THE RESPONSE OF U.S. EAST COAST RIVERS TO GLACIAL-ISOSTATIC ADJUSTMENT

On glacial-interglacial timescales, solid Earth adjustment to variations in ice and water loads produces vertical motion at rates up to 10 mm/yr, matching or exceeding rock uplift rates in these regions. Rivers, because they are highly sensitive to changes in water surface slopes, respond to such surface deformation by adjusting channel dynamics.

Here, we test whether glacial-isostatic adjustment (GIA) explains abrupt transitions in river dynamics observed in the Hudson, Delaware, Susquehanna, and Potomac Rivers from 40-15 ka. During this interval, the eastern coast of North America experienced high rates of GIA-induced crustal deformation along the peripheral bulge, surrounding the Laurentide Ice Sheet. We calculate the perturbation to river channel slope and drainage area due to glacial-isostatic adjustment and find the resulting change in transport capacity is consistent with episodes of erosion and sedimentation in these rivers.

The Potomac River provides a particularly powerful case for GIA-driven incision. The latest period of incision began after ~37 ka, when the river abandoned and exposed a strath terrace (the former river channel) hundreds of meters wide that abuts the river between Great Falls and Black Pond. Cosmogenic dating of the strath terrace along this stretch of the channel does not show upstream younging, suggesting that knickpoint propagation did not cause increased erosion. The timing for this pulse of incision is coincident with fast uplift rates induced by a rapid growth of the eastern Laurentide Ice Sheet beginning ~45 ka. Our modeling of glacial isostatic adjustment predicts that slopes, averaged over a 10 km stretch, in this zone of the Potomac River became 7x steeper over this time interval, potentially explaining the initiation of incision.