PHYSICAL WORLD ESCI 243  -  NICHOLS COLLEGE ONLINE COURSE

Professor Mauri S. Pelto, 508-213-2168 peltoms@nichols.edu   

Rivers-Flooding

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Rivers are both an invaluable resource and a flood hazard.  In our nation we have managed our rivers for hydropower, flood control, irrigation, industrial and municipal water supply.  The result they are often overworked.  The Clean Water Act promises "To maintain and improve the chemical, biologic and physical integrity of our nation's waters".  This lofty goal is seldom met.  the Colorado River usually does not reach the ocean drying up short of its goal, tapped out.  The Columbia River's millions of salmon have been depleted to less than 10% of their former numbers, the stream is free flowing for only miles of its length in the US.  The Mississippi River carries so much sediment and nutrients that it has created a substantial low oxygen "death zone" in the Gulf of Mexico.  Minimum streamflows are dry beds in the case of many streams from the Ipwich River in MA to the Rio Grande in TX.

Note Mt. St.Helens,  The quakes are not deep so it does not have access to the magma chamber for a substantial eruption at this time, will this simmering increase in depth though?

Readings: 129-146

Concept Questions:

1.  What factors lead to increasing flood hazard?

2.  In what situations are dams and levees successful in reducing flood hazards?

3.  How does urbanization impact flood hazards?

Essay:  What is the cause, consequences and solutions to the Ipswich River low flow  problem? 

Rivers fall into three categories: 

Meandering streams occupy a valley with a broader flat bottom.  The stream channel migrates back and forth across the valley bottom.  Their are flat low lying floodplains adjacent to the river all along a meandering stream.  Meandering streams are prone to flooding, they erode the outside of each bend and deposit material on the inside of each bend. 

Braided rivers have multiple interweaving channels and are typically found in desert, mountain and glacier environments where erosion is particularly high.  The streams themselves are seldom a cause of substantial flooding accept in the case of flash flooding in desert areas.  The stream beds are depositional overall, not erosional.  The valleys are slowly being filled up with sediments adjacent to a braided stream.

Valley streams occupy a v-shaped valley.  They are downcutting the valley.  The banks rise steeply away from the river.  The stream gradient is high, rapids and waterfalls common.  There is no flat terrain in the vicinity.