The Minnesota lake you won't find on any map

CATHY WURZER: You are listening to Minnesota Now here on MPR News. I'm Cathy Wurzer. As you know, Minnesota claims to be the land of 10,000 lakes. But there's one lake you won't find on any map, Lake Agassiz.

This gigantic glacial lake, which covered parts of Canada, North Dakota, and Northwestern Minnesota, drained away thousands of years ago. Jim Cotter, geology professor at the University of Minnesota Morris, is back with us on the program to give us the next chapter in his geologist tour of Minnesota history. Professor, how are you?

JIM COTTER: I'm really good, Cathy.

CATHY WURZER: I have to say I've known about Lake Agassiz for a long time, glacial Lake Agassiz. Tell us what it was, and when did it exist?

JIM COTTER: Well, glacial Lake Agassiz was a very large lake. And as you described perfectly, it formed in Western Minnesota, the Dakotas, and into the Canadian Prairie. It existed from about 13,000 to 8,000 years ago. And it formed as the last ice sheet retreated from Minnesota.

CATHY WURZER: Wow. OK. So as it retreated, did the lake grow larger then?

JIM COTTER: That's exactly right. The drainage from about Browns Valley, Minnesota, right in the center Western part of the state, all goes to the north. And so as the ice sheet retreated north of there, the lake grew and grew as the ice sheet retreated all the way to Hudson Bay.

CATHY WURZER: OK. How big was this?

JIM COTTER: It was really big, Cathy. The estimate for the largest extent of the lake was about 170,000 square miles. That would make it larger than any lake that exists today. For scale, the Great Lakes total 94,000 square miles altogether. So this would have been almost double it. It was a really big lake.

There was a rumor for a while that the Guinness Book of Records had it as the largest lake ever to exist, but I've never really followed up on that. Part of the problem with ancient geologic lakes is that it's hard to figure out the boundaries, whereas with Lake Agassiz, you can tell roughly where the boundaries are.

CATHY WURZER: And how do you determine that?

JIM COTTER: It turns out there's a lot of geologic evidence. People always know about the lake sediments in Western Minnesota. It's so great for agriculture. Those exist all from deposition in glacial Lake Agassiz. You can also tell where rivers entered glacial Lake Agassiz from the west. Like rivers everywhere, as they flow into the lake, they formed deltas. And so there's a series of deltas as you go northwards in the Dakotas.

One of the interesting ones is iceberg scours. If you ever fly into Grand Forks or Winnipeg in the early spring or late fall, there can't be snow on the ground or crops, you'll see these lines that are miles long. It's close to 1,000-foot-thick iceberg dragging along the bottom of the lake, floating southward past Grand Forks. It's really, really interesting.

CATHY WURZER: Oh, wow. I had no idea. Wow. OK, so because we're talking about a lake, were there beaches along--

JIM COTTER: There were.

CATHY WURZER: --this lake? OK.

JIM COTTER: There were. Yeah. People always joke in Western Minnesota, we're going to the beach. But it's long tried. There was a series of beaches carved, like any lake, at the shoreline. And sometimes it's erosional, and sometimes it's depositional. But you can trace those beaches from not far from me. A little town of Herman has a beach named after it. And that beach you can trace all the way up to Winnipeg. It just represents where the shoreline was.

CATHY WURZER: Are we talking about sandy sediment? Is that how you would determine that too?

JIM COTTER: That's exactly right. Students are always blown away when I have little coring devices, and you drill down through muck, essentially. And then all of a sudden, you hit pretty nice beach sand. And it's really, really impressive stuff.

CATHY WURZER: Wow. And there were different beaches, obviously.

JIM COTTER: That's right. What happens is as the glacier retreats, it uncovers lower and lower outlets. And so what happens episodically is that the lake establishes a lower level. And at that lower level, it establishes a beach. But what's really interesting about uncovering those lower outlets is that sometimes it can drain portions of the lake catastrophically.

And at different times, the lake was so big, sometimes the lake drained down to the Atlantic Ocean through the Mississippi River. Sometimes it drained out the Great Lakes into the North Atlantic. And it even flowed out the Mackenzie River to the Arctic at one point. It just has this incredible reach.

And when that happens, when a huge amount of very cold water is released from glacial Lake Agassiz, it disrupts ocean circulation. And a lot of people believe that there are two or three short-lived climatic events, cold phases, that are triggered in Europe and Scandinavia, and it's all coming from a lake in Minnesota. It's wild.

CATHY WURZER: That's pretty cool. Say, what happened to glacial Lake Agassiz?

JIM COTTER: So it's inevitable. Once the ice sheet starts to retreat, it eventually goes to the size of Hudson Bay. Once the edge of the ice sheet reaches Hudson Bay, then Lake Agassiz drains out through Hudson Bay. And I guess about the normal drainage patterns that exist today, those were established around there. But every once in a while, you get a hint of glacial Lake Agassiz when things start to melt in Western Minnesota and the Red River is thawed. It starts flowing north.

And all that water reaches places like Winnipeg, where the Red River is still frozen. And if that's the case, then the water can't go anywhere, and you have floods. So that '97 flood, 1997, the river simply couldn't flow northward anymore. And so what happened was that that river spread out over a flat landscape. And you get a short-lived version of a mini Lake Agassiz forming from Fargo to Grand Forks.

CATHY WURZER: Oh, I remember covering that flood in '97 and going up and seeing it from the air. And it was a pretty amazing picture. Wow I know you love history, but I wonder, any big news in the geology world that's grabbing your attention right now?

JIM COTTER: There's volcanoes that are active right now in a couple--

CATHY WURZER: Yes.

JIM COTTER: --different places, I think Indonesia and of course the one in Hawaii. They make great footage for video news. But it's really, really interesting that the Hawaiian one, people are approaching it very, very conservatively. In other words, they're not all that worried about it.

Of course, the Hawaiians are used to it forever. Indigenous people know geologic processes. And what they're doing is just watching and seeing. And it looks like things will develop into a normal volcanic eruption and not impact all that many people.

CATHY WURZER: All right. Yeah. Those pictures are pretty awe-inspiring when you see them, you know?

JIM COTTER: Yes.

CATHY WURZER: OK. Professor, always a pleasure. I know you're busy. Go back to class. We'll talk to you soon.

JIM COTTER: Thanks, Cathy. It was nice talking to you.