Important Pre-requisites for the Existence of Lakes: Basin and Depression Forming Processes

Abstract

Abstract for Sect. 2.1 All lakes are positioned in basin-like forms on continental surfaces. Many lakes are so large it is difficult to identify these depressions such as the Caspian Sea, Victoria Lake, or the Aral and Chad Lake basins, while other depressions are more significant like those of the North American Great Lakes and Lake Baikal. Depressions can be smooth, shallow and very wide while others are narrow and elongated as observed between folded mountain chains or along rift valleys (Eastern Africa). We can surmise that basins/depressions formed by passive or active crustal movements (tectonics/endogenic processes) are comparatively old (up to several millions years). In contrast, a group of volcanic basins (resemble craters) also of endogenic origin are primarily younger and several simply appear and disappear within a few years. In a geological sense ‘young’ translates to a million years or less and includes basin-like forms carved into the landscape by exogenic processes from rivers, ice streams and impact craters (1 billion years to just decades old—not necessarily the age of the lakes in these craters and in any case younger than the impact whereas in many cases a great deal younger). All the basins/depressions described so far originally formed in such a manner. If endogenic forces become inactive, the depressions successively infill with sediments and reduce the size, in particular depth. In addition, basins can be breached and emptied if valleys cut backward into higher ground and in geological time scales, this is the most important process of the degradation and emptying of basins. Many lake-containing depressions result from the unearthing of a structural or petrographic weakness uncovered through denudation or erosive processes (wind, water, or glacial ice) and in particular is true for the geometric configurations of lake clusters overlying fracture and joint systems. In this instance, the overall pattern (fractures, joints) may be old and the transformation into open depressions young. The solubility of rock (limestone, anhydrite, gypsum) in water and weak acid solutions over thousands to millions of years leads to the water disappearing into fissures and underground caves. Many closed depressions form in these landscapes, but because of the permeability of the rock, lakes in these depressions are rare. A large group of lakes (and ponds) develop in basins blocked by depositional processes, for instance by damming a former open valley by rock fall, landslide, glacial moraines, or even a living glacier. Many lakes and swamps can be found along all the shorelines of the world such as lagoons separated from the open sea by accumulative processes of waves and storms. Large numbers of depressions also occur in permafrost regions as thermo-karst lakes or small ponds within the polygons of patterned ground.