A DRY REGION FOR MOST OF THE YEAR. The main river of a system not only cuts its own channel in the mountain side or in the plateau, but each tributary stream does exactly the same thing. Study all of the pictures. Trace the tributary basins. Describe each. All streams have a work to do in cutting down the highlands, wearing out the val leys, and in carrying and depositing mate The deposit is a delta. Find the deltas in the rial to build up the plain or to build out the pictures. seashore. Some rivers flow in the valleys between the mountain folds. Other riv banks and spread out like great lakes on the level land, submerging ers must be older than the moun tains themselves. They flow ed across the everything in their way. This is a flood. fold, and as the mountain slowly arose The land covered at flood-time is a FLOOD-PLAIN. The water which covers the flood-plain is filled with fine soil or with sand. When the flood subsides and the stream returns to its banks, the flood-plain is covered with mud. For a long time the water stands upon the surface in ponds. The floods cause most flood-plains to have a very rich, moist soil. During the flood the valley sides are widened and cut back nearer to the source of the stream. THE HUDSON RIVER, NEW YORK. above the plain, the river cut it down, thus gapping the mountain ridge. RIVERS AND RIVER BASINS. The Delaware gaps the Appalachian Mountains. 85 cause of the rapidly sloping land. In The Potomac gaps the mountains at the place the flood-plains, which are nearly level where you see the rapids in the picture. Can you tell from the picture on page 84 in which direction the Potomac is flowing? The moun tains must have been a long, long time in forming, to have given the river time to cut through. There are rapids usually in a river where it cuts across the highlands. They are formed by the water flowing over the rocky ridge which has not yet been cut to the bed level of the river. If you build up a highland and make a plain on the sand in many places, it loses the power given table or in the schoolyard, and sprinkle it with water, you will see that the water will cut its way directly down the highland by the shortest way to the lower land. On the plain it will work A MINING TOWN BUILT ON THE FLOOD PLAIN OF A RIVER IN THE ROCKY MOUNTAINS. its way down the slight slope, making many curves. In the same way a river cuts a more direct course among the highlands be If you observe two bicycles go around a curve you will see that the one on the outer curve turns around more times in going around than the one on the inner curve. Can you think why? If you throw some shavings into a stream you will see that the water flows much faster on the outer curve than on the inner. This causes the water which is moving so swiftly to cut away the bank on the outer curve, while it helps the slowly moving water on the inner curve to deposit the soil which it carries. See the picture of a mud plain on page 83. Rivers often cut deep gorges in the rocky surface where there are few feeders to widen the valley. Describe the gorge in the picture on page 83. In countries where the weather is usually dry the river valleys are not widened as rapidly as in countries which have a great rainfall. The rivers in these dry sections cut deep gorges called canyons. There are many canyons in the highlands of western United States. Observe those in the pictures. Rivers and smaller streams usually have wooded banks. The richest farming lands in the world lie in the flood-plains of the river valleys. Fine pas and rapids. This enables boats to have a continuous line of transportation and lessens the expense of shipping. How? Rivers help The waterfalls in our rivers have long been utilized by man for turning millwheels. More and more, how ever, steam is taking the place of water power alone. The tremendous force of Niagara is used in generating electricity to light and serve as motor power in cities a the higher land of the valley slopes. Great cities or trade-centers are upon the rivers. These are connected by railways and boats, which serve as outlets for the products of the great farming districts. Canals are usually made around falls long distance from the Falls. Niagara Falls are one of the wonders of the world. The American Fall on the left of the picture is about 165 feet, and the Canadian Fall is about 150 feet high. Carefully study and describe each of the pictures on this page. Observe how the water is wearing down the rocky ledge in the Yosemite Falls. ICE RIVERS OR GLACIERS. Many of the higher mountains of the world have rivers of ice flowing for some distance down their sides. You wonder how they are formed? If you squeeze a snowball very hard and work it for some time with your hands, it becomes almost a ball of ice. This is caused by the pressure and warmth of your hands. In mountainous localities where there is a great fall of snow, the snow does not all disappear through the summer months. More snow falls each winter for several years than can melt during the summer, so that the depth is constantly increasing. The great weight of this accumulation, together with the heat from the sun's rays, causes the snow to change to ice. This ice forms into streams which move slowly down the mountain sides. Ice streams tear up trees, plants, and soil, and break off large rock masses in their descent. They carry these in the ice to lower, warmer sections, where the ice melts, leaving the rock fragments and other material strewn over the ground. Large rivers full of fine soil come from these ice streams. The soil is carried to the floodplain or to the sea. Many gravel and sand hills are formed by the material brought down by these rivers. (P. 88.) Rains wash the sand from these sand hills to the lower land, covering it with a sandy, barren soil. Glaciers, or ice rivers, are in Alaska, Greenland, western Canada and United States, and on the mountains of Europe, South America, and Asia. the After There was a time in the Earth's history when the cold belts extended much farther towards the Equator than at present time. many, many years there was a great accumulation of snow. Through pressure and heat this snow changed to ice, and moved slowly down the slopes in streams, or spread out on the plains in a broad, continuous sheet of ice. This ice sheet covered all of the northern parts of North America and Europe, 87 |