NATURAL LAKE OCCUR WHERE a large quantity of water collects in a hollow in impermeable rock, or is prevented from draining away by a barrier, such as moraine (glacial deposits) or solidified lava. Lakes are often relatively short-lived landscape features, as they tend to become silted up by sediment from the streams and rivers that feed them. Some of the more long-lasting lakes are found in deep rift valleys formed by vertical movements of the Earth’s crust—for example, Lake Baikal in Russia, the world’s largest freshwater lake, and the Dead Sea in the Middle East, one of the world’s saltiest lakes. Where water is able to drain away, it sinks into the ground until it reaches a layer of
impermeable rock, then accumulates in the permeable rock above it; this watersaturated permeable rock is called an aquifer. The saturated zone varies in depth according to seasonal and climatic changes. In wet conditions, the water stored underground builds up, while in dry periods it becomes depleted. Where the upper edge of the saturated zone—the water table—meets the ground surface, water emerges as springs. In an artesian basin, where the aquifer is below an aquiclude (layer of impermeable rock), the water table throughout the basin is determined by its height at the rim. In the center of such a basin, the water table is above ground level. The water in the basin is thus trapped below the water table and can rise under its own pressure along faultlines or well shafts.
