Date of Award

2001

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

W.D. Gosnold Jr.

Abstract

Devils Lake is the largest lake in a chain of approximately 20 interconnected lakes in east central North Dakota. The Devils Lake drainage basin of approximately 8,600 km2, lies in a bowl sculpted by glaciation; thus it is closed and receives and loses water only through precipitation and evapotranspiration. Since 1940, the lake level has risen 14.4 m due to increased precipitation. Geological data show that in the past 10,000 years the lake has alternately dried up and filled due to climate fluctuations (Murphy et al., 1997). During this time it has reached its spill elevation of 444. 7 m msl and has overflowed into the Sheyenne River at least three times.

The rise in lake level has resulted in an increased load of 3.32*1013 N from water on Earth's crust. Flexure models show that, using this load, the lithosphere should be deflecting approximately 30 cm at present, assuming mantle viscosities ranging from 1020 to 1021 Pas, Poissons ratio of 0.22 and an elastic lithosphere thickness of 30 km.

Three high resolution, dual-frequency GPS surveys in a leap-frog array were completed in 1999, 2000, and 2001 . The survey of 59 National Geodetic (NGS) benchmarks in a 300 km east-west profile across the basin has shown the lithosphere is actively flexing due to the water load. The survey also showed a systematic increase in difference between the published and observed elevations of the basin is -35 cm and the signal extends at least 150 km from the center. The reason for the flexure is that over a short time scale the water load flexes the lithosphere and overwhelms isostatic equilibrium. The lithosphere at the center of the basin subsides over a relatively wide area by displacing asthenospheric material and is accompanied by the elevation of the peripheral lithosphere. Currently, the lithosphere beneath the basin is out of isostatic equilibrium due to the water load.

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