Transportafion and Deposifion of action of the water upon the river-bottom is such that in times of flood the coarser materials are carried farther down stream, where, with the diminishing velocity of the current, they are gradually deposited. The coarser and heavier mate rials (cobbles, pebbles, etc.) settle first, next the gravel and sand, and fin ally the fine mud or silt. A velocity of 3 inches per second at the bottom will be sufficient to tear up fine clay; 6 inches per second, fine sand; 12 inches per second, fine gravel; and 3 feet per second, stones of the size of an egg. The more rapid the current of a river, therefore, the coarser will be the composition of the materials forming- the bottom of its bed. The extremely fine mud is held in suspension until the river reaches the end of its course—in a gulf or the sea—where it is deposited. One of the conse quences of this deposition of sediment is the gradual elevation of the river bed and the formation of shoals, especially in those portions where the bed becomes widened and the velocity of its current correspondingly lessened.
formation of deltas at the mouths of many of the great rivers of the world is due to the cause's above indicated. The river, charged with its load of fine sediment, reaching the comparatively still waters of an inland sea or gulf, has its velocity checked, and the sediment, sinking to the bottom and gradually piling up, continually extends the river into the sea. The deltas at the mouths of the Mississippi, the Nile, and the Po afford typical illustrations of these facts, and of the gradual encroachment of the rivers upon the domain of the sea.
and l'elocify of respect of its three conditions of a river are conveniently distinguished—namely, high water level, mean or normal level, and low-water level. The latter occurs usually in winter and in midsummer. High water may be anticipated toward the close of the winter and during the early spring, following upon the melting of the winter's snow. The extreme differences of level vary in different rivers: thus the Ganges, the great river of India, flows at the rate of 55,000 cubic feet per second during the dry months and 5oo,000 cubic feet during- the four rainy months; the Rhine at Kehl flows at the rate of 13,414 cubic feet per second at low water and 165,38o cubic feet at high water; the Elbe at Dresden, 2500 elIble feet at low water and 141,000 at high water. The ratios in these cases, it will be observed, vary from in 9 to in 56. Large lakes exert a pronounced regulating influence upon the water-level of streams of which they are the feeders, as witness the influence of the Lake of Constance in the regulation of the upper Rhine. Floating ice may give rise to ice-jams, and, in extreme cases, to ice-gorg-es, where the ice-masses completely choke up the river-bed, in which case the water may be backed up to a considerable height.
_Rivers Subject to Continual Changes.—The rivers, therefore, when abandoned to natural conditions, are subject to continual chang-es. A well regulated stream even, if left to itself, may gradUally pass beyond its con trolling influences through the undermining of its banks and the deposition of sediment, whereby, on account of the rise of its bottom, the free dis charg-e of its tributaries will be interrupted—a condition which favors the formation of marshes and swamps. The presence of sharp bends and of extreme contractions (or narrows) where the free flow of the stream will be impeded, and where ice may readily effect a lodgment and obstruct the channel, may cause inundations, which, in addition to the direct damag-e they may inflict, frequently cover the overflowed tracts of land with such quantities of stones, grave], and sand as to destroy their value for farming purposes for years. On the other hand, in those sections where the chan nel widens considerably, the diminished velocity of the current will pro mote the deposition of sediment in the river-bed, and this may lessen the depth of water to such an extent as seriously to impede, or even to destroy, its navigability. Again, the natural fall of the stream is frequently so great as to render it difficult or impracticable for vessels to make headway against the current. Too many bends in the channel, likewise, are unde sirable in the interests of navigation.
1Vecessity of Controlling the Streams.—It is the task of the hydraulic engineer to establish and maintain a regimen of the rivers, on the one band, by the construction of defensive works by which their banks shall be protected against injury or destruction, and, on the other hand, by the con struction of offensive works by which their currents shall be so controlled, deflected, or concentrated, that the tendency to shoaling-, and to other irregularities which interfere with the free discharge of the water, impede navigation, and injuriously affect the banks, shall be reduced to a minimum or altogether eliminated.
Stream Measurements.. Charis.—Before the plan for a regimen of the stream can be formed, the character of the stream must be ascertained, to which end certain measurements are necessary. First of al], starting with a definitive low-water level, a plan of the stream must be constructed, and this must be extended to embrace the territory of overflow. In the inter ests of navigation, it is of special importance to determine the trend of the bottom, or " thalweg "—the line of greatest depth. This is done by mak ing a large number of cross-sectional measurements at proper distances apart, the lowest points of which are then marked upon the plan. The line joining these points will be the thalweg, or line of deepest channel.