Mary Ann German, age 12, of Gary, Indiana, for her question:

Do any rivers flow from the sea to the land?

Water has some strict rules of its own. When left to flow freely, it always flows downhill. Except for a few dips here and there, the earth's land masses are higher than the level of the sea. All the streams and rivers flowing freely over the land wind around and around, bending this way and that to avoid flowing uphill over the slopes. Gradually their paths find lower and still lower levels until at last they reach the low shores that dip down to the sea. At the mouth of a wide river, the fresh and salt water are mixed and merged by the tides. High tides may sweep ocean water up the river. And in some cases the swift current of a mighty river sweeps its fresh water many miles out to sea.

Apart from tidal invasions, the sea cannot send rivers inland because the water will not flow uphill to higher ground. But let's suppose the Pacific Ocean cut a ditch through the western mountains to Death Valley, which is below sea level. This water would not be a river. It would be an arm of the sea called an estuary. Estuaries, bays and gulfs are not rivers because they are fed by the sea.