Late Pleistocene Megameanders

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.

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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The Paleo-Apalachicola River

The transition from the Ice Age to the much warmer climate phase that ended it about 15,000 years ago caused a dramatic change in the river systems of southeastern North America.  Average annual temperatures suddenly approached those of the present day, and the glaciers that covered Canada began to melt, resulting in the creation of massive glacial lakes along with a rapid rise in sea level.  More water could now evaporate and enter the atmosphere.  Storm events exceeded what present day inhabitants of earth experience today.  During the Ice Age the water table fell and smaller rivers and creeks disappeared from the surface, while larger rivers suffered a lesser discharge and were clogged with sandbars.  But when precipitation increased decisively as the glaciers melted, larger rivers swelled and began to meander, and for awhile they meandered to a greater degree than they do today.  Geologists refer to these as supermeanders.  Smaller rivers and creeks reached the surface again after the water table rose.

The Apalachicola River drains the Chipola, Chattahoochee, and Flint Rivers.  It was longer and had a larger discharge during the late Pleistocene.

In my blog article last week, I mistakenly stated the Aucilla River in Florida didn’t exist until 7,000 years ago, but when I read the scientific literature in preparation for this article, I learned that I was off by about 6,000 years.  (I’ve since edited the correction.)  The Aucilla River likely existed as a subterranean stream throughout the Ice Age, and it was probably the source for the springs that fed the ponds I discussed last week.  It emerged above ground probably about 13,000 years ago.  By 11,000 BP the paleo-Aucilla River had a greater discharge than it does today.

I wonder if this change was perceptible to the Paleo-Indians who frequented the spring-fed ponds that occurred along the present day course of the Aucilla River.  Some time between ~14,000 BP-~13,000 BP increased rainfall enlarged the ponds to the point that a river began flowing between them.  This might have occurred within the lifetime of a long-lived Indian.

The Apalachicola River is much larger than the Aucilla River, and it too had a greater discharge immediately following the end of the Ice Age.  During the late Pleistocene sea level was still much lower than it is today, and the Apalachicola River flowed over the continental shelf for 50-100 miles–territory that is now the Gulf of Mexico but was dry land then.  Seismic evidence suggests 1 of the now submerged river channels of the Apalachicola was 300 yards wide and 60 feet deep.  The phase of rivers with supermeanders and higher discharges lasted for about 5,000 years before they stabilized to modern day conditions.

Side scan sonar image of a submerged river valleys off the coast of Nova Scotia.  I couldn’t find a sonar image of the paleochannels off the Florida coast on google images.

The Chattahoochee, Chipola, and Flint Rivers join to form the Apalachicola River.  They meet near the border between southwestern Georgia and Florida.  During the Miocene over 5 million years ago this was the site of an enormous bay known as the Apalachicola embayment.  Imagine a bay where 3 major rivers emptied their contents.  Eventually, sea level fell and these rivers met and formed the Apalachicola River.

An aerial photo of the Apalachicola River as it meanders toward the Gulf of Mexico.  The paleo-Apalachicola River was probably clear, not muddy, for most of its existence.  Anthropogenic erosion has ruined the pellucid quality of the river.

Reference:

Donoghue, J.F.

“Late Quaternary Coastal and Inner Shelf Stratigraphy, Apalachicola Delta Region, Florida”

Sedimentary Geology 80 1992