Date of Award

January 2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Philip J. Gerla

Abstract

This thesis 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, drainage networks began to develop and were affected by isostatic adjustment. This adjustment began after the receding 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 one to four mm of uplift per year.

Rivers in the Red River Valley are constantly under continental scale tectonic forces. Little work has been conducted regarding the effect of isostatic adjustment on the pattern of post-glacial rivers and watersheds in the Red River Valley in its entirety. Isostatic adjustment is greatest in the northern Red River Valley where the ice was thickest, which has resulted in greater asymmetry in the watersheds farther north in the valley.

The purpose of this thesis is to determine if watersheds of Red River tributaries within the former glacial Lake Agassiz basin are asymmetric. The study further documented if asymmetry is 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.

Symmetry of each watershed was determined by comparing the following landscape measurements: Transverse Topographic Symmetry Factor (TTSF), Asymmetry Factor (AF), and the total net change in area 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. Twelve of the sixteen watersheds analyzed in this thesis have positive TTSF values indicating the main river channel is in the southern portion 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 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 displayed a northward shift in watershed boundary. Finally, most paleo-channels identified are north of their current river channel showing that rivers have shifted south. This study suggests 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. We believe that these methods can be used to investigate isostatic adjustment on tributaries in other landscape settings.

Download

Share

COinS