Lesson 10 - The River and the Sea
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions
- Take the Week 10 Quiz
By the end of this lesson you should be able to:
- Discuss the importance of fluvial processes in shaping the lithosphere
- Explain the three major forms of stream transport
- Describe the interaction of humans with flood plains
- Compare the effect of wave-action weathering on active and passive margins
- Explain why the ocean is salty
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Fluvial Processes
Recall from Lecture 5 that water in rivers and streams on earth makes up only 0.003% of all fresh water on earth. Yet, this very small fraction of the hydrosphere has a huge influence on the shape of the lithosphere. Water is a primary instrument in weathering processes (Lecture 9) with its unique ability to expand in its solid form, and in its liquid form dissolve nearly every rock-forming mineral. The process by which water wears away at the lithosphere is called erosion. The mechanism by which the eroded material is moved by the stream is called transport. When the material is deposited by the water in a new location, it is called deposition.
Stream related processes are called fluvial processes. The term river is applied to large flows of runoff, while stream applies to smaller flows. When water initially falls on the ground through precipitation, if it is not absorbed by the ground it runs off through sheet flow or overland flow. The water follows the pull of gravity and runs to lower areas, eventually collecting in the stream basins. A drainage basin or watershed is the terrain surrounding a stream valley that contributes to the water and sediments carried by the stream. Drainage basins are determined by topography and gravity. Water always runs down with gravity, so watersheds are always bowl or trough shaped. Watersheds can be divided from large to smaller and smaller units. The largest watershed unit in America is the Continental Divide which runs through the Rocky Mountains. Water that falls on the west of the continental divide eventually drains into the Pacific Ocean, water that falls east of the continental divide eventually drains into the Atlantic Ocean.
You can find the watershed that you live in through the EPA's Surf Your Watershed site.
Watersheds are open systems with inputs of precipitation from the atmosphere and material weathered from the lithosphere. Energy and materials are redistributed as the water and lithosphere material are carried to new locations. The topography of a region determines the drainage pattern of the area. Four basic types of drainage patterns are illustrated in this Thinkquest web site. Look also at Figure 14.8 in the text, then visit the Cerritos College drainage network quiz and test yourself!
Stream Transport
The volume of water that passes through a stream in a given period of time is called the discharge or flow rate. The discharge (Q) of a stream can be calculated by multiplying the width of the channel (w) by the depth of the channel (d) by the stream velocity or speed (v). This formula, Q = wdv , is usually expressed in cubic meters per second, or cubic feet per second (cfs).
The discharge of a river usually increases downstream as more and more tributaries, or small streams, join the major river. The Mississippi River is an example of this phenomenon. In equatorial regions of the world, streams sometimes flow from humid regions to arid regions, causing the discharge to decrease downstream because of high potential evapotranspiration rates. The Nile river is an example of this phenomenon, called an exotic stream.
Streams are an important mechanism in the transport of sediment. If you have ever visited the Mississippi river, the Green River in Utah, the Colorado River, or the American River near Sacramento, you may have noticed that you cannot easily see to the bottom of the river because the water is very cloudy or muddy. These rivers all have a large dissolved load and suspended load, typical of large, placid rivers.
Dissolved load is the material carried in water as dissolved minerals such as calcium from limestone bedrock.
Suspended load is the sediment held up by running water, like silt and clay particles. These particles are so small that they stay suspended in the water until the water is nearly stationary.
Bed load is the larger pieces of sediment which are moved by the river, but cannot be suspended in the running water, such as sand, gravel and boulders. These materials roll and bounce along the bed of the river. The higher the discharge in a river is, the larger the bed load can be. Flood events, where the discharge rate is very high, results in large transport of bedload materials.
When the bed load exceeds the streams capacity (its ability to carry materials as bedload), the stream channel becomes tangled with material which used to be bedload. This results in a braided stream.
Flood Management
A flood is a high water level that overflows the natural (or artificial) banks or levees along any portion of a stream. A flood plain is the region adjacent to a river that is frequently flooded. Floods are a natural part of a riverine environment, and occasional floods enrich soils. For example, the Nile river delta in Egypt experiences flooding nearly every year. This flooding keeps the land around the Nile fertile.
The fertility of frequently flooded land is what draws people to cluster near rivers. Worldwide, many people live within the 50 or 100 year flood plain of rivers. Flood plains are rated statistically based on how often they flood. For example, a 50 year flood plain floods, on average, every 50 years. People who live near rivers often build levees or artificially high banks around the river to contain it and try to avoid periodic flooding events. However, while levees keep one area from flooding, they heighten the flood risk for areas further down stream by constraining the water and 'pushing' more of it downstream.
You can see how prone to flooding your area is using the FEMA/ESRI Hazard GIS.
Where River meets Sea: The Coastal Environment
Nearly all rivers eventually run into the sea. The region where a river runs into the ocean is called a river delta. The river delta is prone to frequent flood events, and is also strongly affected by the tides. To learn more about what causes the tides, check out the Lunar Tides site at the University of Tennessee.
The coastal environment is known as the littoral zone. The littoral zone extends from the highest water line that occurs on shore during a storm, to the point in the ocean where the land is too deep for storm waves to move sediments on the floor (60 m or 200 feet). The point of contact between the ocean and the land is known as the shoreline or the coastline. Mean Sea Level (MSL) is a value based on the average tidal levels recorded hourly at a given site over many years.
The ocean is a powerful force in the shaping of the lithosphere. Ocean waves are continually weathering, or wearing away at the land and changing the shape of the coastline of our continents. Coastlines in areas of active tectonic uplift, such as along the western coast of North and South America, tend to be rugged and high relief. Sea cliffs are formed by the undercutting action of the sea. As the waves wear away, a greater amount of the land along the coast, gravity will often take over and cause a portion of the cliff to fall away into the sea.
While in areas of active tectonic uplift, the ocean is primarily an erosional force, in passive margins, such as along the eastern coast of North and South America, wave are a constructive force. Waves create small islands of sand parallel to the coast line, called barrier islands or barrier spits. If a barrier island grows so large that it connects with the land, it can form a lagoon.
The currents of ocean waves transport sediments (primarily sand) along the coastline in a process known as longshore drift. Beaches are not just made up of white sand. From the standpoint of physical geography, beaches are the portion of the coastline where accumulated sediment is in motion, so to geographers, beaches can be made up of gravel, rocks or boulders. Beaches act to stabilize a shoreline by absorbing wave energy.
The Oceans
We have already discussed the oceans in terms of the hydrologic cycle. Recall that oceans store 97% of water on earth. A common question when we are discussing the oceans in class is: how did the ocean get to be salty? The answer is that the salts in the ocean have accumulated over millions of years. They came from the minerals in the lithosphere. As water percolates through rocks, it dissolves small amounts of minerals. Water dissolves over half of the elements found in nature. The minerals then are carried with the water to the ocean. Over time, water is evaporated from the oceans (recall the hydrologic cycle), but the minerals stay behind. A smaller scale example of this is inland lakes which have no river outlets, such as the Great Salt Lake in Utah, and Mono Lake in the Eastern Sierras. Because these lakes have no outlet, they are like mini oceans, with the dissolved minerals in their waters becoming more and more concentrated each year.
As a result of the dissolving properties of water, most natural elements and compounds can be found in a dissolved form in the ocean. The dissolved compounds are called solutes and the concentration of the dissolved solutes is called the salinity of the ocean. Seven elements account for over 99% of the dissolved solids in seawater:
- Chloride (Cl-)
- Sodium (Na+)
- Magnesium (Mg+2)
- Sulfur (as a sulfate (SO4-2)
- Calcium (Ca2+)
- Potassium (K+)
- Bromide (Br-)
These elements are all considered 'salts'. The salinity of ocean water is 35 g/kg or 35,000 parts per million (ppm). Water that is more salty than 35,000 ppm is called brine, and water that is less salty than 35,000 PPM is called brackish.
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