Saturday, August 9, 2008

Secrets of the Salamanders

The Chattooga River hosts a great diversity of rare salamanders. Don't assume that they just swim around and hide under rocks. These salamanders have a story to tell - an old, old story. Specifically, their DNA is evidence of the geologic changes to the rivers in the Southeast.

As mentioned before, I am a map fiend and, when studying maps, tend to look for the divides between adjoining river basins. Until recently, it had not occurred to me how those boundaries have shifted over time. The Chattooga River is a good example. Today, it joins the Tallulah River to become the Tugaloo and eventually the Savannah River, flowing along the South Carolina-Georgia line all the way to the Atlantic Ocean.

If you go back in time, though, prior to Pleistocene epoch, it was a different story. The Chattooga was a headwaters stream to the Chattachoochee River, flowing southwest into the Gulf of Mexico.

The process that diverted the Chattooga to the Savannah River system is called stream capture. If you visit Tallulah Gorge, south of Clayton, NC, you'll overlook the place where the rapidly down-cutting Savannah carved away at relatively soft rock and, ultimately pulled the Chattooga and Tallulah Rivers away from the Chattahoochee. (See map below.)

This was not the only instance of stream capture in the Southern Appalachians, but is the best known and most dramatic. Clearly, the diversion of a stream from one river drainage to another leaves a mark on the topography of the land. Less obvious is the impact on the distribution and gene flow of aquatic creatures such as mollusks, bog turtles and, yes, salamanders.

The genetic record reveals that the populations of salamanders in the Chattahoochee and Savannah River basins are more closely related to each other than to salamanders in the Tennessee River basin just across the Eastern Continental Divide. The long-ago geologic process that shifted the headwaters of the Chattahoochee to the Savannah is still reflected in the DNA of the salamanders that inhabit those rivers.

So if you start a float trip down the Chattooga today, you'll no longer reach the Gulf of Mexico. You will, however, share the river with salamanders whose ancestors traveled that watery route from the Gulf to the mountains, a long time ago.

SourcesRiver Capture in the Tallulah District, Georgia, by Douglas Wilson Johnson appeared in the journal Science, Volume 25 (March 1907), pp. 428-432, and provides a good explanation of the process.

The study of salamander DNA was published as Allozyme Variation in Neighboring Isolated Populations of the Plethodontid Salamander Leurognathus marmoratus, S. Randal Voss, David G. Smith, Christopher K. Beachy and David G. Heckel, Journal of Herpetology, Vol. 29, No. 3 (Sep., 1995), pp. 493-497.

Illustrations – Top, looking hundreds of feet down at the falls of Tallulah Gorge. Middle, another view of Tallulah Gorge, facing downstream. Bottom, a map showing the course of the rivers before (A) and after (B) the stream capture event at Tallulah Gorge. (Click on map to enlarge.)

From the past to the future - Like most geologic processes, stream capture is inexorably underway right now. The rapidly eroding gorges of the Keowee-Toxaway basin, in the vicinity of Toxaway, are eating their way toward the headwaters of the French Broad River and will eventually divert those waters away from their present course, in what will likely be the next major stream capture event in the Southern Appalachians. I can hardly wait!

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