Date of Award

1995

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geological Engineering

Abstract

Renwick Reservoir, on the Tongue River near Cavalier, North Dakota, was constructed in 1962 as part of a flood control project. The cost at completion was about $350,000. The reservoir was built assuming a 50-year life expectancy. North Dakota's W ater Quality Assessment Plan concludes that sediment accumulation has already diminished storage capacity and negatively affected recreational uses. Primary problems include pollutants consisting of nutrients, sediment accumulation, turbid water and organic enrichment. Forty-one percent of the sediments deposited are in the newly-formed riparian wetland at the headwaters of the reservoir. This problem illustrates a need to characterize wetlands in reservoirs. Sediment samples collected throughout the wetland were analyzed by the N orth Dakota State Health Departm ent (NDSHD) for nitrate and nitrite, ammonia, and phosphate content. Textural analysis was also conducted. Piezometers were designed and installed in the wetland to m onitor ground water movement. Stream gaging and water sampling were conducted one mile upstream from the reservoir to determine the volume and quality of water entering the reservoir. N utrient content was determ ined from the water samples by the NDSHD. Textural analyses revealed that wetland sediments are silty clays and clayey silts with a variably low sand content. Sediment sample nitrate/nitrite levels were found to be greatest near the reservoir. Ammonia concentrations were greatest in the middle and bottom sections of the 1 meter long cores, probably reflecting active denitrification and anaerobic conditions. Phosphate levels are consistent throughout the wetland. Hydraulic head measured in the piezometers indicate upward ground water movement in the wetland. This suggests that nutrients can be released from wetland sediments into the reservoir and may exacerbate eutrophication. River discharge was found to be greatest during spring runoff and seems to be affected by precipitation only in the late summer and fall when evapotranspiration rates have diminished. N utrient concentrations are elevated during periods of greatest river discharge. This indicates that spring runoff and evapotranspiration may be im portant factors that control nutrient load.

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