Many large and small cities on rivers use river water for municipal water supply. The advantage is that it’s there: all you have to do is pump it out of the river rather than drill and maintain deep wells or build and maintain often distant reservoirs. One big disadvantage, though, is pollution. You can chlorinate for pathogenic organisms and filter for turbidity, but you can’t easily extract dissolved chemicals introduced upstream. This is especially a problem for large cities located on the lower reaches of major rivers, like New Orleans.
In many areas, rivers are important sources of irrigation water as well as groundwater. You don’t have to pump: just divert some of the river into irrigation canals and let it flow by gravity into agricultural areas adjacent to the river.
There are problems, however:
• As with municipal water supply, this may use up a large percentage of river discharge by the time the river empties into the ocean!
• That water has to go somewhere. Where does it go? (1) It’s lost by evaporation and transpiration (loss by evapotranspiration is enormous in irrigated agriculture); (2) It infiltrates, then flows as groundwater back into river downstream. Chemical pesticides used on cropland partly remain in the soil and partly are leached with return groundwater flow into the river.
The issue of dams is a complicated one. Humankind has been building dams since way back in prehistory. There are several kinds of dams: earth fill, rock fill, and concrete. The technology of dam construction is well advanced (but, occasionally, there still are dam failures!).
What are the main uses of dams? (Note: these uses are often, perhaps usually, combined.)
• water supply
• flood control
• hydropower generation
Below (in no particular order) are listed some of the aspects of dams that need to be taken into consideration by urban planners and other governmental authorities as well as by environmental scientists and engineers:
• cost–benefit analysis
• displacement of humans
• land loss
• ecosystem disturbance
• evaporation loss
• downstream degradation
If rivers carried no sediment, problems with dams would be far less serious. All rivers, however, carry sediment, and all but the smallest are alluvial rivers. In such rivers, sediment is in one way or another the biggest problem in reservoirs behind dams. Unless special (and very difficult and costly) engineering measures are employed, the reservoir is a sink for sediment arriving at the upstream end. All such reservoirs therefore have a finite lifetime. These lifetimes are usually measured in decades, and often not many decades.
What happens when the reservoir is mostly filled with sediment?
(1) Dig it out—which is almost always impractical. (2) Let it go over the spillway with the water. The problem is most severe with respect to flood control and irrigation: the capacity of the reservoir becomes negligible. The problem is not so severe for hydropower generation, because the difference in water level between the water surface behind the dam and the turbines at the base of the dam is still there, but there are serious engineering problems connected with how to use the water without passing the sediment through the generating facility as well.
Degradation of the river bed below dams is another big problem. If you store the sediment in the reservoir instead of letting it pass, sediment transport rate just downstream of the dam is zero, and the river tends to entrain bed sediment to establish its equilibrium sediment load. The river then cuts down into its bed.
It’s often not realized how much water is lost to evaporation from the water surface of the reservoir behind the dam, especially in summer and in arid climates. Various engineering schemes have been proposed for reducing such evaporation, but none has proved practical.
Water temperature in a given river depends upon a number of natural effects: climate, season, residence time, and heating by energy dissipation. This thermal regime can be seriously disrupted by the need to supply cooling water to power plants, which are often located along rivers just for accessibility to cooling water. The main problem is the biological effect on river fauna and flora downstream.
Rivers, both meandering and braided, have an annoying tendency to shift their channels laterally. (Well, annoying for certain groups of people.) This creates obvious problems for residents along the river. In recent decades the engineering response has increasingly been to make rivers like artificial channels by lining the banks with various kinds of large heavy platy or interlocking objects, first at places of greatest vulnerability and then, increasingly, all along the river banks. Most large rivers in the United States are now partly stabilized in this way. The Mississippi is mostly stabilized. This was not always the case, as a glance at a map of the states adjacent to the Mississippi readily shows: little pieces of one state are now stranded on the other side of the river!