A global understanding of the role of lakes and impoundments in the functioning of any region of the Earth requires quantification of their number and size distribution. Geology, tectonics, and climatic processing have made the Earth's surface an undulating and tilted surface with a hypsometry defined by the amount of relief and the irregularity of the underlying materials. If this undulating surface is cut through by a plane (tilted or not) that represents the groundwater and surface-water table, the water will outcrop in a manner that fills some large depressions and more small ones with water. Depending upon the net rate of replenishment of water by precipitation, the open surface waters of a region may cover much of the land surface or little of it, but the size and shape of depressions in the Earth will determine the regional morphometry or hypsometry. In this way, hypsometry and climate combine to determine the abundance and size distributions of natural lakes and depressions that can be made to hold water through the addition of dams.
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