Further Reading

Campbell, James B. Introduction to Remote Sensing. 4th ed. New York: Guilford Press, 2007. Canty, Morton J. Image Analysis, Classification and Change Detection in Remote Sensing. New York: Taylor & Francis, 2007. Drury, Steven A. Image Interpretation in Geology. London: Chapman and Hall, 1993. Jensen, John R. Remote Sensing of the Environment: An Earth Resource Perspective. 2nd ed. Upper Saddle River, N.J.: Prentice Hall, 2007. Lillesand, T. M., R. W. Kiefer, and J. W. Chipma. Remote Sensing and Image Interpretation. 6th ed. New York: John Wiley & Sons, 2007. Sabins, Floyd F. Remote Sensing, Principles and Interpretation. New York: W. H. Freeman, 1997.

river system Stream and river valleys have served as preferred sites for human habitation for millions of years because they provide routes of easy access through rugged mountainous terrain and water for drinking, watering animals, and irrigation. Most of the world's large river valleys are located in structural or tectonic depressions such as rifts, including the Nile, Amazon, Mississippi, Hudson, Niger, Limpopo, Rhine, Indus, Ganges, Yenisei, Yangtze, Amur, and Lena. The soils in river valleys are also some of the most fertile that can be found, as they are replenished by yearly or less frequent floods. The ancient Egyptians, whose entire culture developed in the Nile River valley and revolved around the flooding cycles of the river, appreciated this characteristic of river systems. Rivers now provide easy and relatively cheap transportation on barges, and the river valleys are preferred routes for roads and railways as they are relatively flat and easier to build in than over mountains. Many streams and rivers have also become polluted as industry has dumped billions of gallons of chemical waste into our nation's waterways.

Stream and rivers are dynamic environments— their banks are prone to erosion, and the rivers periodically flood over their banks. During floods, rivers typically cover their floodplains with several or more feet of water, dropping layers of silt and mud. Ancient civilizations relied on this normal part of a river's cycle for replenishing and fertilizing their fields. Now that many floodplains are industrialized or populated by residential neighborhoods, the floods are no longer welcome and natural floods are regarded as disasters. On average, floods kill a couple of hundred people each year in the United States. Dikes and levees have been built around many rivers in attempts to keep the floodwaters out of towns. This exacerbates the flooding problem because it confines the river to a narrow channel, and the waters rise more quickly and cannot seep into the ground of the floodplain.

Streams are important geologic agents critical for other Earth systems. They carry most of the water from the land to the sea, they transport billions of tons of sediment to the beaches and oceans, and they erode and reshape the land's surface, forming deep valleys and floodplains and passing through mountains.

Streams and rivers are dynamic systems that constantly change their patterns, the amount of water (discharge) they carry, and the sediment transported by the system. Rivers can transport orders of magnitude more water and sediment during spring floods compared to low-flow times of winter or drought. Since rivers are dynamic systems, and the amount of water flowing through the channel changes, the channel responds by changing its size and shape to accommodate the extra flow. Five factors control how a river behaves: (1) width and depth of channel, measured in feet (m), (2) gradient, measured in feet per mile (m/km), (3) average velocity, measured in feet per second (m/sec), (4) discharge, measured in cubic feet per second (m3/ sec), and (5) load, measured as tons per cubic yard (metric tons/m3). These factors continually interplay to determine the behavior of the river system. As one factor, such as discharge, changes, so do the others, expressed as:

Q = w x d x v where Q represents discharge, w represents channel width, d represents channel depth, and v represents the velocity of the water in the channel.

All factors vary across stream, so they are expressed as averages. If one term changes then all or one of the others must change too. For example, with increased discharge, the river erodes and widens, and deepens its channel. The river may also respond by increasing the number of bends, known as meanders, and their curvature (measured as sinuosity), effectively creating more space for the water to flow

Kuskokwim River, just up from Bethel, Alaska, showing distributary channels, meanders, oxbow lakes, point bars, sand bars, and cut banks (Paul Andrew Lawrence/Alamy)

in and occupy by adding length to the river. The meanders can develop quickly during floods because the increased stream velocity adds more energy to the river system, and this can rapidly erode the cut banks enhancing the meanders.

The amount of sediment load available to the river is also independent of the river's discharge, so different types of river channels develop in response to different amounts of sediment load availability. If the sediment load is low, rivers tend to have simple channels, whereas braided stream and river channels develop where the sediment load is greater than the stream's capacity to carry that load. If a large amount of sediment is dumped into a river, it will respond by straightening, thus increasing the gradient and velocity and increasing its ability to remove the added sediment.

When rivers enter lakes or reservoirs along their path to the sea, the velocity of the water suddenly decreases. This causes the sediment load of the stream or river to be dropped as a delta on the lake bottom, and the river attempts to fill the entire lake with sediment in this manner. The river is effectively attempting to regain its gradient by filling the lake, then eroding the dam or ridge that created the lake in the first place. When the water of the river flows over the dam, it does so without its sediment load and therefore has greater power to erode the dam more effectively.

Rivers carry a variety of materials as they make their way to the sea. These materials range from minute dissolved particles and pollutants to giant boulders moved only during the most massive floods. The bed load consists of the coarse particles that move along or close to the bottom of the river bed. Particles move more slowly than the stream, by rolling or sliding. Saltation is the movement of a particle by short intermittent jumps caused by the current lifting the particles. Bed load typically constitutes between 5 and 50 percent of the total load carried by the river, with a greater proportion carried during high-discharge floods. The suspended load consists of the fine particles suspended in the river that make many rivers muddy. The particles of silt and clay move at the same velocity as the river. The suspended load generally accounts for 50-90 percent of the total load carried by the river. The dissolved load of a river consists of dissolved chemicals, such as bicarbonate, calcium, sulfate, chloride, sodium, magnesium, and potassium. The dissolved load tends to be high in rivers fed by groundwater. Rivers also carry pollutants, such as fertilizers and pesticides from agriculture and industrial chemicals, as dissolved load.

The range of sizes and amounts of material that a river can transport varies widely. The competence of a stream refers to the size of particles a river can transport under a given set of hydraulic conditions, measured in diameter of largest bed load. A river's capacity is the potential load it can carry, measured in the amount (volume) of sediment passing a given point in a set amount of time. The amount of material carried by rivers depends on a number of factors. Climate studies show erosion rates are greatest in climates between a true desert and grasslands. Topography affects river load, as rugged topography contributes more detritus, and some rocks are more erodable. Human activity, such as farming, deforestation, and urbanization, all strongly affect erosion rates and river transport. Deforestation and farming greatly increase erosion rates and supply more sediment to rivers, increasing their loads. Urbanization has complex effects, including decreased infiltration and decreased times between rainfall events and floods.

Was this article helpful?

0 0
How To Survive The End Of The World

How To Survive The End Of The World

Preparing for Armageddon, Natural Disasters, Nuclear Strikes, the Zombie Apocalypse, and Every Other Threat to Human Life on Earth. Most of us have thought about how we would handle various types of scenarios that could signal the end of the world. There are plenty of movies on the subject, psychological papers, and even survivalists that are part of reality TV shows. Perhaps you have had dreams about being one of the few left and what you would do in order to survive.

Get My Free Ebook


Post a comment