Stabilize Riverbank to Protect Archeological Resources and Infrastructure

The National Park Service is proposing to stabilize the Elbee Bend of the Knife River, an important step in the implementation of the ARMP/EIS.

The project is anticipated to include the following features:
• Installation of void filled longitudinal peak stone toe protection (LPSTP) up to the Ordinary High Water elevation throughout the bend

• An constructed embankment above the LPSTP with a 3.75 ft layer of soil riprap with earthen cover for vegetation growth

• A bench incorporated to facilitate construction and future maintenance and enhance recreation opportunities

• Riverward of the bench, the soil riprap will have slopes of 3H:1V for improved stability, above the bench varied slopes of 2.8H:1V to 3H:1V to accommodate existing bank variances

• All earthen slopes fitted with erosion control matting and seeded to establish natural vegetation post construction.

• Partial fill of two small drainage swales (resultant of head-cutting due to overbank flooding) at the northern half of the site to facilitate proper drainage

• A 50 ft buried refusal installed at the upstream end of the project to address flanking risk

• A transition to a simple revetment at the existing grade at the downstream end of the project

• Excavation on the opposite bank to offset encroachment on the existing river section

• A 10 ft average width, 3 ft deep blanket of riprap installed at the toe trenched into the riverbed to mitigate scour forces at the toe of the LPSTP

Riverbank erosion has been an ongoing problem since park establishment, and archeologists have identified flooding and erosion as the greatest threats to the park's archeological resources. The Knife River runs the length of the 1,758 acre park and is a tributary to the Missouri River downstream of the Garrison Dam Project. It is estimated that significant amounts of riverbank along the Knife River running through the park have been lost over the past few decades, leading to the loss of archeological and historic resources. Village remnants and archeological sites adjacent to the Knife River have experienced measurable erosion.

In 2018, the park completed an Archeological Resources Management Plan/Environmental Impact Statement (ARMP/EIS) to address threats to archeological resources, including riverbank erosion (PEPC # 34314). The plan identified a suite of treatment options for riverbank erosion and prioritized archeological sites based on importance and level of risk to inform management decisions. The Elbee Village located in the Elbee Bend of the river was identified as the highest priority riverbank stabilization need for the park. In addition to fundamental park resources, erosion in the Elbee Bend threatens County Road 37, park fencing, and utility poles.