DRAINAGE AND IRRIGATION.
The Object of Drainage is to dry swampy or low-lying lands which may be saturated or wholly covered with water, or which may be subject to overflow. By this means the ends of agriculture are served by winning for cultivation the lands reclaimed; and there is also accomplished the no less important result of improving the sanitary condition of localities by removing the cause of noxions exhalations.
Swamps owe their origin to various circumstances, among which, nat urally, insufficient draina,ge is the principal cause. Wherever the rain water, or that flowing in from surrounding localities, or the water of springs, collects in a basin the floor of which is formed of impenetrable strata, this accession of water, if not counterbalanced by evaporation, the growth of vegetation, or efflux, will form either a lake of considerable magnitude or a marsh in which will flourish a rank growth of aqnatic \vegetation. In process of time the annual development of the plants and the accumula tion of their partially-decomposed remains give origin to the so-called `` peat bogs." Extension of Swamps.—By the gradual deposition of sediment brought by streams discharging into ponds or lakes, the latter, in time, may become almost completely silted np; in such cases the former lake or pond is con verted into marsh-land by the rank g-rowth of aquatic and semi-aquatic vegetation. At the same time, in consequence of the silting up of the out let channel, the ontflow is restricted, and the damming back of the inflow ing water causes it to spread out laterally, thns laying the foundation for the further extension of the marsh.
flindrances to Drainage.—Generally speaking-, the rise of the river bottoms by reason of the gradual deposition of the sediment carried by the flowing water, and the damming back of the waters thus occasioned, are the principal causes of the hindrance of the natural drainage of rivers and creeks, and of the consequent conversion of extensive districts into swamps. Apart from this, hindrances to drainage are caused when, for example, the waters of tributary streams are dammed up by the occurrence of floods in the main river, or when the mouths of rivers discharging into the sea are closed by sand-banks or bars thrown up by the action of the waves. The
river must in such cases make a detour around or pass over the obstacle, its eourse being consequently lengthened, its fall diminished, and the rapid efflux of its waters impeded.
Drainage making plans for the drainage of a g-,iven locality, there must first be gathered a thorough and accurate knowledge of the causes leading- to the formation of the swamp or body of water, also of the nature of the bottom and of the sub-soil, of the annual rainfall, etc. Fur thermore, accurate observations of the level of the locality in relation to that of its surrounding territory must be made, that the amount of avail able fall may be determined.
Drainage actual operation of drainage consists either in lowering- the level of the water or in elevating the ground. The lowering of the water-level is the more usual and the simpler plan, and is effected in various ways, according to circumstances. Should the swamp be caused bv the silting of the bottom of an adjacent river, the attempt will be made to lower the water-level of the stream by shortening its course or by other well-known expedients (see River Engineering, p. 298), and drainage canals communicating with the stream will be clug, through which the waters of the swampy or submerged district will be drained into the latter. To obtain the needful fall, it will frequently be found necessary to give the drainage-canals considerable length and a circuitous route, and to locate at some distance down the stream the outlets, which must be provided with locks, to prevent the entrance of tide-water from the river. The lower ing of the level of many lakes in mountainous regions is expeditiously accomplished by cutting through the rock-barrier which like a nat ural dam retains the waters (for example, the Lake of Lugano, in Switz erland).