Date of Award

1993

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

P.J. Gerla

Abstract

Agnes Marsh is a semipermanent wetland in the Red River Valley of northeastern North Dakota. The marsh overlies and is in direct hydraulic connection with the unconfined Elk Valley aquifer. Recent irrigation development nearby raised concerns about the hydrological impact to the marsh by a declining water table in the aquifer. The purpose of this study was to determine how Agnes Marsh interacts with the underlying Elk Valley aquifer and how sensitive the marsh is to declining water levels in the aquifer.

Three methods were used to assess how the marsh interacts with the aquifer. First, stratigraphic analysis revealed the nature of marsh and aquifer sediments and provides data on the spatial distribution of hydraulic conductivity. Second, piezometer nests were installed to characterize the temporal and spatial variation in the ground-water flow system. Third, an estimate of the wetland water budget was used to determine the wetland hydrologic function and sensitivity to hydrological changes in the aquifer.

The estimate of the water budget for the marsh and an analysis of the ground-water flow system revealed the sensitivity of this hydrological relationship. Precipitation and open water storage initially supported evapotranspiration. As the open water receded, precipitation and aquifer storage supported evapotranspiration. The marsh functioned as a closed system, neither contributing water to nor deriving water from the ground-water flow system. Snowmelt and precipitation create ground-water mounds which are consumed by evapotranspiration before dissipating laterally. A decline of the water table as a result of irrigation may affect pond permanence, but the marsh plant community may still be supported by periodic water table mounds.

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