Freshwater (Lakes and Rivers) and the Water Cycle Completed

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One part of the water cycle that is obviously essential to all life on Earth is the freshwater existing on the land surface. Just ask your neighbor, a tomato plant, a trout, or mosquito. Surface water includes the lakes, reservoirs (human-made lakes), ponds, streams (of all sizes, from large rivers to small creeks), canals (human-made lakes and streams), and freshwater wetlands. The definition of freshwater is water containing less than 1,000 milligrams per liter of dissolved solids, most often salt.

As a part of the water cycle, Earth's surface-water bodies are generally thought of as renewable resources, although they are very dependent on other parts of the water cycle. The amount of water in rivers and lakes is always changing due to inflows and outflows. Inflows to these water bodies will be from precipitation, overland runoff, groundwater seepage, and tributary inflows. Outflows from lakes and rivers include evaporation, movement of water into groundwater, and withdrawals by people. 

So, the amount and location of surface water changes over time and space, whether naturally or with human help. During the last ice age when glaciers and snowpacks covered much more land surface than today, life on Earth had to adapt to different hydrologic conditions than those which took place both before and after. And the layout of the landscape was different before and after the last ice age, which influenced the topographical layout of many surface-water bodies today. Glaciers are what made the Great Lakes not only "great," but also such a huge storehouse of freshwater.

Surface water keeps life going

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Water on the land surface really does sustain life, and this is as true today as it was millions of years ago. Dinosaurs likely gathered at the local watering hole 100 million years ago, just as antelopes in Africa do today. And, since groundwater is supplied by the downward percolation of surface water, even aquifers are happy for water on the Earth's surface. You might think that fish living in the saline oceans aren't affected by freshwater, but, without freshwater to replenish the oceans they would eventually evaporate and become too saline for even the fish to survive.

As we said, everybody and every living thing congregates and lives where they can gain access to water, especially freshwater. Just ask the billions of people living on Earth! Here's a satellite picture of the world at night. The most obvious thing you can see is that people live near the coasts, which, of course, is where water, albeit saline, is located. But the interesting thing in this picture are the lights following the Nile River and Nile Delta in Egypt (the area within the purple-outlined box). In this dry part of the world, surface-water supplies are essential for human communities. And if you check the price of lakefront property in your part of the world, it probably sells for much more than other land.

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Usable fresh surface water is relatively scarce

To many people, streams and lakes are the most visible part of the water cycle. Not only do they supply the human population, animals, and plants with the freshwater they need to survive, but they are great places for people to have fun. You might be surprised at how little of Earth's water supply is stored as freshwater on the land surface, as shown in the diagram and table below. Freshwater represents only about three percent of all water on Earth and freshwater lakes and swamps account for a mere 0.29 percent of the Earth's freshwater. Twenty percent of all fresh surface water is in one lake, Lake Baikal in Asia. Another twenty percent (about 5,500 cubic miles (about 23,000 cubic kilometers)) is stored in the Great Lakes. Rivers hold only about 0.006 percent of total freshwater reserves. You can see that life on Earth survives on what is essentially only a "drop in the bucket" of Earth's total water supply! People have built systems, such as large reservoirs and small water towers to store water for when they need it. These systems allow people to live in places where nature doesn't always supply enough water or where water is not available at the time of year it is needed.

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The World's Water - Distribution of Earth's Water

The Earth is a watery place. But just how much water exists on, in, and above our planet? About 71 percent of the Earth's surface is water-covered, and the oceans hold about 96.5 percent of all Earth's water. Water also exists in the air as water vapor, in rivers and lakes, in icecaps and glaciers, in the ground as soil moisture and in aquifers, and even in you.

This bar chart shows how almost all of Earth's water is saline and is found in the oceans. Of the small amount that is freshwater, only a relatively small portion is available to sustain human, plant, and animal life.

• In the first bar, notice how only 2.5 percent of Earth's water is freshwater.

• The middle bar shows the breakdown of freshwater. Almost all of it is locked up in ice and in the ground. Only a little more than 1.2 percent of all freshwater is surface water.

• The right bar shows the breakdown of surface freshwater. Most of this water is locked up in ice, and another 20.9 percent is found in lakes. Rivers make up 0.49 percent of surface freshwater. Although rivers account for only a small amount of freshwater, this is where humans get a large portion of their water from.

Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp. 817-823.

Do you really like lakes? Or maybe you'd rather sit by the ocean, or maybe a waterfall is your favorite? 
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Sources and more information:

• NASA Earth Observatory - The Water Cycle