April 2025 Draft and Questions/Comments

Replacing the low-head dam with rock arch rapids should have no affect on invasive species distribution:

• There have been no reports of bighead carp, silver carp, or grass carp either upstream or downstream of Crissy Lake.
• Common carp already have established populations along the Pomme de Terre. For example, Artichoke Lake in Big Stone County and  Ten Mile Lake in Otter Tail county both documented common carp in their latest lake surveys.
• Zebra mussels have already been documented in Crissy Lake and multiple other waterbodies along the Pomme de Terre.

For more information, visit the DNR’s webpage on Aquatic Invasive Species.

Before construction, engineers use detailed flood modeling and calculations to understand the river’s power during extreme events. This modeling helps determine the correct rock size so the structure stays in place even during the highest floods. The large boulders used in the main parts of a rock arch rapid—usually 3 to 4 feet wide or more—are too heavy for floodwaters to move. These sizes are chosen specifically to withstand the river’s strongest flows

Rock arch rapids have already proven themselves in larger rivers than the Pomme de Terre and have successfully endured high flows and major flooding:

• The Mid-Town Dam on the Red River in Fargo/Moorhead was replaced with rock arch rapids in 1999. The river’s peak flows there are three times higher than those at Appleton on the Pomme de Terre, and the structure has handled record floods without needing repairs.
• The Crookston Dam on the Red Lake River was replaced in 2005. It also sees peak flows three times higher than those on the Pomme de Terre. Some rocks at the top shifted during early floods and were repaired in 2007. That site also deals with very large ice and debris jams due to a narrow bridge upstream. After minor repairs and an added weir, the structure has held up well—even with nine feet of water drop.
• The Willow River Dam in Pine County failed in 2016 and was replaced with a rock arch rapids in 2022. The new design was made specifically to withstand flows greater than the flood that destroyed the original dam.

• Houston Engineering webpage on dredging. 
• Pigs Eye Lake restoration project where dredged material was used to create 40 acres of island habitat, featuring sand benches, terrestrial plantings, and protected marshes. in St. Paul, MN.
• UMN-Duluth study done on the soil fertility, floristic inventory, and survivability of restoration using dredged material.

  

Rock arch rapids are designed to safely handle ice and reduce the kinds of problems often seen with traditional dams. Unlike dams, they don’t have structures above the water—like pilings, abutments, or catwalks—that can trap ice and create blockages or debris jams. Their sloped design also helps. As water levels rise, the banks allow more room for water and ice to pass through. This makes it easier for ice, logs, floating bogs, and other debris to move downstream without getting stuck.

Rock arch rapids have also been successfully used in much larger lakes, where ice conditions are far more extreme than at Crissy Lake (114 acres). For example:

• Otter Tail Lake (14,117 acres – 124 times larger): Built in 2023, no issues with ice.
• Shell Lake (3,147 acres – 27 times larger): Built in 2017 on the windy side of the lake, no problems since.
• Norway Lake (519 acres – 4.5 times larger): Built in 2022 with a steeper design than what’s proposed for Crissy Lake—again, no ice issues reported.