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MOVING A RIVER- UNCOVERING THE PAST

| The Parklands
MOVING A RIVER- UNCOVERING THE PAST

Arguably, the only rule in nature is that change is constant. The Floyds Fork landscape is no exception. For this landscape clearly presents a case study in the interactions of climate, vegetation, and human land use and the story is waiting to be uncovered by anyone who is willing to pay attention.

The confluence of Broad Run and Floyds Fork is a dynamic area with number of gravel bars and side channels that form due to the complex currents and converging water. This happens at nearly all tributary confluences on Floyds Fork. Here, however, Broad Run is pushing Floyds Fork to the west causing the high bank of Floyds Fork to erode, in this case quite rapidly.

The bank is eroding so rapidly, in part, because a powerline crosses at this site. The power company prevents trees from growing beneath their lines so no sycamores or box elders hold the bank together. Thus with every flood it crumbles. From 1993 to the present the bank has retreated 2 to 3 feet per year— this is very fast in geologic terms. Tree roots would have prevented this.

The high level of erosion allows us a bit of a window into the past. At summer base flow one can see a system of old gravel bars buried under 8 feet of fine-grained valley bottom sediment, itself covered in corn or soy, depending on the year.

We call one of these buried gravel bars a “lens.” The gravel bar lens contains generally dime-sized gravel sediment, but also old mussel shells and wood fragments. Most of these wood fragments are small, but a few reach stick or log size. The largest of them, a log approximately 15 inches in diameter, shows clearly by the ray-lines in the cross-section of the wood that it was an oak.

Because the wood has been buried in an oxygen-poor environment, very little decomposition has occurred. When I found this wood buried beneath 8 feet of sediment what I first wanted to know was how old it happened to be. If the wood pieces were just 200 years old, this would tell us that deforestation during early Kentucky agriculture caused an enormous amount of erosion, burying the wood under 8 feet of sediment. But on the other hand, if the wood was 8000 years old it would show us that deposition has been quite slow indeed. Because the wood is oak we know it can’t be older than about 8000 years because oak did not grow here prior to then— Kentucky was still climbing out of the last glacial cycle and warm-sited trees such as oak had not yet reached this far north.

To age the pieces of wood I sent two samples to a lab in Boston to have them dated. Since all biological life is made of carbon, measuring the amount of decay in certain carbon isotopes can fairly accurately estimate the age of carbon-based life. This process is good for up to 60,000 years. I sent two samples so that we could have a greater sample size and thus a more reliable record of results for dating the wood and the deposition of the gravel bar.

The first sample, which was the oak log, dated to 3500 years. The other wood sample was 2200 years old. These dates tell us a few things. First, they tell us that there was oak growing around Floyds Fork 3500 years ago. We can’t tell what species it is or how far upstream it occurred, but not unlike today, there was an oak tree. It fell into the water, and floods carried it downstream until it landed on a gravel bar and was buried in sediment. It is possible that a Native American or a beaver cut it down.

We also know that the 8 feet of sediment occurring on top of the gravel bar layer was most likely not deposited in the wake of European settlement. The deep bottomland soils of the Floyds Fork valley are likely a few thousand years old. But nevertheless, for 8 feet of fine-grained sediment to become deposited on top of a gravel bar over 2100 years there would need to be a lot of slow moving water with a high load of fine-sediment. It is likely that the broad Floyds Fork valley once hosted some very large beaver dams which would have allowed for such deep deposition, but a beaver dam is not necessarily a requirement for deposition such as that of Broad Run.

Paddling is really the only way to appreciate this 2000 year old gravel bar.  Gravel bars occur regularly along The Parklands section of Floyds Fork, and just downstream of the Fisherville Canoe Launch there is another series of buried gravel bars.  Lower water levels allow the best opportunity for viewing.  While you’re out there paddling, ponder this: What will this valley look like 2000 years from now?

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