Depletion of ground water reserves is equally alarming. All of them are being pumped out much faster than the recharge rate. Annual net depletion of the High Plains System, for example, amounts to more than 325 billion cubic meters, which equals about 15 times the annual flow of the Colorado River. The other two mentioned aquifers have comparable histories. The same pattern applies in every other aquifer of the world where irrigation agriculture is practiced.
Pollution is as great a threat to aquatic ecosystems as river and lake depletion.
Mercury, P.C.B.'s, chlordane, dioxins, and commonly used farm chemicals are the common known offenders. About 30,000 different chemicals are known to enter rivers and lakes, and only a few of them are monitored. Loss of fish population is a common result of chemical pollution. The loss of salmon in the Columbia River is an example. The Columbia is only one of the many rivers similarly afflicted, and salmon are only one of the species affected.