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Unformatted text preview: This page last updated on 28-Mar-2011 EENS 111 Physical Geology Tulane University Prof. Stephen A. Nelson Streams and Drainage Systems Streams A stream is a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel . Thus, streams may vary in width from a few centimeters to several tens of kilometers. Streams are important for several reasons: z Streams carry most of the water that goes from the land to the sea, and thus are an important part of the water cycle. z Streams carry billions of tons of sediment to lower elevations, and thus are one of the main transporting mediums in the production of sedimentary rocks. z Streams carry dissolved ions, the products of chemical weathering, into the oceans and thus make the sea salty. z Streams are a major part of the erosional process, working in conjunction with weathering and mass wasting. Much of the surface landscape is controlled by stream erosion, evident to anyone looking out of an airplane window. z Streams are a major source of water, waste disposal, and transportation for the world's human population. Most population centers are located next to streams. z When stream channels fill with water the excess flows onto the the land as a flood. Floods are a common natural disaster. The objectives for this discussion are as follows: 1. How do drainage systems develop and what do they tell us about the geology of an area? 2. How do stream systems operate? 3. How do streams erode the landscape? 4. What kinds of depositional features result from streams? 5. How do drainage systems evolve? 6. What causes flooding and how can we reduce the damage from floods? Streams and Drainage Systems 3/28/2011 Page 1 of 18 Drainage Systems Development of Streams - Steamflow begins when water is added to the surface from rainfall, melting snow,and groundwater. Drainage systems develop in such a way as to efficiently move water off the land. Streamflow begins as moving sheetwash which is a thin surface layer of water. The water moves down the steepest slope and starts to erode the surface by creating small rill channels. As the rills coalesce, deepen, and downcut into channels larger channels form. Rapid erosion lengthens the channel upslope in a process called headward erosion . Over time, nearby channels merge with smaller tributaries joining a larger trunk stream. (See figure 17.3 in your text). The linked channels become what is known as a drainage network . With continued erosion of the channels, drainage networks change over time. Drainage Patterns- Drainages tend to develop along zones where rock type and structure are most easily eroded. Thus various types of drainage patterns develop in a region and these drainage patterns reflect the structure of the rock....
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