55 Waterfalls-Mid

Waterfalls FOCUS Question How are landforms and waterfalls interrelated?

Wonderful Water It seems as if the dry desert canyon will never end. The sun beats down on your skin as you trudge up the steep slopes. Soon the walls of the canyon grow tall and narrow, casting cool shade. You hear the most wonderful sound— rushing, flowing, gurgling water. Despite the heat you hike on, and around a corner you find a beautiful surprise. A shimmering waterfall roars and splashes into a clear blue pool. Waterfalls are some of the most beautiful and inspirational natural features. You can find them in mountains and rainforests, and near beaches. Even in deserts, wherever a river meets a drop-off, a waterfall is born. How do waterfalls form, and why do they occur in some places but not others?

What Makes a Waterfall? A waterfall needs two parts—a stream or river and a steep section of land. The water flows over a slope, ledge, or cliff. Some waterfalls, called falls, drop vertically through the air. Others, called cascades tumble over several drops or steps. Water always flows downhill. It takes just the right combination of flowing water and dramatic landforms to make a spectacular waterfall. Word Wise A rapid is a section of a river where the water flows swiftly and turbulently over a rocky surface. but it is not considered a waterfall.

Vertical Drop-Offs True falls require land that drops away suddenly, such as a steep slope or cliff. These landforms can form in many ways. Uneven Erosion Different types of rock wear away at different rates In some places very hard rock lies next to or on top of softer rock. Wind and water wear away the soft rock more quickly. The hard rock remains forming a steep cliff called a scarp. Mountains Where the large plates of Earth's crust meet, the edges can collide and push upward to form mountains with steep slopes and canyons. Faults Faults are enormous cracks in Earth's surface. The rock on one side of a fault can push up or slide down as plates move, forming a steep slope or cliff.

Volcanic Flows When a volcano erupts, thick, slow-flowing lava may move across the land. These lava flows can form tall piles. After the lava cools the rock often cracks apart and falls away in big chunks, leaving behind slopes and cliffs. Glacial Valleys Glaciers are enormous rivers of ice that creep downhill. They bulldoze huge U-shaped valleys with high steep sides. Sea Cliffs Ocean waves wear away soft rock, forming tall sea cliffs. Travertine Cliffs Travertine is a kind of rock that forms from minerals dissolved in water. When water carrying certain minerals flows across a surface some of the minerals get left behind. Over time the minerals build up, forming a sloped shelf of travertine rock.

Just Add Water Once a steep or sloping landform is present flowing water goes to work. A stream flows down a slope. Or it might flow over a drop, such as a fault or a boulder. The flowing water begins to wear away the underlying rock. The softer layers of rock wear away faster than the harder ones. If the top layer is the hardest, a ledge forms. The water flows faster as it drops over the ledge. This faster- moving water erodes the softer rock at the bottom of the drop even more. As the slope gets steeper, the water flows even faster. The water erodes the drop even more. As this process continues, a large waterfall eventually forms. Over time, the waterfall shifts upstream due to erosion.