Water2 Essay Research Paper WATER IT MOVES — страница 2

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than they are deep. The stream now runs in a narrow channel on the valley floor. Years of erosion have transformed boulders and cobble into sand and gravel . Old valleys are extremely wide compared to their depth. The stream flows slowly through a winding channel on the valley floor. The stream is now so slow that erosion is scarce. The stream work consists mainly of transportation and deposition. An extremely old valley can be miles wide and only a few yards deep. The walls can still be as steep as they were in the young valley stage. If you were traveling through a valley you can sometimes see young, mature, and old valleys in order, in only a few miles. A valley can only go as deep as sea level or base level which is the lowest point at which erosion can occur. Geologists have

coined the term, Local Base Level which is the lowest land in the region being considered. While valleys may be formed by erosion, there are a variety of other natural occurrences which can interfere with this process: a valley damin occurs when a landslide dams up the stream. This will create a lake until the water finds a way to escape. This can also occur by a lava flow. If seismic activity occurs, causing a portion of the valley to sink, much or all of the river?s power could be lost. A plate in the valley?s floor could be raised, shifting the stream into another valley. Uplift can also change the gradient of the stream, causing it to be stronger; this is called rejuvenation. In a wide valley, rejuvenation can cause the stream to cut a narrow gorge (or young valley) in the

old valley floor. If a coast sinks, sea water can travel into the coastal valleys. This can cause sea water to travel far inland and create bays and estuaries. When a stream is flowing rapidly in a narrow valley, it can create interesting sculptures. One type of sculpture is called an alcove. This is a cave made in the valley?s side by the stream, at a turn in the stream. When rounding the turn, water digs hard into the outside channel and makes a wide cave—the alcove. The natural bridge is another kind of sculpture. It is formed when water in a meandering or “looping” stream breaks through the bedrock wall which separates the loop. This forms a tunnel, leaving the older route dry. Years of erosion will deepen and erode most of the tunnel but a small portion may remain as a

natural bridge. WATERFALLS Cliffs that form waterfalls have various origins. If crucial blocks shift they can cause steep cliffs. According to Geology: our changing earth through the ages by Jerome Wyckoff, at Africa?s great Victoria Falls, the water plunges 420 feet into a graven–a depression formed by the sinking of a block. In Utah?s Wasatch Mountains and Wyoming?s Tetons, streams pour over cliffs formed by the rise of mountain blocks. Most waterfalls are formed by a resistant rock layer overlaying a weaker layer. As the river reaches the falls, it is flowing over a layer of resistant limestone. Under it are weak shale and sandstone. As the water falls it crashes against the cliffs, eroding the shale and sandstone much faster than the limestone. The limestone now slightly

overhanging, creates the waterfall. Sometimes an overhang breaks off, more shale and sandstone are eroded away and a new overhang is formed. Since falling water is the faster water can travel, erosion is extremely strong below waterfalls. Large sandy pools are usually formed below waterfalls which are empty of rocks except for resistant rocks from former overhangs near the foot of the falls. FLOODLANDS In extremely wide valleys, streams may overflow their banks and deposit alluvium (stream carried sediments) over most of the valley floor. This level alluvium surface is called flood plain. Rivers on flood plains will move side to side looking for a way through the up to 5 yards deep sediment plains. Individual loops are called meanders. During floods, high waters can cut across

from one loop to another, forming channels called cut-offs. This is how old meanders become abandoned as new ones form. As water flows by abandoned meanders, sediment builds up and dams it at both ends: this creates an oxbow lake. This oxbow lake may fill up in time with sediment, forming a “meander scar”. Humans consider floods to be disasters–destruction of land, property, bridges, etc. But flooding spreads silt, which, when mixed up with organic wastes, creates fertile soil which farmers are grateful for. OCEAN Another type of erosion is that caused by the ocean on the shoreline. The ocean can shape the shoreline, enlarge it, make it smaller, or smooth the cliffs above it. Irregular shoreline are a series of large coves along the shore. The bar of land between the coves