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Thalwegs, Point Bars, & Cut Banks (Oh my!)

A cache by foxult Send Message to Owner Message this owner
Hidden : 1/27/2015
Difficulty:
1.5 out of 5
Terrain:
2 out of 5

Size: Size: other (other)

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Geocache Description:

This Earthcache takes you along the Six Mile Creek Greenway. The path is flat and paved until the final 30 feet, when you will have to leave the pavement to bushwhack to the edge of the creek. Don’t fall in!

This Greenway floods easily. It's not recommended to do this Earthcache during flooding.


I. The Mecklenburg County Greenway System

The Mecklenburg County Greenway system is quickly becoming one of the finest in the country. Greenways are vegetated natural buffers that improve water quality, reduce the impacts of flooding, and provide wildlife habitat. Greenway trails provide recreation, transportation, fitness, and economic benefits for all to enjoy. There are over 35 miles of developed greenways in Mecklenburg County. [http://charmeck.org/mecklenburg/county/ParkandRec/Greenways/OpenGreenways/Pages/default.aspx]

 

II. Thalweg

 

(Note: In the write up below, “stream” and “river” will be used interchangeably.)

 

The fastest and deepest part of any stream is called the thalweg, which is a great word that definitely isn’t used enough in casual conversation. (Dude 1: “Dude, I got caught in a thalweg yesterday. Super stoked!”   Dude 2: “Dude!”)

 

It’s no accident that the fastest part of a stream tends to be the deepest. Faster water picks up more sediment and bigger chunks of sediment.

 

Fast moving water has enough energy to carry a large amount of materials, known as the stream's load. Raging floodwaters can pick up small-to-large sized particles and carry them until they no longer have the energy to keep them suspended. [from http://www.atlas.keystone.edu/edu/virtual/water_discovery/Station15.html]

 

When the slope a stream flows down is steep, most of the energy of erosion cuts downward into the stream bed. Streams in mountainous areas, where there are big drops in elevation, are usually found in steep-walled (V-shaped) valleys without much of a floodplain (flat land next to a stream).

 

But when a stream flows along a coastal plain where there is not much change in elevation, there is not much downward erosion. Instead, a stream meanders its way back and forth across the plain with most of the erosion being sideways.

 

The location of the thalweg, in general, depends on the straightness of the stream. When a stream flows in a straight line, the thalweg is usually towards the center of the stream bed. When a stream bends, the thalweg is more toward the outside of the bend because most of the water is forced along the outside of the curve.

 
 
 

III. Point bars

 

A point bar is a depositional feature made of sediment that accumulates on the inside bend of streams. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend.

Point bars have a very gentle slope and an elevation very close to water level. Since they are low-lying, they are often overtaken by floods and can accumulate driftwood and other debris during times of high water levels. Camping on a point bar can be dangerous as a flash flood that raises the stream level by as little as a few inches can overwhelm a campsite in moments. [from http://en.wikipedia.org/wiki/Point_bar]

 

IV. Cut banks

A cut bank is the outside bank of a stream, which is continually undergoing erosion. Cut banks are found in abundance along mature or meandering streams, they are located on the outside of a stream bend. They are shaped much like a small cliff, and are formed by the erosion of soil as the stream collides with the river bank. As opposed to a point bar which is an area of deposition, a cut bank is an area of erosion.

Typically, cut banks are nearly vertical and often expose the roots of nearby plant life. Often, particularly during periods of high rainfall and higher-than average water levels, trees and poorly placed buildings can fall into the stream due to mass wasting events. Given enough time, the combination of erosion along cut banks and deposition along point bars can lead to the formation of an oxbow lake. [from http://en.wikipedia.org/wiki/Cut_bank]



 

V. Oxbow Lakes

 

An oxbow lake is a U-shaped body of water that forms when a wide meander from the main stem of a river is cut off, creating a free-standing body of water. This landform is so named for its distinctive curved shape, resembling the bow pin of an oxbow.

 

An oxbow lake forms when a river creates a meander, due to the river's eroding the bank. After a long period of time, the meander becomes very curved, and eventually the neck of the meander becomes narrower and the river cuts through the neck during a flood, cutting off the meander and forming an oxbow lake. [from http://en.wikipedia.org/wiki/Oxbow_lake]


Once two cut banks meet creating the ox bow lake, the stream now has a shorter path to its destination. Along the Mississippi River some meanders were intentionally cut off in the 1800s to save many miles of boat travel.

Here’s a timelapse video of a stream winding its way across a plain.

http://imgur.com/Uak4YU3


Unfortunately, there are no oxbow lakes in the Six Mile Creek area.

 
Questions

Go to the coordinates, which puts you on the edge of Six Mile Creek. Look upstream about 100 feet where you will see a point bar on the left and a cut bank on the right.

Email me the answers. Don’t put them in your log.

1) From your vantage point, estimate the width of the point bar.

2) Estimate the height of the cut bank.

3) Why is sediment deposited on a point bar?

4) Why does erosion take place on a cut bank?

5) [Optional] Feel free to include photos of your Earthcache experience.

Additional Hints (No hints available.)



 

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