Posts Tagged ‘Uvalda Paleomeander’

Late Pleistocene Megameanders

December 30, 2018

Glacial recession during the end of the last Ice Age caused dramatic changes in the climate and river drainages of southeastern North America.  All that melting Canadian ice released moisture into the atmosphere and average annual precipitation in the region tripled to an estimated 47 inches.  But average temperatures were still cooler than they are today and as a result evapotranspiration rates were lower. This increased precipitation and reduced evaporation caused rivers to meander more than they do today.  These supermeanders eroded scars that are still visible in satellite photographs.  Recently, scientists studied 6 paleomeander scars adjacent to the Oconee, Ogeechee, Black, Neuse, Pee Dee, and Congaree Rivers.  These rivers are located in Georgia, North Carolina, and South Carolina.

Image result for megameander paleochannels of the southeastern United States

Satellite photo of a paleomeander scar located along the Oconee River.  From the below referenced paper.

The scientists found these paleomeanders dated to between ~17,000 years BP-~11,000 years BP.  The scar next to the Oconee River was radio-carbon dated using a 17,000 year old pine log.  The supermeanders cut through the former braided channels that existed during the Last Glacial Maximum when rivers shrank and became clogged with sandbars due to aridity.  Eventually, when present day climatic conditions began to predominate, the supermeanders became cut-off from the main flow of the river.  For awhile they existed as oxbow lakes but then filled with clay and sand.  Scientists estimate the supermeanders were 2-5 times larger than modern meanders and the discharge was up to 4 times larger.  The typical flow was equal to a modern day 5 year flood event.  Scientists aren’t sure of the exact mechanism that caused supermeanders.  It was likely a combination of increased precipitation, low evapotranspiration rates, and seasonal monsoons.  The distance between the frigid air over the Laurentide Ice Sheet and tropical air was much smaller during the Ice Age, and this could have caused an increase in major storm events.

I hypothesize canebrakes and river bottomland forests really expanded during the supermeandering phase.  Canebrakes are a now nearly extinct environment consisting of pure stands of bamboo cane.  Canebrakes formerly occupied hundreds of square miles of river bottomland in the southeast, but European settlers cleared them for agricultural purposes.  They were the most fertile pieces of land in the region.  Canebrakes depend upon a complex regime of flood and fire.  Suppression of either results in the growth of river bottomland forests that shade bamboo out.  During the late Pleistocene canebrakes attracted herds of bison and horses which fed on the nutritious bamboo.  Newly arrived humans facilitated the spread of canebrakes by setting fire to the landscape.  However, canebrakes must have also existed along the braided rivers of the Glacial Maximum, perhaps growing on the sandbars in the middle of partially  dried up rivers. The supermeander oxbow lakes likely hosted the last North American capybaras and giant beavers (Casteroides sp.) before they were hunted into extinction by people.

Reference:

Suther, Bradley; David Leigh, George Brook, and L. Yann

“Megameander Paleochannels of the Southeastern Atlantic Coastal Plain, USA”

Paleogeography, Paleoclimatology, and Paleoecology July 2018

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