Sedimentation in the Upper Missouri River Basin
By John Remus
U.S. Army Corps of Engineers
Since construction of the six mainstem dams and filling of the Missouri River mainstem system in 1967, sediment continues to deposit where streams enter the reservoir pools.
These deposits increase water surface elevations and groundwater problems. As a result, the U.S. Army Corps of Engineers (Corps) has increased its project boundaries at a number of locations and settled damage claims at nearly every mainstem reservoir due to sedimentation/erosion-related problems.
The entire upper Missouri River mainstem system has increased flooding and groundwater problems, bank and shoreline erosion, and legal claims over government boundary lines, water quality issues and loss of hydroelectric power generation capacity as a result of sedimentation in the headwaters areas of the mainstem reservoirs.
Shoreline erosion threatens dozens of cultural sites and contributes to water quality problems for at least three municipal water intakes. Stream bank erosion downstream of the dams threatens residential areas, causes loss of agricultural and forested land, degrades wetlands and damages irrigation intakes.
Problem areas are located throughout the entire basin and include the White River delta that has effectively cut Lake Francis Case (Fort Randall Dam) in half, resulting in increased flooding upstream of the delta and decreased head for hydropower downstream of the delta. The decrease in head is as much as 12 feet.
Flooding and power constraints in the Bismarck-Mandan, ND area, where potential buyouts are estimated at over $100 million.
Flood and power constraints at Pierre/Ft. Pierre, where acquisition, relocation, and/or flood proofing of just a portion of the affected area is estimated to cost more than $70 million.
Aggradation-related problems include raising Highway 12 near Niobrara at an estimated cost of more than $30 million. Property claims along the Niobrara River have resulted in $100,000 in payments with pending lawsuits valued at more than $10 million.
Claims along Ponca Creek total $600,000 and land acquisition in the Buford-Trenton area is estimated at more than $35 million.
Cost of lost hydropower generation varies depending on time of year and the value of power; however, restrictions are generally associated with severe cold weather, when the need for power is highest.
Cultural resource losses have required the Corps to alter reservoir operations. These adjustments reduce flexibility to meet project purposes resulting in loss of hydropower, decreased flood control capacity and impacts to endangered species.
Recent and ongoing efforts within the Corps to address sediment-related problems on the mainstem reservoirs are:
1. Niobrara and Missouri River, SD and NE, Reconnaissance Study Update.
Primary purpose was to determine if a plan exists with federal and non-federal interests to serve as the basis for initiating a feasibility study. A plan of study was developed and coordinated with local interests, and ranges of alternatives were studied including dredging, canals, diversions, sediment transport methods and a plan to maintain access to recreation areas. Study findings indicate there is no cost effective plan with federal interest. No non-federal sponsor was identified, thus the study has been terminated. Although this study was terminated, sedimentation problems still exist and will continue to worsen. Because of the significance and severity of this problem, the Corps pursued additional investigations into sediment flushing. A concept-level study indicated there might be a technically viable sediment-flushing plan. Sediment flushing is also included as an element of the Missouri River biological opinion. The Corps has included additional study of the sediment flushing at Gavins Point Dam in the Missouri River recovery plan. If recovery funding is sufficient additional sediment flushing studies will begin in fiscal year 2006.
2. Pierre/Ft. Pierre Flood Mitigation Project.
Since Oahe Dam was put into operation in 1958, flooding in the Pierre and Fort Pierre area has been confined to low-lying lands adjacent to the river. This shallow flooding in the low-lying areas has been caused primarily by water backing up in the storm sewer system of Pierre and out into the streets, businesses and homes, most frequently in the southeast Pierre subdivisions. During the winter ice cover in the Missouri River downstream causes additional problems from Oahe Dam resulting in significantly higher stages for a given discharge than during open flow conditions. This causes increase in operation constraints on hydropower production during the winter. Additionally, sediment, primarily from the Bad River, continues to accumulate in the Missouri River adjacent to and downstream of the Pierre/Fort Pierre area and continues to contribute to higher stages in water surface during summer and winter conditions increasing the severity and reoccurrence of flooding problems. The Corps’ Omaha District is in the final stages of completing a $35 million buyout/flood proofing project in the Pierre/Fort Pierre area that will alleviate a significant portion of this problem. However, the problem will not be completely solved. If no further action is taken the present constraints to power generation will continue to gradually increase. Ultimately, power generation could be limited to 25 percent of capacity during the three-week constraint period of highest demand in the winter.
3. Buford-Trenton Land Acquisition (Williston, ND Area):
The project consists of the acquisition of permanent flowage and saturation easements on approximately 11,750 acres from about 55 landowners. The primary problem is increased groundwater levels making it impossible to grow sugar beets. Sugar beets are a very high dollar cash crop compared to corn, wheat and alfalfa. High groundwater is caused by sediment deposited in the headwaters of Lake Sakakawea (Garrison Dam) just west of Williston, ND.
