Grindstone River

The Minnesota Department of Natural Resources is proposing to remove the dam on the Grindstone River in Hinckley. The dam impounds the Grindstone Reservoir, a 26-acre public water basin within the state-owned Hinckley Aquatic Management Area.

Removal of the dam would drain the reservoir and restore the channel of the Grindstone River in that area. This project will eliminate the ongoing costs of dam maintenance, remove a public safety hazard, and restore natural river flows and connectivity.

Dams have been at this location for various uses such as logging, hydropower, and water storage since the late 1800s. The former Minnesota Department of Game and Fish, now part of the DNR, built the current dam in 1931 to provide a water supply for fish rearing ponds. The dam failed in 1944 and again in 1954 due to high water events. Several major repairs have been necessary since then, with the most recent in 2014.

In 2017 the DNR Dam Safety Unit inspected the dam and found it to be in stable but poor condition. Keeping the dam in place would require costly ongoing maintenance and is not considered an option. Rebuilding the dam would also be expensive. The reservoir could be maintained after dam removal by creating a series of rapids downstream of the dam site, but bridges and natural features in that area present engineering challenges that could be costly. For these reasons, the DNR decided that dam removal was the best option.

The project will be completed in three phases:

1. Drawdown of water from the reservoir.
2. Demolition of the dam.
3. Restoration of the river channel.

Heavy equipment such as backhoes will be used to gradually lower the level of the dam, allowing a controlled drawdown of the water in the reservoir. When the drawdown is complete, the remains of the dam will be demolished and removed. These actions will likely take place during periods of low flows in late summer.

Restoration of the river channel may occur naturally or some excavation and grading may be needed to ensure that the channel is stable and not eroding. Drawdown and demolition will take place over a period of weeks, while restoration of a stable river channel may take a year or longer.

For more details on these project phases, please see the scoping environmental assessment worksheet.

Ecological benefits of dam removal

Scientists have long recognized the negative impacts that dams have on river ecosystems. The most obvious impact is the barrier a dam creates for fish migration. Fish in river systems often migrate many miles in a year to find the best habitat for different stages in their life cycle. Removal of the dam at Hinckley would reconnect 24 miles of the North and South Forks of the Grindstone River with the lower Grindstone and Kettle River.

Mussels also depend on fish being able to migrate freely in rivers. Adult mussels are sedentary, but the larvae spend part of their life cycle attached to the gills of fish so they can move to different areas of the river. There are several threatened mussel species in the Grindstone River.

A free flowing river also offers better habitat for more species of fish and the insects that fish feed on than a small shallow reservoir. Fish surveys have shown more species in the Grindstone River below the dam than in the reservoir or upstream in the north and south forks of the river.

Dams can also affect water temperatures in the reservoir and downstream. The longer water is held back on warm sunny days, the higher the surface temperature becomes. This warmer water can affect fish communities downstream from the dam, favoring warm water species such as smallmouth bass over cool water species such as northern pike and walleye.

A lesser known but ecologically important benefit of dam removal is the restoration of normal sediment transport within the river. A flowing river moves fine particles of sediment along with the water. The amount and size of sediment is determined by how fast the water is moving. When a dam holds back water, the reduction in flow causes sediment to settle out and build up behind the dam. The water that flows over the dam has lost this load and is “sediment hungry”—that is, not carrying fine particles. This water has more energy and is more likely to cause erosion in downstream areas than if the normal flow was not interrupted by a dam.

Change in recreational opportunities

Removal of the dam will change the nature of recreation in the reservoir area. Currently there is a boat launch and fishing pier that receive light use. These will be removed, but people will still have access to the river for fishing, swimming, and kayaking without the safety hazard the dam creates. Fishing is expected to improve due to the restored connection with the lower Grindstone River and the increased diversity of habitat that will be available. There will be interpretive displays at the Hinckley AMA describing the history of the dam site.