PROBLEMS FROM DEVELOPMENT 137 



ground water are moving toward the same general 

 destination. 



3. The watercourse may cross other ground- water reser- 

 voirs, in which case the other reservoir may discharge 

 water into the ground-water reservoir and stream of the 

 watercourse, or vice versa, depending upon the hydraulic 

 gradient. 



4. In the watercourse, impermeable beds provide no more 

 than local isolation of surface water from Ground water, 

 or of the water in individual aquifers of the ground-water 

 reservoir. In general there is intimate relationship to the 

 extent that water traveling in the watercourse may be 

 classed successively as ground water, surface water, and 

 "diffused surface water" (see page 248). 



Precipitation at any point along the watercourse may in- 

 crease the quantity of water in the stream and also in the 

 ground-water reservoir wherever conditions are favorable for 

 recharge. Recharge to the ground-water reservoir is not lim- 

 ited to local precipitation but may come from the stream. 



In segments of some watercourses, especially where precipi- 

 tation is abundant, the water table of the ground-water reser- 

 voir is at all times higher than the stream level, and there is 

 continuous movement of ground water into the stream. In 

 other places, more likely to be in arid areas, conditions may 

 be reversed, and water is always moving from the stream into 

 the underlying ground-water reservoir. In many watercourses 

 the stream gains from the ground-water reservoir when it is at 

 low stage and recharges the ground-water reservoir at high 

 stages. Recharge to the ground-water reservoir is commonly 

 greatest during flood stages of the stream, not only because of 

 the favorable hydraulic gradient, but because the finer and less 

 permeable sediments are cleaned from the channel at that 

 time. Also, the stream may overflow and saturate the materials 

 of its flood plain for a considerable distance from the channel. 



Characteristically in watercourses there is slow movement 

 of water, called "underflow," through the ground-water res- 



