Fellow: Benjamin York
Advisor: Phil Gerla

This research investigates the relationship between glacial isostatic adjustment and watershed asymmetry of tributaries in the Red River Valley, North Dakota, U.S.A. After the draining of glacial Lake Agassiz, channel networks began to develop and were affected by isostatic adjustment. This adjustment began after the recession of the Laurentide Ice Sheet and is still occurring today, but on a lesser degree. Adjustment in the Red River Valley, which has varied since the ice sheet retreated, is determined from differences in the elevation of the horizontally deposited beach ridges which are the ancestral beaches of glacial Lake Agassiz. The Red River Valley is currently experiencing 1 to 4 mm of uplift a year.

Untested in the Red River Valley is the concept that large-scale isostatic adjustment influenced the pattern and development of post-glacial rivers and watersheds. Because isostatic adjustment was greatest where ice was the thickest in the northern valley and least in the southern valley, watersheds should be more asymmetrical, with the main river channel offset from the center of the watershed, the farther north they are positioned in the valley. The purpose of this thesis is to determine if tributaries within the former glacial Lake Agassiz basin are asymmetric. The study further documented if asymmetry was the result of 1) changing watershed boundary; 2) a shifting river channel position; or 3) a combination of both a changing watershed boundary and a shifting river channel. Asymmetry of each watershed was determined by comparing the following landscape measurements: Transverse Topographic Symmetry Factor (TTSF), Asymmetry Factor (AF), and the total net change between pre-adjustment watersheds and current watersheds. Along with the measurements listed above, paleo-channels were identified in the Red River Valley to determine if there has been a uniform shift in drainage between Lake Agassiz stages and isostatic adjustment.

Significant of Research

The results of this study suggest that asymmetry in the watersheds is the result of a changing watershed boundary and a shift in river position, likely associated with glacial isostatic adjustment. The combination of TTSF and AF values, pre-adjustment watershed net change, and location of paleo-channels could help quantify the relationship between glacial isostatic adjustment and watershed development. I believe that these methods can be used to investigate isostatic adjustment on tributaries in other landscape settings.

Significant Findings

Twelve of the sixteen watersheds (Forest, Goose, Otter Tail, Park, Rush, Sand Hill, Snake, Tamarac, Turtle, Two Rivers, and western Wild Rice) analyzed in this thesis have TTSF values that are positive, while the remaining four watersheds (Buffalo, Maple, Red Lake, and Wild Rice) have negative values. A higher positive value indicates a river that lies farther north relative to the center of the watershed. Watersheds displaying the most asymmetry based on TTSF are farther north in the Red River Valley. Similarly, AF values reveal that the most asymmetric watersheds are also located near the northern part of the Red River Valley and suggest greater tilting has occurred, compatible with isostatic adjustment. Furthermore, analysis of the change in watershed boundaries revealed that all but one displays a northward shift in watershed boundary. Finally, paleo-channels were identified and associated with the current Turtle, Elm, Sand Hill, Buffalo, western Wild Rice, and Maple rivers, which flowed into the paleo-Red River. Six of the seven paleo-channels identified lie north of their current river channel, showing that rivers have shifted south. These results suggest that the asymmetry identified using the TTSF and AF values would result from a shifting river rather than a shifting watershed. This does not imply that a shift in watershed did not take place, but rather that the asymmetry observed is not solely the result of a change in watershed boundary.