HOW COME? Rivers may flow, but oceans will never overflow

Since rivers flow down into the ocean, why don't the oceans ever overflow? asks a reader.

From space, rivers look like running faucets, flowing into the sea.

The 4,000-mile-long Amazon River pours continuously into the Atlantic; the 1,200-mile Columbia River empties day and night into the Pacific.

Meanwhile, the 1,700-mile Zambezi River gushes nonstop into the Indian Ocean. Yet, unlike a bathtub with its faucets left on, overflowing onto the bathroom floor, the oceans never seem to fill.

Water from rain and melting snow flows down the towering mountains and gentle hills of Earth in streams; streams collect together into rivers. Rivers widen as they are joined by streams along the way. Smaller rivers feed big, deep rivers, like the mighty Mississippi. And all of that water flows down, down, down, toward the sea.

The biggest rivers dump stadiums full of water into the oceans. Runoff is greatest in places with the most yearly precipitation, like the torrentially rainy tropics. Some tropical rivers can pour out 700,000 cubic feet in a second. The Amazon alone drains one-fifth of the Earth's total runoff into the Atlantic Ocean.

So why don't seas simply overflow? Think of an ocean as a fountain in a public square. In the fountain, water continuously sprays into a wide basin. But the basin doesn't overflow, because water is pulled up to spurt out of the fountain's top again. The water in the fountain continuously recycles.

A similar kind of recycling happens in the oceans. Water rains down on the land and flows into the sea. But on the surface of the oceans, water continuously escapes into the sky. Molecule by molecule, water breaks free from the oceans, saturating the air above and forming clouds. As a cloud's water droplets or ice crystals grow larger, gravity tugs them to Earth in the form of rain and snow. And water makes its way back to the oceans, in pouring rivers.

But the water level in ocean basins isn't unchanging. Earth's periodic ice ages have a big effect on sea level. When glaciers locked up much of the land, the average sea level was much lower than it is today.

Water continued to evaporate from the oceans, forming clouds. But rain and snow falling on the continents quickly froze onto the massive sheets of ice, rather than flowing back into the sea.

However, as centuries passed and the ice slowly melted, sea level rose again, increasing at least 400 feet since the depths of the last Ice Age (about 20,000 years ago). Scientists say that global warming is now making sea level rise even faster.