When Is a River Also a Lake?

Fluvial lakes—that is, lakes formed by moving water—are the hamburgers of the hydrological world. (Stick with me here.) Just as a patty in a bun is both singular foodstuff and sandwich, a fluvial lake is a standalone body of water as well as a part of a broader river system.

Lake St. Croix is one such “burger.” Located on a 25-mile stretch of the St. Croix River running from Stillwater to Prescott, this impounded fluvial lake formed when river sediments dammed the lower St. Croix Valley. As a kid, I worked at a marina in Afton and remember standing on the gas dock, wondering what was up with the doublewide span of the St. Croix stretching out before me. When someone finally explained that the river of my childhood was also a lake, my mind was blown. Part of growing up, it turns out, is realizing that two things can be true at the same time.

Arguably the most impressive fluvial lake in Minnesota is good old Lake Pepin, which occupies roughly 30,000 acres of the Mississippi River in southeastern Minnesota. At its widest, the lake balloons out nearly two miles, a massive pool in the middle of bluff country.

I remain interested in riverine lakes, which explains our visual breakdown of the concept. This article examines different types of fluvial water bodies and explains how they act as important habitat for birds and fish. But this isn’t the only niche, lake-themed story in this issue. Former MCV associate editor Joe Spring interviews Randy Carlson, an experienced surfer and surf instructor whose watery playground is the North Shore of Lake Superior.

Carlson caught his first wave in Hawaii, which just so happens to be where I’m visiting as I write this editor’s note in early March. As I admire the roiling swells of the South Pacific, I picture Carlson or one of his students ripping through a cold, slushy wave on Superior, thousands of miles away. Water connects us in more ways than we can imagine.