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Billy Rowley, age 9, of Des Moines, Iowa, for his question:

Why doesn't lake water sink into the ground?

Lake water sometimes does sink into the ground. It percolates down through the soil and rocks below. It seeps down and down, until it reaches the level of the buried ground water. This tends to happen in dry climates. Here, there is not enough rainfall to keep the ground water high.

Every lake is a basin, a hollow in the earth's crust. It is fed by streams, or springs welling up from below. And there is always at least one stream or creek draining the water away from the lake. In some respects a lake resembles a bath tub. There is a tap to run the water into the basin and another to drain away the surplus. You may have to hunt all around the lake to find the stream which drains it. This stream may be small, but it is there.

The ground water is a buried reservoir of rainfall. Rain which does not run away seeps down into the earth and remains there, saturating the rocky layers below. In damp, rainy climates there is a lot of water stored in the earth. The ground water level, or water table, is high. In dry climates there is less water, or maybe none, stored below. The water table is low or non‑existent.

A lake may be deep or shallow. The Great Lakes are so deep that their floors bite into the ground water below. The water table is always higher than the floors of these lakes. It is not possible to empty them. For, as fast as the water flows out, new water seeps in from the saturated water table.

Other lakes may be shallow, their floors high above the water table. Hero water is constantly sinking through the soil. If the climate is dry, more water evaporates from the surface. Such a lake must be constantly filled with new water from streams, creeks and rivers. Otherwise it would soon dry up.

Lakes are perhaps the most changeable items of geography. Even the Great Lakes may be ___ in a million years. This is because a lake floor tends to fill with sediment. Piles of mud and silt are brought in by streams and dumped there. New lakes are forever being born, but all the existing lakes are busy filling up with layers of mud and silt.

Lake Mead, behind Hoover Dam, is a man‑made lake. But it too behaves like a natural lake. It is filled by the Colorado River. Its outgo, however, is controlled by the dam and its power is put to use. The river which foods it totes tons of silt and deposits it on the floor of this man‑made lake.

A careful check is kept on the increasing silt in Lake Mead. The water is gradually being replaced by layers of mud on the floor of this lake. It is estimated, on the basis of the first 12 years, that it will take about 400 years before the whole of Lake Mead is filled up with sediment. By the year 2400 A.D. the useful life of our great Hoover Dam will be over.

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