Water does move the world, but not always in a noticeable or dramatic way. Erosion is a key element in making the world possible to live in. Erosion forms the valleys, literally dissolves solid rock, moves 3 ton boulders, and builds bridges. Water is the most abundant and therefore most powerful resource on the planet. Although water creates landmarks the size of the Grand Canyon, man cannot see the process, only the result. RAIN EROSION According to Rain Erosion: graphics.lcs.mit.edu, rain erosion is a result of kinetic energy developed by water as it flows over the surface of the material. Material particles are detached from the mass and transported to another location. Although one raindrop has little effect on the earth, the accumulated effect of rainfall over long periods of time can accomplish large amounts of erosion. Erosion begins when rainfall hits materials traveling at a high velocity and forces the materials to crumble into small particles. This continues until the rain creates small grooves. Continuing rain causes these grooves to overflow with a combination of water and particles.
This is called run-off. Continuous run-off breaks up the surface into smaller and smaller particles. Accumulating run-off on the surfaces moves down the slopes. This causes sheet erosion. This down slope run-off detaches these particles and moves them with the water. These moving particles strike against other particles on the surface which sets them into motion: this process is called abrasion. The velocity and turbulence of the run-off affect the degree of the sheet erosion; also some materials are less abrasive than others which also affects the degree of sheet erosion. Matter can be transported in three different ways: heavier particles will roll along the surface–these are called rolling matter. Smaller particles will bounce along at a faster rate than rolling matter—-these are called bouncing matter. The smallest particles are completely suspended by the water and travel the fastest—–these are called suspended matter. The particles comes to rest when resistance forces are greater than the velocity of the run-off. RIVER EROSION There are three types of river erosion: chemical erosion; hydraulic erosion, and abrasion.
Chemical erosion occurs when chemicals in the water react with minerals in the surface materials and cause them to dissolve and break apart; the dissolution can cause chemical compounds to form such as salts. The salts are carried in the form of ions. Rock gypsum, for example, contains the compound Calcium Sulfate. When this is dissolved in water it creates calcium ions and sulfate ions without several other salt compounds. It does not effect the taste of the water. Hydraulic erosion is based completely on the force of the moving water. A rapidly moving stream can widen cracks and break off large chunks of fractured rock. According to Lakes and Streams by Laurence Pringle, . . .in 1923 a stream at flood stage in the Wassatech Mountains of the northwestern United States wrenched loose boulders weighing up to 90 tons and carried them over a mile downstream. Abrasion is the third and most powerful form of river erosion.
A young valley is usually a gully or ravine on a mountainside. It will usually zigzag and have steep sides. The bottom is usually rock with potholes and boulders. The brooks have many waterfalls, rapids and pools. Downstream from a young valley there is usually an older, or mature valley. Mature valleys streams are longer, straighter and wider, due to years of erosion. They have a smooth bottom liminating rapids and pools. Down sloping in the valley has reduced the steepness of the slopes, therefore slowing down the flow of the stream and slowing also the process of erosion—so the bigger a valley gets, the longer it takes to get any bigger. Mature valleys are much wider 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 rivers power could be lost. A plate in the valleys 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 valleys 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 Africas great Victoria Falls, the water plunges 420 feet into a graven–a depression formed by the sinking of a block. In Utahs Wasatch Mountains and Wyomings 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 is called a headland. The irregular coastline is caused by waves beating against the shore. When waves reach shallow water, the waves begin to curve, and as they reach the shore they push the sand into the shape of the wave. The back flowing water drags the sand which was not previously moved back out to sea.
Arches are formed when waves hit headlands head on, due to the curving shape of the wave. The wave wraps around the headland and hits hardest against the sides. This action causes the waves to break away a large hole through the rock, creating the arch. If the roof of the arch breaks off, waves will cause the remainder to look like a tall cylinder sticking out of the water–this is called a sea stick. Retreating cliffs are cliffs along a shoreline where the waves continuously erode the rocks. This occurs in areas too deep for beaches. The rocks, sands, and gravel are pulled off the rocks directly out to sea. Beaches are formed when the rock, sand and gravel taken from the cliffs is pushed up onto the shallower surfaces.
Under beaches are large terraces of loose rock which waves have pushed up against the shore above. This layer is the gravel. When the gravel is pushed up by waves and is also pulled back , this forms bars which keep the sand and gravel from slipping off the terrace. The final layer is the sand, the thinnest layer of all. CONCLUSION 70 % of the earth is water. There are more than 330 cubic miles of ocean water on our planet . Therefore it is no surprise that water, being the most abundant substance on the planet has had such a strong role in creating or forming its shape. Although erosion by water may not be clearly visible, the long term effects are some of the biggest landmarks on earth.