prevailing air temperatures and the water temperatures of the flow
downstream.
Current practice with most dams and water storage facilities is
to release either cold bedwater from the bottom sluices or warm
surface water over the top of the dam wall, down the spillway. This
can have disastrous consequences unless the temperature of the
water released or its possible effect on the downstream flow regime
is taken into account. Warm water, for example, discharged into a
stretch of river where the temperature gradient is only slightly pos-
itive, will effectively cancel the effect of the positive gradient, result-
ing in the automatic and almost simultaneous deposition of silt and
sediment. The result will be flooding.
On the other hand if only the cold bedwater is released, it may
overcool the lower reaches, causing excessive scouring and the
transport of very heavy sediment loads which the lower flow
regime may be unable to handle. This may be because of the slope
of the bed-gradient and thereby the speed of flow, the width of the
channel — wide, shallow channels dropping sediment more
quickly, the temperature gradients operative lower down, etc. Each
Fig. 11.2. Groundwater recharge through
river bank reforestation.
The trees act like a refrigerator, cooling the
ground, which allows a positive temperature
gradient to draw waterfrom the river to recharge
the water table.- RIVERS AND HOW THEY FLOW