The Missouri River Basin experienced a significant drought from 2000 to 2007. During these years, Lake Sakakawea reached its lowest reservoir level (1805.8 feet), which resulted in the largest observed difference of 41.1 feet in water levels between the Lake Sakakawea and Lake Audubon. This difference in lake levels, described as a pool differential, increases the amount of seepage through the dam foundation. If not properly drained, this seepage can result in a large uplift pressure which initiates erosion of the dam foundation. If left unchecked, this can progress to a point at which the dam collapses into the eroded foundation.
USACE increased monitoring and surveillance of the Snake Creek Embankment during the drought and identified evidence that the foundation drainage system was no longer performing as designed. At the Snake Creek Embankment, relief wells are the primary form of foundation drainage, and instruments named piezometers measure the foundation uplift pressure. The relief wells were determined to be in poor condition, and the piezometer readings were consistent with this observation. There was no direct observation that erosion beneath the embankment had initiated or progressed, but from the available information USACE decided it was necessary to complete risk reduction measures to ensure the dam integrity. Recently in 2021, USACE inspected relief wells and found evidence they are not functioning as designed.
Even though the probability of failure is remote, risk reduction measures were warranted because loss of Snake Creek Embankment would result in significant economic damages to the four-lane highway, railroad, utilities, and require costly repairs. In addition, critical local and state water supply projects would be impacted, and environmental impacts would occur to the National Wildlife Refuge, the North Dakota Game and Fish Wildlife Management Area, and surrounding area.
Risk reduction measures included additional relief wells and piezometer installations through 2004 to 2008. An engineering underseepage analysis concluded that a pool differential threshold of 43 feet would additionally reduce risk by limiting uplift pressure. The pool differential restriction required a temporary deviation from the Lake Audubon Water Control Manual that expired when the drought ended in 2007. (A Water Control Manual is the guiding document that specifies how USACE operates a reservoir.) Basically, if an extended severe drought results in Lake Sakakawea being lowered to the 43-foot threshold, then Lake Audubon would be lowered as needed to maintain the 43-foot differential.
In 2019, USACE implemented a formal change to the Lake Audubon Water Control Manual to ensure safe and efficient operation of the embankment by adjusting the difference in water levels on either side of the Snake Creek Embankment. USACE adopted this change following the completion of an Environmental Assessment in accordance with the National Environmental Policy Act to evaluate impacts of various alternatives and the finding of no significant impact. The amended Water Control Manual stipulates a maximum pool differential between Lake Sakakawea and Lake Audubon of 43 feet during drought conditions until further dam safety studies and potential modifications can be evaluated. If an extended severe drought results in Lake Sakakawea being lowered greater than the 43-foot differential, then Lake Audubon would be lowered to maintain the 43-foot differential.
This Water Control Manual update prompted concern from North Dakota state agencies and the public about potential impacts to local water supply projects caused by a pool restriction during extreme drought conditions. The BOR completed an Environmental Impact Statement’s for the Eastern North Dakota Alternate Water Supply Project in 2021 which allocated water supply via Lake Audubon to McClusky Canal for various rural and urban water supply purposes in eastern North Dakota. Reliability of the project is dependent on the pool elevation of Lake Audubon. They also completed an Environmental Impact Statement for the Northwest Area Water Supply Project (2015) which is intended to meet future drinking water needs for local communities and rural water systems in northwest North Dakota. The Northwest Area Water Supply Project will withdraw water from Lake Sakakawea utilizing the Snake Creek Pumping Plant. A Snake Creek Pumping Plant pump unit will be modified to pump water north through a 36-inch pipeline. Reliability of this project is tied to performance of the Snake Creek Embankment in the vicinity of pump plant.
In response to state and local concerns, USACE initiated a Dam Safety Modification Study to evaluate opportunities to further reduce risks to embankment infrastructure and the significant economic and environmental consequences should the embankment fail. The Study also will evaluate reasonable structural and operational alternatives for future adaption to climate change in North Dakota, as well as consider the economic and environmental benefits Snake Creek Embankment provides. USACE is using current survey and geotechnical data collected in 2021 to assess the dam safety concerns. As part of the study, an Environmental Assessment will evaluate potential environmental and social effects of any proposed actions or modifications. Additionally, USACE is coordinating the early analysis of risk reduction alternatives with the BOR and GDCD.
It is important for interested organizations, agencies, community groups, and the impacted public to be involved in this Study. The proposed actions or modifications to address the embankment’s safety concerns, including potential impacts, will be available for review and comment prior to completion of the study.