Erosion and sedimentation in Lake Ashtabula, southeastern North Dakota

Erosion along the shore of Lake Ashtabula and nearshore and offshore sedimentation were studied from May 1969 to December 1970. The lake, located in the southeastern part of North Dakota, was formed when the Baldhill Dam stopped the flow of the Sheyenne River in 1950. Maximum depth of the lake is 45 feet in the inundated river channel immediately above the dam. The lake is 27 miles long and ranges in width from 1/3 to 1/2 mile. Shoreline erosion, measured at stations located at 100-foot intervals around the margins of the lake, was found to be a major source of sediment filling the lake. The shorelines have attained 6 percent of the projected shoreline erosion based on a stable shelf profile. All incoming wave energy is expended in internal turbulence when a stable shelf is attained. The shape of the profile is dependent on the wave form of deep-water waves, the angle of repose of sediments building the terrace, the slope of the exposed banks adjacent to the shoreline, the valley-wall slope, and the percentage of sand-sized and larger particles in the bank material. The main erosional processes along the shore are slumping, frost weathering, block separation, and collapse of overhangs. The most important conditioning factors are groundwater regime, shoreline orientation, shoreline use, and organic activity. The swelling and contraction of montmorillonitic clays in bank material is important in erosional processes. The role of the conditioning factors was analyzed for all shoreline stations using a number of specially written computer programs. Many turbid-water currents were traced using temperature, specific electrical conductance, and suspended solids. Suspended sediment settling out of turbid water fills the depressions on the lake bottom. The Sheyenne River channel, with 6 percent of the total area, has 19 percent of the total sediment accumulating in the lake. Sediment samples taken along· coring ranges were analyzed for particle size, organic content, total carbonate, mineralogy, and com paction. The total amount of sediment between each coring range was calculated. The accumulating sediment is gelatinous in appearance and has an initial water content of 80 percent. Compaction reduces the water content to 60 percent at 8 inches. Dominant minerals are dolomite, calcite, potassium feldspar, plagioclase, quartz, disordered cristobalite, and the clay minerals. Oxidizable organic content ranges from 8 to 12 percent. At the present 1.5 X 109 pounds (dry weight) of sediment has accumulated in the lake, with at least 5 X 108 pounds coming from shoreline erosion. A thin blanket of sediment is being built into the lake by the Sheyenne River. The lake will be completely filled in 5,000 years, based on the present rate of sediment accumulations. Because erosion rates will decrease as shoreline and other adjustment are made, a life expectancy of 10,000 years is suggested