Science for beginners . the soil to the watertable. When the water table gets higher than the surface ofthe river or lake, it tends to move horizontally into the riveror lake. But it has to find its way through the soil, betweenthe soil particles. The soil particles constantly block its pas-sage just as rocks block the free passage of water in the bed of 386 GROUND-WATER AND GROUND-AIR a shallow stream. The ground water, therefore, moves muchmore slowly down hill through the soil than it does down themore open bed of the river. The water level in the lake orriver is, therefore, constantly at a
Science for beginners . the soil to the watertable. When the water table gets higher than the surface ofthe river or lake, it tends to move horizontally into the riveror lake. But it has to find its way through the soil, betweenthe soil particles. The soil particles constantly block its pas-sage just as rocks block the free passage of water in the bed of 386 GROUND-WATER AND GROUND-AIR a shallow stream. The ground water, therefore, moves muchmore slowly down hill through the soil than it does down themore open bed of the river. The water level in the lake orriver is, therefore, constantly at a lower level than the sur-face of the ground water in the soil near its banks. Thefarther back we go from the lake or river, the higher the levelof the ground water stands. 458. Drainage through the Soil.—Since a rivers channel isusually considerably lower than the soil on either side, andalso lower than the water table, we see that not only does thesurface water flow into the river but that the soil-water is G*4z^z?y. Fig. 231.—Ground-water and river-water. also constantly flowing through the soil into the river. Thishorizontal movement of soil-water, or lateral percolation,is usually very slow but varies with the nature of the soilthrough which it flows. Different tests show the rate of move-ment of this soil-water to vary from 5 feet to 100 feet per day. Roughly speaking, about % of the rainfall in the UnitedStates is carried off by the streams. Fig. 229 shows theamount of this annual run-off in the various sections of thecountry. Compare this map with Fig. 174, the annual rain-fall map. The remaining % of the rainfall evaporates fromrivers, lakes, from vegetation and from the soil itself. 459. Relation of the Ground-water to the River-water.— GROUND-WATER 387 It is interesting and important to note the relation of theground-water near a river to the water in the river who has followed the course of a river far is awarethat, in most cases, springs are common
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