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Rh large floating débris 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 being closed against floods at their upper end and having their entrance at the lower end facing down-stream. Many such ports have been provided on several German and North American rivers; where the port, being near a town, is lined with quay walls, it -can also be used for river traffic, a plan adopted at the refuge port on the Main just below Frankfort (fig. 8). Regulation of Rivers for Navigation.

As rivers flow onward towards the sea, they experience a considerable diminution in their fall, and a progressive increase in the basin which they drain, owing to the successive influx of their various tributaries. Thus' gradually their current becomes more gentle and their discharge larger in volume and less subject to abrupt variations; and, consequently, they become more suitable for navigation. Eventually, large rivers, under favourable conditions, often furnish important natural highways for inland navigation in the lower portion of their course, as, for instance, the Rhine, the Danube and the Mississippi; and works are only required for preventing changes in the course of the stream, for regulating its depth, and especially for fixing the low-water channel and concentrating the flow in it, so as to increase as far as practicable the navigable depth at the lowest stage of the water-level. Regulation works for increasing the navigable capabilities of rivers can only be advantageously undertaken in large rivers with a moderate fall and a fair discharge at their lowest stage; for with a large fall the current presents a great impediment to up-stream navigation, and there are generally great variations in water-level, and when the discharge becomes very small in the dry season it is impossible to maintain a sufficient depth of water in the low-water channel.

Removal of Shoals.—The possibility of securing uniformity of depth in a river by the lowering of the shoals obstructing the channel depends upon the nature of the shoals. A soft shoal in the bed of a river is due to deposit from a diminution in velocity of flow, produced by a reduction in fall and by a widening of the channel, or to a loss in concentration of the scour of the main current in passing over from one concave bank to the next on the opposite side. The lowering of such a shoal by dredging merely effects a temporary deepening, for it soon forms again from the causes which produced it. The removal, moreover, of the rocky obstructions at rapids, though increasing the depth and equalizing the flow at these places, produces a lowering of the river above the rapids by facilitating the efflux, which may result in the appearance of fresh shoals at the low stage of the river. Where, however, narrow rocky reefs or other hard shoals stretch across the bottom of a river and present obstacles to the erosion by the current of the soft materials forming the bed of the river above and below, their removal may prove a permanent improvement by enabling the river to deepen its bed by natural scour.

The deepening of the bed of a non-tidal river along a considerable length by dredging merely lowers the water-level of the river during thelow stage; and though this deepening facilitates the passage of floods in the first instance, it does not constitute a permanent improvement even in this respect, for the deposit of the detritus brought down by the river as the floods abate soon restores the river to its original condition. Nevertheless, where sand-banks obstruct and divert the low-state channel of a river at its low stage, as in parts of the Mississippi below Cairo, it has been found possible before the river has fallen to its lowest level to form a channel through these sand-banks, with a depth of 9 or 10 ft. and 250 ft. wide, by suction dredgers, aided by revolving cutters or water-jets (see ), which discharge the sand through floating tubes into a part of the river away from the channel; and the navigation can thus be maintained throughout the low stage at a reasonable cost. Though, however, these channels across the shoals, connecting the deeper parts of the river, can be easily kept open on the Mississippi till the return of the floods, they are obliterated by the currents in flood-time, and have to be dredged out again afresh every year on the abatement of the floods. Regulation of the Low-Water Channel.—The capability of a river to provide a waterway for navigation during the summer or throughout the dry season depends upon the depth that can be secured in the channel at the lowest stage. Owing to the small discharge and deficiency in scour during this period, it is important to restrict the width of the low-water channel, and concentrate the flow in it, and also to fix its position so that, forming the deepest part of the bed along the line of the strongest current, it may be scoured out every year by the floods, instead of remaining an undefined and shifting channel. This is effected by closing subsidiary low-water channels with dikes across them, and narrowing the channel at the low stage by low-dipping cross dikes extended from the river banks down the slope, and pointing slightly up-stream so as to direct the water flowing over them into a central channel (figs. 4 and 5). The contraction also of the channel is often still more effectually accomplished at some parts, though at a greater cost, by low longitudinal dikes placed along either side of the low-water channel, some distance forward from the banks but connected with them generally at intervals by cross dikes at the back to prevent the current from scouring out a channel behind them during floods (figs. 4 and 6). By raising these dikes only slightly above the surface of the bed of the river, except where it is expedient to produce accretion for closing an old disused channel or rectifying the course of the river, the capacity of the channel for discharging floods is not affected; for the slight obstruction to the flow produced by the dikes at the sides is fully compensated for by the deepening of the low-water channel in the central course of the river.

This system of obtaining a moderate increase in depth during the low stage of a river, whilst leaving the river quite open for navigation, has been adopted with satisfactory results on several large rivers, of which the Rhone, the Rhine and the Mississippi furnish notable examples. Regulation works were preferred on the Rhone to canalization from Lyons nearly to its outlet, in spite of its large fall, which reaches in some places 1 in 250, on account of the considerable quantities of shingle and gravel carried down by the river; the comparative regularity of the discharge, owing to the flow being derived from tributaries having their floods at different times of the year, has aided the effects of the works, which have produced an increase of about 3 ft. in the available navigable depth below Lyons at the lowest water-level. Owing, however, to the unfavourable natural condition of the river, the depth does not exceed 5 ft. at this stage; and the rapid current forms a serious impediment to up-stream navigation. The Rhine is much better adapted for improvement by regulation works than the Rhone, for it has a basin more than double the area of the Rhone basin, and its fall does not exceed 3·1 ft. per mile up at Strassburg and 2·5 ft. per mile through the rocky defile from Bingen to Kaub, and is much less along most of the length below Strassburg. These works systematically carried out in wide shallow reaches between the Dutch frontier and Mainz, aided by dredging where necessary, have secured a navigable depth at the low stage of the river of 10 ft. from the frontier to Cologne, 8 ft. from Cologne to Kaub, and 6 ft. through the rocky defile up to Bingen, beyond which the same depth is maintained up to Philippsburg, 22 m. above Mannheim. Works, moreover, are in progress by which it is anticipated that the minimum depth of 6 ft. will be extended up to Strassburg by 1916. The Mississippi also, with its extensive basin and its moderate fall in most parts, is well suited for having its navigable depth increased