RIVER ENGINEERING.
Source of the atmospheric precipitation of moisture in the form of rain, snow, or hail are due the existence and maintenance of the supplies of water -upon the land. The larger portion of the precipipted moisture remains upon the surface, and, collecting- into watercourses, con stitutes the rivulets, brooks, larger streams, and rivers of their respective drainage-areas; a smaller portion sinks into the soil, and, penetrating into the crevices and cavities of the rock-strata, forms the subterranean streams, which reappear on the surface in the form of springs. The watercourses find their way -ultimately to the oceans, and the series of changes here de scribed repeats itself endlessly. The explanation of these operations lies in the fact that the air at a certain temperature is capable of taking tip and retaining only a certain quantity of aqueous vapor. If a body of air charged to saturation with moisture be cooled, it will part with a portion of its vapor, which will make its appearance as masses of clouds, and under cer tain conditions will be condensed and fall upon the earth as rain, snow, or hail. ln thickly-wooded districts, where the air is always cooler than over stretches of bare g,round, the passage of a body of warm air is commonly attended with the precipitation of a part of its moisture. This explains why in such rcg,ions the rainfall is abundant and more uniformly distributed throughout the year.
Rainfall.—It is of much practical value in the study of the hydratilics of rivers to know the amount of the atmospheric precipitation in a given period of time—say in a year. This factor is determined with close approxi mation to accuracy by the udonieter, or rain-gauge. The quantity of rain fall varies greatly, certain regions of the earth being almost rainless, while others receive enormous quantities. For any given locality, however, the annual rainfall is very constant. The amount of the annual evaporation of a given locality is measured with the atinometer. Of the total rain fall, it is estimated that about one-third sinks into the ground; the rest runs off from the impervious or thoroughly-saturated surface, and, seeking the lowest situations, collects in pools or lakes, or, feeding the watercourses, finds its way to the sea through the channels of the brooks, creeks, and rivers. Of the third that finds its 1N-ay through an infinite number of ave nues beneath the surface, a portion reappears in the form of spring,s gush ing from the rocks, while another portion sinks until it reaches an imper vious stratum, where it collects to form ground-water, which may rise to a considerable height by continued accumulation, and, overflowing, may contribute to the supply of the superficial watercourses.
Rivers and their brooks, creeks, and lesser streams by their union form the rivers, and these by their union in turn form the great river-systems. That portion of the earth's surface which
contributes its rainfall to a river or river-system is termed its drainage-area or "catchment-basin." The elevated ground or ridge forming the boundaries between adjacent drainage-areas is termed the watershed, or " divide." From the area of a given drainage district and the quantity of its annual rainfall the volume of water received and discharged by the river may be determined with close approximation to accuracy. The Amazon and its tributaries drain an area of 2,000,000 square miles; the Mississippi system drains an area of 1,200,000 square miles, the Dannbe 269,187 square miles, the Rhine 65,28o square miles, etc. A convenient distinction is made between the upper course, the middle course, and the lower course of a river; the first has the greatest fall, and the last the least fall. The Elbe, for example, has an average fall in Bohemia of I 2670; in Saxony, of in 376o; and at Hamburg„ I in ri.,400 to in 28,Soo at ebb and flood tides respectively.
The River river-bed is the channel in which the water is confined and through which it is carried off; and, in respect of the bed, we may distinguish the bottom, the more or less precipitous sides, and the upper border of the latter—the water's edge. Thc land lying adjacent to the riN-er-bed is called the "shore," and that area which is liable to be intin dated in times of freshet is designated the territory of overflow. By the term " banks" is to be understood the limit of an area approximately par allel with the bed and cut out by the stream. The area contained between the foot of the banks and the water's edge is called the " beach;" and if this is of considerable extent, it is called the bottom-land or "lowlands." of RiVel- Channels: Erosion and bed of the stream is subject to constant and manifold alterations by the action of the water flowing over it. The yielding materials of the bed and of the banks are washed away, and the more resisting- materials—even the hard est rocks—are ground away by abrasion or dissolved, so that ravines and gorges are formed, some of these so colossal as to rank among the greatest of natural wonders (the great Caiion of the Colorado River, the great gorge of the Niagara River from the Falls to Queenstown, etc.). The concave portions of the river-bank are most deeply indented by the washing and undermining action of the current, while the convex portions assume more and more the characteristics of peninsulas. Then comes a flood, in which the river breaks through the projecting:, arm, creating for itself a new and shorter channel, and in time the abandoned channel, filling up with silt and debris, becomes dry land.