Page:Oregon Historical Quarterly vol. 26.djvu/12

2 The conditions necessary for the existence of lakes are (1) Depressions without outlets, and (2) a sufficient supply of water. By depressions without outlets it must not be understood that lakes have no outlets. It means that below the level of the lake outlet there is in each case a depression which has no outlet. It is this depression without an outlet that makes the lake, and when the water reaches the elevation of the outlet it overflows.

Lake basins originate in many ways, but most of them are the result of vulcanism and diastrophism. Diastrophism includes movements of the earth's crust up or down. Vulcanism produces crater lakes, and in Oregon particularly, streams of lava have caused a considerable number of our water bodies. Other common causes are landslides which dam up streams, glacial deposits, and frequently glacial action, gouging out depressions in soft rocks. Lakes are also produced by dams built of windblown sand.

Lakes are considered to be the most short lived of all important geographic features. Every lake's fate is certain and its ultimate destruction has led to the famous epigram, "Rivers are the mortal enemies of lakes." The main causes of lake destruction are the lowering of outlets by stream cutting which results in ultimate drainage of lakes, and by sedimentation from streams entering lakes which results in the depressions being filled up. Lake basins are being filled constantly with sediment from inflowing water, and also by wave action along the shores and particularly by organic matter deposited by shell-bearing animals, plants and other growths. In some cases wind-blown dust accomplishes the end.

Oregon has four well defined lake regions. While there are lakes in most of the counties of the state, nature has concentrated her efforts as far as lakes are concerned in definite areas. The first area is along the main axis of the Cascade Range. This part of the state is well supplied with lakes, some of which are large and impressive,