive inclination of the river-bed. Thus, even on the Seine, with its moderate fall, a depth of 1 o 2 f t. has been obtained on the Lower Seine by weirs placed on the average 13-1-m. apart, but on the Upper Seine weirs are required at intervals of only about 43m. to attain a depth of 6-1-ft.
