In looking at the Harpeth River, we saw a river, which, for the most part ran freely.
Most rivers in Tennessee, however are dammed. Nashville's waterways are framed by dams. The Cumberland River is dammed just upstream from Nashville (Old Hickory Dam) and downstream from Nashville (Cheatham Dam). One of the two major tributaries, Stones River, is dammed at the eastern edge of Nashville by (Percy Priest Dam.) The three dams are part of a system of dams on the Cumberland River and its tributaries.
I had hoped that it would be possible to take tours of the dams, to learn how they work. Unfortunately since 9/11/01, this is no longer possible, and we will have to be content with exploring around the dams.
This field trip visits the Percy Priest Dam on Stones River, to show the impact that dams have on rivers. The Harpeth and Stones Rivers are quite comparable in size, but the dam has formed a lake on the Stones River.
Rivers move water, but they also move sediment. Undammed rivers tend to move quickly at their source, and this rapidly moving water picks up sediment. As the river gets closer to its mouth it slows down gradually dropping its load of sediment: first the large sediment, gravel and sand; and finally the mud and clay.
Dams change this pattern. Above the dam the speed of the water is slowed as it approaches the lake. Streams slow down and release their sediment long before they would have without the dam. This creates small deltas with wetlands at the mouths of tributaries, and gradually fills the lake in with sediment.
Below the dam, the water is released with a great deal
more force than it would have had without the dam. It is also released
without any sediment. This water picks up sediment rather than depositing
it, leading to a phenomenon called scour. Find out more about consequences
of dams.
Stop 1: Percy Priest Dam
Percy Priest Dam because it is the simplest dam of the three dams in Nashville. Although there was commercial navigation on Stones River in the early nineteenth, there hasn't been any recently, and therefore the dam does not need locks to allow boats to get through it as Nashville's other two dams do.
Directions:
The lake provides many opportunities for swimming, fishing
and boating.
It also provides habitat for lake dwelling animals like
ducks.
Take a look at some of the rocks.
Do you recognize these fossils?
Have you seen them before?
Can you identify what stratum is exposed?
If you have taken the Earthquakes and Volcanoes field
trip you will recognize these as backfilled trace fossils which are typical
of Carters Limestone.
Now let's go to look below the dam.
Directions:
Before you approach the river, be sure to look at the
sign. DON'T go down onto the river bed, no matter how safe it may
look.
Now take a look at the river below the dam.
Is this what you expected?
Why do you think there is no water?
There are two reasons: one natural; one human.
Nashville's climate is one of rainy winters and dry summers. Remember it is a humid, sub-tropical climate. All streams get lower in summer.
Because Stones River is not used for navigation, a continual
flow of water in the river below the dam is not needed by people.
Because the lake us used by people for recreation, particularly in the
summer, we have a motive to keep the lake level from falling, and therefore
don't release any water for most of the summer.
Notice that the stream bed is mostly just rock.
There is very little sediment, and that which there is is made up of fairly
large rocks. Can you explain why?
Now let's go look at a tributary. We will go to the Vivrett Boat Ramp
Directions
Vivrett Boat Ramp is on the last tributary to enter Stones River before the dam.
Compare the edge of the stream to the Harpeth River at Rt. 100.
Can you explain the difference?
Home | Exercises | Fossils | Field Trips | Natural Resources | Ordovician Period | Rocks | Strata
