A reader of the Nantahala Gorge post asked a very relevant question: Is there a stream capture in the Appalachians that is going to happen soon? While “soon” can be a very relative term in geology, there is most definitely a good answer to the question. At Canton, North Carolina, the headwaters of Hominy Creek, a French Broad River tributary, are VERY close to capturing the Pigeon River. In human terms, this is still probably a long way off, but it is most certainly geologically “imminent.”
The future capture will occur where the headwaters of Hominy Creek, which flows to the French Broad River, appear to nearly touch the Pigeon River just left of center. This color-coded elevation map does a nice job of showing the elevation contrast between the two neighboring river systems, which sets the stage for stream capture. The map area is about 25 miles (40 km) across. Canton, North Carolina is located where Hominy Creek and Pigeon appear to nearly touch on this map. Note the weird umbrella/anchor/grappling hook/whatever shape of the Hominy Creek headwaters…this is discussed further down.
Much like the Nantahala scenario, this is a capture between neighboring river valleys that are both part of the Tennessee River system. Today, their waters meet in the Tennessee Valley within 10’s of miles of the imminent capture point. Even so, the Pigeon River valley is “perched” (yes, a real geologic term) about 400 ft (125 m) higher than the neighboring Hominy Creek/French Broad valley in the area where the capture will occur. Because these neighboring rivers have strongly contrasting elevations, the lower river system (French Broad) has the potential to capture from the higher river system (Pigeon).
This capture is, geologically speaking, “inches” away from happening. The headwaters of Hominy Creek are about 1,500 ft (450 m) ground distance from the banks of the Pigeon River at the Evergreen Packaging paper mill. There is no dividing ridge between the two streams; the actual drainage divide is only about 20 ft (6 m) higher than the Pigeon itself, and it is so subtle as to be nearly invisible in the landscape.
Paper mill buildings are visible at the center of the image. The yellow line is the drainage divide between Hominy Creek and the Pigeon. I have not actually been to this spot, but I would be curious to see how discernible the features are due to development. Right now, the point where the railroad tops out and enters the mill is the leading edge of Hominy Creek’s attack.
It probably would not be too difficult to construct a canal to link the Pigeon and French Broad River systems through Hominy Creek. When the future stream capture creates this link, the introduction of the Pigeon’s flow across the 400 ft descent to the French Broad River valley will produce impressive rapids and falls and carve a deep gorge. Would-be engineers of the “Canton-Asheville Canal” would have to carefully control the water over this descent with dozens of locks and dams. Fortunately, Interstate 40 provides a much better means of traveling between the two river systems.
The headwaters of Hominy Creek show that it has been advancing into the Pigeon River system through the recent geologic past. Many stream networks develop “accordant junctions” between tributaries and the main stream, meaning that the angle between the tributary and main stream is well below 90 degrees on the upstream side of the junction. In other words, the layout of a stream system looks like a tree with branches, with the flow direction of the stream being from the tips of the branches towards the base of the trunk.
In the first panel, where the river systems are labeled, the “normal” branching patterns and flow directions are represented. As Hominy Creek eats its way into the Pigeon system, it inverts and reverses the normally branched stream network to make the odd drainage pattern. Peter Vogt called this pattern a “grapnel” shape in a paper in the 90s, and I used that term in my own subsequent work.
Upper Hominy Creek shows the opposite pattern. Two of its tributaries are angled as if they still flow to the Pigeon, but Hominy Creek now flows the opposite direction. These tributaries did indeed flow to a stream that once continued to the Pigeon. Hominy Creek’s “ancestor” ate its way into the elevated Pigeon Valley, carving right up the existing stream channel and progressively inverting it to flow towards the French Broad.
The inversion process is possible because Hominy Creek and the now-inverted Pigeon tributary flow down a very weak zone in the bedrock that is easily eroded. This weak zone is probably a fault or area of very intense fracturing of the rock, which makes it more susceptible to erosion and thus a good place for stream courses to localize.
... continue reading