Week 7 PowerPoint Lecture

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Objectives:

4.01 Evaluate erosion and depositional processes: Formation of stream channels with respect to the work being done by the stream (i.e. down-cutting, lateral erosion, and transportation).

Nature and characteristics of sediments.

Effects on water quality Effect of human choices on the rate of erosion.

Groundwater and Chemical Weathering

 As groundwater passes through permeable rock, minerals in the rock dissolve. The warmer the rock is and the longer it is in contact with water, the greater the amount of dissolved minerals in the water.

 Water that contains relatively high concentrations of dissolved minerals, especially minerals rich in calcium, magnesium, and iron, is called hard water.

 Water that contains relatively low concentrations of dissolved minerals is called soft water.

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Results of Weathering by Groundwater

Watch this video to view how caves form. Click on worksheet. /form.html

“ How Caves Form you go to the website. Use the information to help with your http://www.pbs.org/wgbh/nova/caves ” on the left side of your screen once Caverns - a natural cavity that forms in rocks as a result of the dissolution of minerals; also a large cave that commonly contains many smaller, connecting chambers

Results of Weathering by

Groundwater, continued

Stalactites and Stalagmites

 When water containing dissolved calcite drips from the ceiling of a limestone cavern, some of the calcite is deposited on the ceiling.

 As this calcite builds up, it forms a suspended, cone-shaped deposit called a stalactite.  When drops of water fall on the cavern floor, calcite builds up to form an upward-pointing cone called a stalagmite.

Results of Weathering by

Groundwater, continued Sinkholes - a circular

depression that forms when rock dissolves, when overlying sediment fills an existing cavity, or when the roof of an underground cavern or mine collapses

Results of Weathering by

Groundwater, continued

Natural Bridges

 When the roof of a cavern collapses in several places, a relatively straight line of sinkholes forms.

 The uncollapsed rock between each pair of sinkholes forms an arch of rock called a natural bridge.  When a natural bridge first forms, it is thick, but erosion causes the bridge to become thinner.

Results of Weathering by

Groundwater, continued

Karst topography -a type of irregular topography that is characterized by caverns, sinkholes, and underground drainage and that forms on limestone or other soluble rock  Common features of karst topography include many closely spaced sinkholes and caverns.

 Generally, karst topography forms in regions where the climate is humid and where limestone formations exist at or near the surface.

Parts of a River System

 A river system is made up of a main stream and tributaries.

 Tributary - a stream that flows into a lake or into a larger stream Watershed - the area of land that is drained by a river system  The ridges or elevated regions that separate watersheds are called divides.  

Can you find the tributaries that are part of this watershed?

Can you identify the divide in the diagram?

Parts of a River System, continued

 The relatively narrow depression that a stream follows as it flows downhill is called its channel.

  A stream channel gradually becomes wider and deeper as it erodes its banks and bed.

River systems change continuously because of erosion.

 The edges of a stream channel that are above water level are called the stream’s banks.

 The part of the stream channel that is below the water level is called the stream’s bed.

Stages of a River System

   Youthful  V-shaped valley with steep sides   Erodes beds faster than banks Most rapid water flow Mature  Less steep gradient   Erodes banks more than beds Meanders often form Old    Deposits sediment, does not erose Broad, flat plain forms in river valley Slowest water flow  Click to view formation and evolution of a river: http://highered.mcgraw hill.com/olcweb/cgi/pluginpop.cgi?it

=swf::640::480::/sites/dl/free/00724 02466/30425/10_22_10_23.swf::Fig s.%2010.22,%2010.23%20 %20Meander%20Cut Offs%20and%20Oxbow%20Lakes

Channel Erosion

Stream Load - materials other than the water that are carried by a stream 1.

The suspended load consists of particles of fine sand and silt. The velocity, or rate of downstream travel, of the water keeps these particles suspended, so they do not sink to the stream bed.

2.

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The bed load is made up of larger, coarser materials, such as coarse sand, gravel, and pebbles. This material moves by sliding and jumping along the bed.

The dissolved load is mineral matter transported in liquid solution.

Channel Erosion, continued Stream Discharge - volume of water that flows

within a given time  The faster a stream flows, the higher its discharge and the greater the load that the stream can carry .

Channel Erosion, continued

Stream Gradient - the change in elevation over a given distance  Near the headwaters, or the beginning of a stream, the gradient generally is steep. This area of the stream has a high velocity, which causes rapid channel erosion.

 As the stream nears its mouth, where the stream enters a larger body of water, its gradient often becomes flatter.

Evolution of River Channels

Meandering Channel - one of the bends, twists, or curves in a low-gradient stream or river  When a river rounds a bend, the velocity of the water on the outside of the curve increase. However, on the inside of the curve, the velocity of the water decreases.

 This decrease in velocity leads to the formation of a bar of deposited sediment, such as sand or gravel.

Evolution of River Channels, continued

Meandering Channels, continued  Meanders can become so curved that they almost form a loop, separated by only a narrow neck of land.

 When the river cuts across this neck, the meander can become isolated from the river, and an oxbow lake forms. (See Sims Lake, Wedgeworth Lake to right)

Evolution of River Channels, continued

Braided stream - a stream or river that is composed of multiple channels that divide and rejoin around sediment bars   direct result of large sediment load, particularly when a high percentage of the load is composed of coarse sand and gravel.

can cause much erosion, affecting water quality.

Deltas and Alluvial Fans

Delta a fan-shaped mass of rock material deposited at the mouth of a stream   Alluvial fan a fan-shaped mass of rock material deposited by a stream when the slope of the land decreases sharply Deltas form where streams flow into the ocean at the edge of a continent A stream may deposit sediment on land or in water.

  Alluvial fans form when streams flow from mountains to flat land When a stream descends a steep slope and reaches a flat plain, the speed of the stream suddenly decreases. As a result, the stream deposits some of its load on the level plain at the base of the slope.

Alluvial fans differ from deltas in that alluvial fans form on land instead of being deposited in water.

Floodplains

A floodplain an area along a river that forms from sediments deposited when the river overflows its banks  The volume of water in nearly all streams varies depending on the amount of rainfall and snowmelt in the watershed.

 When a stream overflows its banks and spreads out over the floodplain, the stream loses velocity and deposits its coarser sediment load along the banks of the channel.

 The accumulation of these deposits along the banks eventually produces raised banks, called natural levees.

 Finer sediments are carried farther out into the floodplains by the flood waters and are deposited there.

 A series of floods produces a thick layer of fine sediments, which becomes a source of rich floodplain soils.

Human Impacts on Flooding

  Human Impacts on Flooding   Vegetation, such as trees and grass, protects the ground surface from erosion by taking in much of the water that would otherwise run off. Logging and the clearing of land for agriculture or housing development can increase the volume and speed of runoff, which leads to more frequent flooding.

Flood Control   Indirect methods of flood control include forest and soil conservation measures that prevent excess runoff during periods of heavy rainfall.

More-direct methods include the building of artificial structures that redirect the flow of water, such as dams or artificial levees.

Do you remember a recent natural disaster that resulted from failed levees? Hint: New Orleans, LA

Bibliography

Pictures and information gathered from:

http://www.nps.gov/cave/naturescience/images/entrance_jones_556.jpg

http://www.blm.gov/education/expert/3/small/cave4.jpeg

http://www.blm.gov/education/expert/3/small/cave9.jpeg

http://ga.water.usgs.gov/edu/pictures/sinkhole1.jpg

http://www.dcr.virginia.gov/natural_heritage/images/ccvup56.jpg

www.adoptawaterbody.utah.gov/what_is.htm

http://salvinia.er.usgs.gov/MS_site_Forrest_County_web.JPG http://3dparks.wr.usgs.gov/3Dbayarea/images2/ArroyoSeco2.jpg

http://www.fws.gov/.../bhg/images/Ebutte_delta2.jpg http://wrgis.wr.usgs.gov/parks/deva/Badwater_fan08.jpg

http://www.water.ky.gov/NR/rdonlyres/5D74D194-3780-4B2A-8C1A-6A4840B2CE20/0/02goose.JPG

https://www.llnl.gov/str/June07/gifs/dam1.gif

Allison, DeGaetano, Pasachoff, Holt Earth Science. 2008, Holt, Rinehart and Winston, New York