David Plyley, age 11, of Niagara Falls, Ontario Canada, for his question:

Why are lakes fresh and seas salty?

Billions of years ago, the global oceans were as fresh as our fresh water lakes and streams. Scientists estimate that several billion tons of salty chemicals are added to the seas every year—and this has been going on at a steady rate through the ages. Hence the seas grow saltier—while lakes and rivers remain more or less fresh. This remarkable situation involves a robbery that goes on and on.

We live on a watery world and this has been so since the first rains deluged down on the earth some four billion years ago. These fresh water rains filled and stayed in the great ocean basins. Since then the total amount of the world’s water has remained more or less the same. However, an annual quota is recycled back and forth between the earth and the air. Water covers about three quarters of our planet’s surface and the total amount is estimated to be more than 330 million cubic miles.

Some 97 percent of the total is salty sea water and about two percent is fresh water frozen in icy glaciers. Less than one percent is the so called fresh water found in lakes, streams and ground water, some of which is buried three miles deep. We refer to it as so called fresh water because it is not perfectly pure water. Actually it contains a tiny trace of all the 100 or so chemicals dissolved in the salty sea.

All the fresh water lakes are drained by rivers that finally empty themselves into the sea. Water is a great solvent, which means that it tends to dissolve molecules of almost everything it touches. And all the streaming surface water dissolves molecules from all the different sub¬stances in the earth’s rocky crust. It carries away all this loot from the land and dumps it into the sea.

Meantime the sun beams down and evaporates countless tons of moisture from the surface of the sea. But only the water molecules become gaseous vapor and go up into the weathery atmosphere. The salty chemicals and other loot stolen from the land are left behind, which is why the sea gets saltier day by day.

The water in a lake dissolves traces of this and that from everything it touches. True, it tastes fresh and not at all like the salty sea. But this is because sea water contains a very much higher percentage of dissolved chemicals. It is estimated that 166 million tons of looted chemicals are dissolved in every cubic mile of sea water. And almost 78 percent of this is ordinary salt, called sodium chloride.

A fresh water lake is fed by fresh water streams and springs and other streams drain its mildly salty water back to the sea. But if its drainage to the sea gets cut off, in time it becomes a salt water lake. The same old sun evaporates its surface moisture and leaves its dissolved chemicals behind. As streams drain in, more chemicals are added and in time the lake water may become even saltier than the sea. This is just what happened to the Great Salt Lake of Utah.