Cracked mud of a lake bottom exposed by drought.

<< Back   Next >>

Agents of Natural Change in Watersheds

Natural Change Processes: Drought
In addition to its obvious adverse effect of significantly reducing overall water volume or flow, drought can have a major impact on water chemistry by altering the relative contribution of groundwater versus surface water. This can in turn result in changes in the water chemistry, transparency, light regime, and thermal characteristics of lakes and rivers. Drought also may totally dry up temporary water bodies such as woodland ephemeral pools, small streams with marginal flow, or seasonal, pothole-type wetlands. Many organisms are uniquely dependent upon these ephemeral waters that are absent from the landscape during periods of drought. For several species of birds and amphibians, the disappearance of seasonal water bodies during times of drought can have a detrimental effect on the population due to temporary elimination of breeding or feeding grounds. Many years of depressed population numbers in these species can follow droughts. Other species, including most microcrustaceans, many other amphibians, and even some species of fish can wait out periods of drought by going through a stage of dormancy that, in the case of some crustaceans, can last up to hundreds of years.

Beyond affecting the water body and its aquatic organisms, drought significantly affects upland areas of watersheds. Severe drought can cause dieback of less-tolerant tree and shrub species and lead to shifts in dominant vegetation. Crop failures from sustained droughts can eventually lead to abandonment of some areas' agricultural land uses and communities. As witnessed in the "dust bowl" years, droughts can also liberate huge volumes of topsoil that is no longer stabilized by crops or natural vegetation. This may have severe effects on water quality and on productivity of the topsoil-deprived lands as well as the new sites of deposition.