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Go outside and look for water. Seventy-five percent of the crust of the Earth is covered with water, so there must be some nearby. Water can be tremendously forceful. Water in motion can cause great changes in the appearance of the crustal plates. Over millions of years, the forceful rush of water can cut deep canyons into dry desert rock. The Grand Canyon, in Arizona, for example, was carved by water.

Falling water, also, can shape the landscape. The pounding force of a waterfall erodes, or wears down, the rock beneath the water. The broken pieces of the rock are carried by the water and deposited in new places where, over time, heat and pressure meld the pieces together to form new rock. Meanwhile, the waterfall is actually retreating because the rock beneath it is crumbling. For example, scientists think that Niagara Falls has worn away about eleven horizontal kilometers (seven miles) of rock during its lifetime of about twelve thousand years. That means that the location of the waterfall today is eleven kilometers upriver from its original location.

If moving water is forceful enough to carve the Grand Canyon, imagine what it can do to wooden buildings. In the United States, flooding is the most destructive natural event because nearly fifty-percent of the nation's cities and towns are built along waterways. In 1993, for example, the Mississippi, Missouri, and Illinois Rivers all rushed over their banks, spilling into the cities and towns along their boundaries. The floods caused billions of dollars in damage to homes, businesses, water treatment facilities, bridges, roadways, and farmland. Thousands of people were left homeless. What caused the flood? Unusually heavy rain had fallen steadily throughout the spring of 1993, and, by June, the rivers had swollen to their highest levels.

Across the United States, thousands of homes are built on floodplains—the land surrounding rivers and other waterways where flood waters naturally pool. So, flooding is inevitable. Browse these websites to find out where floods may happen next.

Floods and Flash Floods
Flood - NOVA Online
Fargo Flood Homepage
Mississippi Flood of 1993
The Hows and Whys of Floods

The force of water rushing over riverbanks is minor, however, in comparison to the force of a tsunami—a Japanese word that means "harbor wave." Imagine an ocean wave, as high as a ten-story building, moving across the ocean's surface at jet speed, and crashing into the coastline. The tremendous force of the tsunami can instantly recreate the landscape. The crash of the water pulverizes the rock and sand of the beach. Houses and businesses along the coast are instantly destroyed. In 1964, a tsunami slammed into the coastal town of Seward, Alaska, and neighboring communities. With almost no warning, the huge wave came streaking into view, sending terrified residents running for higher ground. When the water calmed, 106 Alaskans had lost their lives, and $84 million dollars of damage was left behind.

A tsunami is born when the crustal plates beneath the ocean release stored energy and cause an underwater earthquake. Sometimes, underwater earthquakes are hardly noticed, but, when a strong quake shakes the ocean floor, the water shakes with it, setting off a ripple effect of waves that head toward land. As the waves reach the shallow waters along the coast, they collect and form the giant wave of the tsunami.

Today, the National Weather Service monitors tsunami conditions in the Pacific Ocean. Residents of countries with Pacific Ocean coastlines depend on early warning systems for their survival. However, since a tsunami can streak across the water at the same speed that a jet plane streaks across the sky, the danger is still very real.

Browse these websites to learn about tsunami of the past as well as to monitor current conditions and forecasts.

1998 Papua New Guinea Tsunami
Recent Tsunami Events
A Survey of Great Tsunamis
The Physics of Tsunamis
Tsunami Warning System
Tsunami Hazard Mitigation
Tsunami Research
Tsunami Movies and Simulations
Tsunami Animations
Tsunami: The Big Wave
Pacific Tsunami Museum
Double Whammy

If liquid water in motion can carve canyons and destroy cities, how about solid water? The frozen force of water can be just as powerful. In the coldest parts of the world, snow piles up and forms huge blocks of ice called glaciers. As more snow and ice attaches to the glacier, it grows bigger and heavier. The bigger it gets, the more forceful its movement becomes. Its weight, plus the force of gravity on Earth, causes it to shift and move. Anything in the glacier's path is sure to notice. Trees, rocks, and even mountains can be pushed out of the way.

Not all snow forms glaciers, however. In some parts of the world, snowdrifts freeze solid without forming glacial ice. Most often, the deepest snowdrifts are in mountain areas, at high elevation. Of course, the mountain is also a steep slope. As a huge drift of snow clings to the side of a mountain, it resists gravity and the force of its own weight. Eventually, the huge snowdrifts weaken their grip and tumble down the mountain in a thunderous wave called an avalanche. The force of the frozen snow's movement is strong enough to destroy anything that stands in its way.

People who live in mountain areas learn how to notice the warning signs of an avalanche. Browse these selected websites to learn more about avalanches.

Teacher Activity: Slip Sliding Away
Avalanche Glossary
Elements of an Avalanche
Avalanche Resources and Links
National Snow and Ice Data Center
Snow on the Web