Mississippi River Lock and Dam 1, Ambursen Dam Downstream Repair

U.S. Army Corps of Engineers, St. Paul District
Published Feb. 26, 2015
Updated: May 3, 2023

The purpose of this project is to re-establish armor downstream of the concrete apron and prevent further scour.


Lock and Dam 1 is located on the Minneapolis, Minnesota side of the Mississippi River.


The existing rock protection consists of grouted derrick stone. The grout has failed resulting in the derrick stone being broken up and washed downstream, exposing bedding material and risking failure of the wooden piles and sheet pile located underneath the existing concrete apron.

The St. Paul District performed an analysis in fiscal year 2013 and determined that placing bedding and riprap in the scoured area from the concrete apron extending 150 feet downstream is the most efficient and effective solution. Construction involves a removal of the remaining grouted riprap and derrick stone, excavating excess material immediately downstream of the concrete apron, and using the material to smooth out the downstream topography and placing bedding and rip rap into the scour/repair area.


The challenge with this project is that in recent years high flows over the spillway have not allowed for construction work downstream of the spillway. There are no alternative flow paths through or around the dam. For the most part, work is limited to low flow periods, which typically occur in the September – February time frame.

The St. Paul District awarded the initial construction contract in January 2015. Persistent high water (flow greater than 6,000 cubic feet per second) has prevented the contractor from accessing the area.

In November 2018, site surveys were conducted to evaluate the conditions as a result of the continued high flows since contract award. Surveys indicated that there has been some additional deposition in the area, but the scour has not progressed.

The original repair method was reviewed in late fiscal year 2021. The St. Paul District evaluated a more efficient way to manage spillway flows to increase the range of discharge conditions under which the work can be performed. The evaluation noticed the scour hole has not been active. Therefore, the updated repair will be able to use some of the rock already there with the addition of a smaller quantity of rock. The rock will be repositioned to address any future possible scour.

The contract was terminated in July 2019. Contingent upon funding, plans and specifications will be prepared for re-solicitation.


Congress authorized the project as part of the River and Harbor Act approved July 3, 1930.


Estimated repair cost                                   $700,000