Welcome to the Obed Wild and Scenic River. If you've just pulled up to the posted coordinates you are not looking at the Obed River, but rather one of its tributaries, Clear Creek. The bridge in front of you, known as Lilly Bridge, crosses over a nice example of an Alluvial Bar, which will be the focus of this Earthcache. Parking is available at the posted coords, as well as toilets and a trail head. This area is popular with rock climbers, white water enthusiasts and nature lovers of all kinds, for more information on recreational opportunities visit the park head quarters in Wartburg, TN.
CAUTION
If you happen to visit the river when it is flooding, please do not go near the water for any reason, lest of all answering questions for this earthcache. A flooding river is a dangerous thing that requires the utmost respect.
Alluvial Bars
What is an alluvial bar?
Alluvial bars are discrete deposits of stream sediment or
alluvium along the length of a river channel. They can come in many different forms, depending on the composition of the alluvium, which can vary from tiny particles and clay, to large boulders. They also vary greatly based on the water forces present. Some alluvial bars were laid down during great floods that may have occurred thousands of years ago, others can be intermittent and shifting like sandbars in a shallow river.
Upstream view from Lilly Bridge showing large alluvial bar on the right of the main channel
Alluvial bars can be colonized quickly by fast growing flora which further serves to hold the bar together. In situations with relatively little seasonal flooding, the vegetation can quickly cover the bar and soil lead to the establishment of new riverbank. In other cases, such as the one below the Lilly Bridge, frequent high flood events prevent accumulation of substantial soil deposits, and limit the quantity and variety of vegetation that is able to subsist on the bar.
How do alluvial bars form?
The alluvial bar seen below the Lilly Bridge is composed mostly of boulders and large cobbles, with smaller sediments filling in the spaces. Surface deposits of sand and silt are uncommon here indicating regular flooding events. These characteristics are typical throughout the Obed River watershed. The streams and rivers here are in deep narrow gorges, bordered by sandstone cliffs. Sandstone blocks, periodically dislodged from the cliffs, fall and roll to the bottom of the gorge and provide the main material for the bars.
The size of the alluvium in the boulder bar is indicative of the hydraulic forces necessary to move such material. By taking a look at the shapes and sizes of the boulders, estimates for the speed and flow of the river can be made. However, it is not a simple matter. Several factors must be assessed; The shapes and arrangements of boulders and other particles in the channel complicate the movement of course material; Rounded boulders require less energy to move than flat ones; Isolated boulders are easier to move than ones that are clumped together; local variations in channel slope, water velocity and bed roughness. Generally, the more compounding factors that exist, the higher the water depth and velocity is needed to transport a given particle size. Just looking out at the size of the boulders in the bar beneath the bridge one can guess that serious flooding occurs here. Indeed, a
paper published by the USGS, suggested that the minimum conditions to move 3-foot boulders along the channel beds in the Obed River are common.
Chart showing Obed River discharge and thresh-hold lines for when a 3 foot boulder could be moved
Signs of large flooding events are usually not too hard to pick out. Look for flotsam (bits of flood debris) hanging in branches of trees in the boulder bar. The higher the flotsam is, the larger and more powerful the flooding event was.
Evidence of a high flood event
Now that the hydraulic forces necessary for moving the large boulders you see below you are established, it is time to investigate the location of the deposits. In general, alluvium is deposited in low-energy areas of the river system. These can be any sections of the area where the energy of the river system decreases, and commonly includes major confluences, eddies or deflections of flow caused by large non-alluvial boulders and bends in the channel. Alluvial bars form as boulders accumulate in these low energy locations. Each time boulders are deposited at a location, the energy required for additional deposits is increased as is the energy required to remove existing deposits. Because of this effect, flows capable of transporting boulders across bar surfaces are likely to deposit more material than they remove.
Why study alluvial bars in the Obed River Watershed?
Understanding what an alluvial bar is and how it forms is all well and good but why take an interest in them at all? The dynamic characteristics of flooding over moderately stable bars produces a somewhat unique ecological environment. While the alluvial bars make up only a small part of the land surface in the Plateau gorges, they support a large number of rare, threatened and endangered (RTE) plants. In this way they are important to the local biodiversity, that enriches the river system. Recent aerial surveys have identified over 200 alluvial bars along the 43 miles of the Obed River and Clear Creek within the Obed WSR. Protecting these habitats is an important part of the mission of the Obed WSR, thus an understanding of the factors that play a role in the geology underlying the habitats is vital to understanding how to protect them. At least that's the official answer. But perhaps you will appreciate this lesson the next time you see a relatively ordinary looking gravel bar in your own local stream. Taking a careful look at the composition of that bar may yield fountains of information about the river that formed it.
Required Questions
- Notice a set of concrete stairs near the posted coordinates. there are 42 steps* in total leading down to the base of the boulder bar. How high is the river from the last concrete step? Measure lower or higher.
- Describe the shape and size of the cobbles located near the bottom of the steps. How do they fit together if at all?
- Look for signs of flood in the vegetation on the boulder bar. How high above the alluvial bar is that highest piece of flood debris that you can find? OPTIONAL: Take a picture of the flood debris to show how high it is.
- From the bridge, estimate the length of the boulder bar downstream from the bridge.
- What are the features of the river in this spot that may have resulted in the boulders being deposited here?
Please email me your answers for verification. I reserve the right to delete logs from those who have not contacted me. That said, I am pretty reasonable about what is an acceptable answer. Mainly I just want people to have fun out there and learn a little about this gorgeous area. As always, pictures are appreciated.
*The number of steps, 42, is given in case the water is higher than the last step. By knowing the total number of steps, you should still be able to come up with a measurement even if the water is quite high.
Bonus Questions
Answer these for fun and to get a little more out of your visit. I encourage you to answer the bonus questions in your online log. It will be fun to see what different flow rates were observed. For the bonus questions you will need to visit the
USGS River Gauge for the Lilly Bridge.
- What is the current discharge/height of the river?
- What was the highest discharge/height of the river in the last 180 days?
- Using the chart above, were any of the flood events in the last 180 days capable of transporting a 3 foot boulder? Assume a slope of 0.0045.
Resources
- Obed River WSR website
- Hydrologic Data for the Obed River Watershed, R. Knight, W. Wolfe, G. Law, USGS Publication 2014-1102.
- Alluvial Bars of the Obed Wild and Scenic River, Tennessee, W. Wolfe, K. Fitch, D. Ladd, USGS Publication 2007-2972.
- National Water Information System: USGS 03539778 CLEAR CREEK AT LILLY BRIDGE NEAR LANCING, TN