Where the water begins… Mountains are cloud-blockers and rain-makers. They are nature’s water towers for the world. Does the river nearest you start its journey on a mountain? There’s a good chance that it does. All the major rivers in the world - from the Amazon to the Nile - have their headwaters in mountains. More than half of the world's people rely on mountain water to grow their food, to make electricity and, most important of all, to drink. In arid regions of the Andes and in parts of Africa, mountains provide 80% of precious fresh water.

Waterfall in the Bolivian Andes (photo by Alton Byers)

What is a rain shadow? When moving clouds meet a mountain, they are forced to rise. The rising air cools, condenses, and rain or snow falls. By the time the clouds have moved over the mountain, they have less moisture in them. As they sink down the other side, the air warms, and it is less likely to rain or snow. So, one side of a mountain is usually the “wet” side, and the other side is dryer. We say that the dry side is in the rain shadow.

Mountain glaciers are shrinking

When snow falls on high, cold mountaintops, it doesn’t melt. Instead, it piles deeper and deeper until it is compressed into ice, making a glacier. The ice flows ever so slowly down the mountainside. Finally, the river of ice reaches the lower, warmer parts of the mountain, and melts. Melting water from glaciers can keep mountain rivers flowing even when no rain has fallen for a long time.

But there is trouble on the mountaintops! Global warming is melting glaciers in most mountain ranges. The glaciers on the summit of Mt Kenya have shrunk by 92% in the last century. In the Alps, some ski lifts that used to take skiers to the glaciers are now sitting on dry slopes, far below the ice and snow. These photos show the shrinking of a glacier in the Cascade Mountains of the western United States between 1928 and 2000.

   
Photos courtesy of the United States Geologic Survey

Would you like to see animations of mountain weather, rivers, and glaciers?

How a rain shadow forms
(McGraw-Hill Higher Education’s “The Living World – eBridge”)

Hydrologic cycle
Quicktime movie (Bigelow Laboratory for Ocean Sciences)

What is flooding?
(Geology Labs On-line, created by Gary Novak at the California State University at Los Angeles)

Glacier fly-by animation
3-D fly-by of Tarr Inlet and the Grand Pacific Glacier using a 1986 satellite image. 1 MB Quicktime movie (Glacier Bay Movie Theater, NASA)

Find more great learning resources by searching our on-line guide. Or, click on the keywords to do an automatic search for mountain learning resources related to water resources, climate, glaciers, or global change.