Study to review effects of retiring a stretch of the Mississippi River in Minneapolis

An 18-mile stretch of the Mississippi River, mostly in Minneapolis, has been prodded, poked and measured with an eye toward seeing how it changes now that it's no longer a working river.

The state-backed effort will attempt to measure over three to five years how shuttering the Upper St. Anthony Falls lock, stopping the dredging of the navigation channel and ceasing shipping affect the river.

Will the channel silt in so much that recreational boating is more difficult? Will the variety of mussels in the riverbed diminish? Will new islands emerge?

All are possibilities because of the change in river use wrought by a congressionally mandated closing of the upper lock last June. It's something environmental groups lobbied for to deter upriver migration of invasive carp, but it also boosts the city's plans to remake the upper river with parks, housing and business parks. Barging interests fought the change.

The Minneapolis Riverfront Partnership found no previous research nationally documenting how other lock closings have affected their rivers, according to Executive Director Kathleen Boe. The advocacy group is the lead agency for the $190,000 first phase of the study, documenting physical, chemical and biological conditions in the river as of the lock closing. The Mississippi Watershed Management Organization, based on the northeast riverfront, the University of Minnesota's St. Anthony Falls Laboratory and the Minnesota Department of Natural Resources, are cooperating in the work.

Researchers are cautious in describing their expectations, due both to the lack of precedent and the length of time since the river ran unfettered.

"They're very, very complex systems without humankind even messing with them," said Stephanie Johnson, a watershed staff member.

The new river study started with a survey of data that was already being collected. Boe said data was being harvested by multiple agencies, but there was a lack of consistency in where and how often that was done.

Then, some special studies were done. The watershed agency built a profile of the riverbed with a device like a depth finder that compiled riverbed depth readings along a grid of locations along the river. That will be repeated next summer to see how the submarine terrain changes.

"That's one of the debates we're having — how quickly will you see change?" Boe said.

But with the Army Corps of Engineers previously dredging an average of 43,000 cubic yards annually, stopping that likely means changes in water depth.

The watershed also upped its previous sampling of water chemistry, providing a baseline for any changes there. A special DNR survey documented the mussel population. Another special survey this fall sampled the river's invertebrates — crayfish or dragonfly larvae, for example.

The work plan for the study suggests that lessons learned about how the river responds to changing conditions can provide insight on how to manage it, and other rivers.

One question researchers are curious about is how more sediment deposits on rocks in the river will affect fish habitat. They also wonder if riverbank erosion will lessen with fewer barge tows generating wakes.

And plans to open the upper lock at St. Anthony Falls only when high flow requires bypassing some water could affect mussel populations. That's because the lock previously allowed fish bearing larval mussels to bypass the falls, making for a more diverse mussel population than before the lock opened in the 1960s.

"This will be the most significant transformation of the river in our lives," Whitney Clark, executive director of Friends of the Mississippi River, told a forum last winter anticipating the lock closing.

Steve Brandt • 612-673-4438