Reporter's Notebook: Covering the impact of rock riprap on Minnesota lakes

NINA MOINI: Have you ever noticed how many lake homes have large rocks lining their shorelines? Well, those layers of rocks, known as riprap, are often meant to fight erosion and keep soil from crumbling into the lake due to waves or high water. But rock riprap brings a whole raft of other problems with it. Kirsti Marohn has been reporting extensively on the subject and is here to share what she's learned as part of a check-in with our reporters in greater Minnesota. Thanks for being with us, Kirsti.

KIRSTI MAROHN: Hi, Nina. Thanks for having me.

NINA MOINI: Kirsti, tell us how you got interested in looking into this topic.

KIRSTI MAROHN: Sure. Well, I actually live in the Brainerd Lakes area. I grew up there and have lived there now again for a couple of decades. So I try to spend a lot of time on the lakes. We don't live on a lake, but try to get out as much as I can on the water. And if you do that, if you've done that over the last decade or so, you've probably noticed there's a lot of changes to the shorelines of Minnesota lakes, not just in the Brainerd area, but throughout the state.

And we wrote about this last summer in our series "Trouble by the Water," about how natural shorelines are changing. And one of the things we really noticed was that a lot of shorelines that used to be kind of natural and woodsy, they now have these rock edges, like, lined with rock. And we started wondering, how does this affect the lake? And so we decided to hone in and look more closely at that topic of rock, which is called actually rock riprap, and why people are choosing this option.

NINA MOINI: Yeah, a lot of alliteration there. But what did you find out about why people are putting the rock on their lakeshore?

KIRSTI MAROHN: Well, there's a few different reasons. One is some people just like that look. It's just clean and tidy. It's also kind of almost like a status symbol of that up north lake life, the big home and the patio and the boats and the docks and all of that.

But then, of course, a of people are just really also worried about erosion. And there are some real concerns. I mean, we've got climate change affecting these-- causing these extreme rainfalls and causing lake levels to fluctuate, creating these ice heaves that push up on the shore in the winter. So there are people who are really concerned about losing that valuable lakeshore.

And we also have larger recreational boats that are creating these big waves that kind of pummel the shore. So some people think putting this rock riprap is a solution that will hold their shoreline in place.

NINA MOINI: But not a perfect solution, right?

KIRSTI MAROHN: That's right. There's a few reasons why. Number one, it just doesn't slow the water that runs off your lawn and the landscape into the lake. It doesn't filter out any of the nutrients in that water, like phosphorous. And that can cause algae blooms in the lake and actually can warm the lake, because those rocks, they absorb the heat from the sun. And then that warmer water also creates better conditions for algae growth. And then, also, it doesn't provide any habitat for fish or turtles or amphibians or loons nesting or anything like that along the shoreline.

NINA MOINI: And so when you were doing your reporting, you spent a lot of time actually on the water, which makes sense. Tell me about that.

KIRSTI MAROHN: Yeah, I actually went out on a boat with a DNR hydrologist. We went out on the Whitefish Chain of Lakes, which is just north of Brainerd, very popular chain. There's still quite a bit of natural lakeshore, but also some very large, expensive homes and a lot of big lawns and a lot of this rock riprap. So he walked me through how homeowners could maybe think about their shorelines differently and some better ways to prevent erosion, actually, before it happens so you don't have to resort to this rock.

NINA MOINI: Oh, and what did he have to say?

KIRSTI MAROHN: Well, he said, a lot of erosion could be prevented if homeowners kept their shoreline more natural. And that means not cutting down the trees, having deep-rooted native plants and trees and other natural vegetation that will help hold the soil in place. And it also kind of buffers those waves so that your shoreline doesn't start to crumble.

But then he also talked about-- there's a question of whether you really have erosion or not on your property. There is some natural erosion that occurs along lakes, lakeshore, and it doesn't always require immediate action, like putting on rock riprap. Sometimes property owners can protect their shorelines or repair erosion just by using some of these more natural methods.

NINA MOINI: And how does that work?

KIRSTI MAROHN: Well, I got to see firsthand one example. There was a shoreline restoration project using something called a willow wattle, which I had to learn what that was. They used bundles of dried willow branches, and they kind of bundle them together, tie them with twine into these long-- it's almost like a natural fence, and it's called a wattle. And then they put that up against the shoreline and use stakes to pound it in place.

And then they plant a ton of native plants, and those kind of hold the soil in place. The willow wattle protects the plants and gives them a chance to grow so they don't get washed away. And then the wattle also allows the water to filter through, but it holds back the dirt and organic matter. So it actually kind of helps rebuild the shore.

And then, of course, it's better for fish and wildlife too.

NINA MOINI: So Kirsti, if I live on a lake, which I don't, but if I did and I was worried about my property eroding, what would I do?

KIRSTI MAROHN: Well, you can call your area DNR hydrologist, and they can help you out and tell you, if your property is really eroding, what your best option is. They'll actually come out and look at your property. And then before you do any work on your shoreline, you should check with your local city or county to find out if you need a permit, because a lot of times you do.

NINA MOINI: Sure.

KIRSTI MAROHN: The state's public waters rules do allow riprap projects without a permit if there's a demonstrated need due to erosion. But even then, there are still rules about how long it can be and how high and what kind of rock and all of that. If you don't follow the rules, you could face a fine or even have to tear it out and restore the lakeshore, which can be very expensive.

NINA MOINI: Sure. And so this series that you've made, "Trouble by the Water," been looking at issues facing Minnesota lakes, many issues. What else have you reported on?

KIRSTI MAROHN: Well, we've looked at the impact of wakesurfing boats. Those are the boats that are designed to create this big wake for people to surf behind. You don't even need a rope. You just kind of ride the wave. Those have been really controversial around the state, and some lake residents are really concerned about what those big waves are doing to shorelines and lake bottoms.

So we took a closer look at that and looked into a recent study from the University of Minnesota that looked at what those waves are doing to lake bottoms and then also looked at some recommendations that you should be in at least 20 feet of water if you're going to be wakesurfing.

And then my colleague, Dan Kraker, also reported on how some of the state's most pristine lakes in the Boundary Waters are polluted with mercury and why that is and what impact that's having on those lakes and the fish that we like to eat from those lakes. So we're continuing to dive into these topics, and we do plan to do more reporting on these issues.

NINA MOINI: I really appreciate that, Kirsti, because we all how important our lakes are in the state of Minnesota and just all of the beauty and the quality of life that they bring. You mentioned you live in the Brainerd Lakes area, and I am curious to know what your favorite thing to do out on the water is when you're not reporting on the water.

[LAUGHTER]

KIRSTI MAROHN: Well, my favorite thing is probably-- I have a small kayak, and I just love being out on the water. It's so peaceful and gets you an up-close look at the wildlife and the birds and everything. So that's my favorite. When I have time to do it, I love to be out there.

NINA MOINI: Well, we thank you so much for your reporting, Kirsti.

KIRSTI MAROHN: Thanks, Nina.

NINA MOINI: That was MPR News correspondent Kirsti Marohn, reporting from Central Minnesota.