Luke J. Reusser of the University of Vermont and his colleagues investigated how quickly the rivers cut through their rocky basins at two deep gorges--Holtwood Gorge on the Susquehanna (see image) and the Great Falls of the Potomac. Taking several samples from terraces above each river, the scientists measured trace amounts of 10-beryllium, an isotope created by cosmic rays smashing into oxygen atoms within the rocks. The more 10-beryllium present, the earlier the river's carving exposed the surface. The team reports today in Science that, around 35,000 years ago, both rivers started stripping away about a meter of bedrock every thousand years. This incision rate is between 10 and 100 times higher than the long-term averages from the same region. In fact, it is comparable to those of rivers in the more mountainous Himalayas.
The period of greatest erosion, which subsided about 13,000 years ago, coincides with the timing of the last glacial maximum of the late Pleistocene era. Because the rapid wearing away of rock requires intense flooding, the natural assumption is that glacial melt was responsible for the rivers' swelling. But over the given time period only the Susquehanna Basin--and not the Potomac--was partially ice-covered. "If glaciers mattered, we would have seen big differences in the two rivers," explains co-author Paul R. Bierman, also of the University of Vermont. Because the two gorges were carved out equally fast, the geologists speculate that the implied flooding was instead the result of an increase in storms. This hypothesis is supported by evidence from Greenland ice cores: a rise in the cores' sodium concentration during the glacier maximum indicates an increased amount of sea spray in the atmosphere and hence more storm activity.