RUSSELL.] SPRINGS. 1 49 
HILLSIDE SPRINGS. 
Examples of this type occur where thick beds of porous material, 
such as sand, loose-textured soil, disintegrated rock, etc., occur on 
the border of valleys or form uplands on which precipitation takes 
place and the water percolating downward through the interspaces in 
the receiving layer meets a less pervious layer and flows laterally along 
its surface, becoming more or less concentrated, and emerges at or 
near the base of an escarpment as a stream. The presence of a reten- 
tive layer in the uplands bordering a depression, at a horizon above its 
bottom, however, is not essential. As is well known, when precipita- 
tion is sufficiently abundant the subsoil is saturated below a certain 
horizon. During heavy or long- continued rains the soil and subsoil 
may be saturated from the surface downward, although under certain 
conditions there is a lower limit to saturation above the base of the 
porous layer. After the supply of water at the surface ceases the 
upper limit of saturation or the " water table," as it is termed, sub- 
sides. The water table is not a plane, but an undulating surface, 
which rises beneath hills and is depressed beneath valleys. As water 
escapes by seepage, or when more concentrated, as springs, the water 
table is lowered and at the same time flattens, tending to become a 
plane surface where it subsides below all surface depressions. So long- 
as the water table is above the bottom of a surface depression, such as 
a valley, hillside springs are possible. 
These conditions are indicated in fig. 2, which is an ideal vertical 
section across a valley with a stream flowing down it, excavated in 
porous material. 
Fig. 2.— Section showing relation of hillside springs to water table. 
The water falling on the surface aa penetrates downward, and at a 
certain stage after the supply ceases the upper surface of the saturated 
subsoil may be assumed to have the position hh. At such a stage the 
water flows toward the valley along the courses indicated by the dotted 
lines and escapes most commonly by seepage, but if concentrated in more 
definite channels forms springs. Should the water table be depressed 
to the line cc, the springs would cease to flow and lateral seepage from 
the stream occupying the valley would occur. Under these conditions 
the surface of saturation is higher beneath the valley than beneath the 
adjacent uplands. So long as a stream continues to flow, unless its bed 
is impervious, the water table can not be depressed below its bottom. 
