Pennsylvanian Paleofloods

Hello all! This post marks two years of From Physics to English!
Today we’ll be looking at Pennsylvania–more specifically, the West Bank of the Susquehanna River there. This river has evidence for floods that swept through during the Pleistocene, 1.8 million to 11,700 years ago. Evidence of large floods is not easily erased from the geologic record, so information still remains. Along with evidence of large floods, the West Bank also shows evidence of river tributaries blocked by ice, creating lakes, which probably flooded once in a while. These floods would have washed water, sediment, and ice downstream, probably changing the landscape in its rushing water. So what did these ancient paleofloods look like, and what effect did it have on the river?
Enter today’s article, “Hydraulic modeling of glacial dam-break oods on the West Branch of the Susquehanna River, Pennsylvania”, by Jessica T. Newlin and Benjamin R. Hayes. Published in 2015 in the American Geophysical Union journal Earth and Space Science, these authors began by mapping the current riverbed and its features. Using data from previous studies, they set up a model of the past river conditions.
The geologic time scale. Taken from Portrait of a Planet, page 443.
The geologic time scale. Taken from Portrait of a Planet, page 443.
Present day, this section of the river averages 270 meters (295 yards) wide and 4 meters (13 feet) deep. It is mostly free of silt and sand, with a rocky bottom ranging from pebbles to small boulders; the sizes centered around 32 to 64 mm (1.25 to 2.52 inches). Nearby, there is evidence of river channels that could not have been formed by the modern river, hinting at how the glaciers influenced the area. These old riverbeds are now wetlands; previous studies showed some of them are 46 meters (50 yards) wide and the same depth as the current river. The sediments in the area vary, but some are notably sediments brought down in the water from a melting glacier. There is evidence of the glaciers advancing, then melting, then advancing when the climate turned colder again, from 10,000 to 770,000 years ago. 
Somewhere in the area of 770,000 to 970,00 years ago, Lake Lesley was formed when an ice dam blocked the West Branch. It lasted possibly 4,000 years and is estimated at over 100 kilometers (62 miles) wide, 150 kilometers (93 miles) deep, holding about 100 cubic kilometers (24 cubic miles) of water. A flood would have occurred whenever the ice dam melted and failed. This could have been periodic, as during the flood, ice could have moved to re-create the ice dam and temporarily stop the flooding, until the next one occured. A flood from this kind of ice dam scenario is also called a jökulhaup; for the ease of reading (and writing), I am simply going to use the term ‘megaflood’.
The authors set up a model of the past river conditions, modelling a flood where 8 percent to the entirety of Lake Lesley was drained. Various features from erosion and the deposition of sediments during the Pleistocene can be explained with the modeled megafloods. The simulated palefloodplains reach past the floodplains of a hundred year flood in the modern river, although not all areas currently containing sediments from the actual paleofloods were reached in the simulations. These discrepancies have several possible sources; the exact location of the ice dam is unknown as well as how much of Lake Lesley drained, and how often such floods happened. In addition, the model only looked at the main river, not including the contribution from tributaries. The authors, of course, call for further study, their work being an addition to what has already existed on this topic.

 

 

Bibliography
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Company. Print
My geology textbook, used for reference on glaciers and floods.
Molnia, Bruce F. Glossary of Glacial Terms. United States Geological Survey. 12 January 2013. Web.
A dictionary of terms used when speaking of glaciers. Used for reference and clarification of some terms.
Newlin, J. T., and B. R. Hayes (2015), Hydraulic modeling of glacial dam break oods on the West Branch of the Susquehanna River, Pennsylvania, Earth and Space Science, 2, 229243, doi:10.1002/2015EA000096
Today’s main article.
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