Weir Dam EarthCache
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In North East Tennessee, west of the Blue Ridge Province, is found the Valley and Ridge Province. The Valley and Ridge Province is well studied for its geological characteristics. It is characterized by long linear ridges with elevations between 1,000 and 4,500 feet. Between these ridges are wide valleys with a trellis drainage pattern. Small streams and creeks drain rainfall from the ridges to a valley floor to form a river. The direction of flow of the rivers is to the southwest. When one ridge ends rivers from two valleys may meet in what is called a confluence to form a larger river. A great example of this is the area of South West Virginia and North East Tennessee where the Holston River is located. The Holston River is made up of three forks called the North Fork, Middle Fork, and South Fork. Further south, at the confluence of the Holston and French Broad rivers, the Tennessee river begins.
The Valley and Ridge Province was a hindrance to westward movement of settlers. Mountain crossings and river crossings were a big obstacle to early settlers. However, after crossing those obstacles they found broad valleys in which to settle. As time passed people started working with the geology of the area to their great benefit.
In 1933 the Tennessee Valley Authority was established to provide navigation, flood control, electricity generation and fertilizer manufacturing. Dams were the means of making rivers navigable, to control floods, and generate electricity. Dams also fit into the geology of the Valley and Ridge Province. Dams could be built between two ridges across the valley floor to create a reservoir. In 1947 the South Holston Dam was completed for this purpose. South Holston Dam was built 50 miles above the confluence of the North Fork and South Fork of the Holston River. South Holston Dam is an earth-and-rock dam also known as an embankment dam. It is 285 feet high and extends 1600 feet across the valley floor. Embankment drains are almost always used because of the geology of the area. They are built across broad valleys and near areas where there is an abundance of construction material. Large amounts of rock may be quarried from the ridges.
In some lakes created by dams some unwanted consequences may develop. Most lakes are holomictic which means the surface and deep waters mix at least once a year. In monomictic lakes the mixing is once a year. In dimictic the mixing occurs twice a year. In polymictic lakes mixing occurs several times a year. Meromictic lakes have little mixing and the lower levels become very oxygen poor because of decomposition of organisms settling from above. In some meromictic lakes no mixing occurs at all for years, decades, or centuries.
Lakes and large reservoirs behind dams tend to have colder water at the lower levels. This tends to lower mixing of the water from different levels. With no mixing lower levels have less oxygen and more carbon dioxide. When this water is discharged from the dam it has a great effect on the river below the dam. This water below the dam is colder and with less oxygen than the original river which changes the types of organisms which may survive in this water.
In 1991, TVA built a weir below South Holston Dam in order to add oxygen to the river water. As the water tumbles through the weir it picks up oxygen which diffuses through the water. The cold water makes a good environment for trout and other organisms.
Geology has been the determining factor for all aspects of South Holston Dam. The Valley and Ridge Province provides a good location for the dam itself. The ridges provide a large amount of rock material which can be quarried. The Valley provides the land area for the reservoir. All these geological features make for a great location for an earth-and-rock type of embankment dam.
The efforts of the TVA has increased dissolved oxygen levels in more than 300 miles of river downstream of TVA dams and improved water flow in 180 miles of river. These changes have resulted in significant growth in tailwater fish and aquatic life. A footbridge from the parking lot below the dam leads to Osceola Island and its one-mile-loop wildlife trail. Early in the morning or late in the evening are the best times to sight waterfowl, deer, and other wildlife.
Directions: From Bluff City, TN, take Hwy 394 East to Hwy 421. Turn right. Drive approximately 3 miles and take a right onto Emmet Rd. You will come to a fork in the road. Take the right fork. Follow the signs to South Holston Dam. Once you cross the bridge over the river, turn into the 2nd parking area on the right.
It is open to the public from daylight to dark. Visiting this location will allow you to marvel at the impact the Weirs have on the environment. We hope you enjoy!!
There is a wealth of information on the weirs, fish and plant life of the river, as well how the weirs has impacted the area. Take the time to read the information and e-mail us the answers to the following questions:
1. The South Holston is a earth-and-rock dam, but what else is it called?
2. What geological features do you see present at the posted coordninates?
Optional: Take a picture with your GPS device at the weir.
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