Devil's Kettle

At Devil’s Kettle, the Brule River divides around a rib of rhyolite, volcanic rock as hard as granite, and does something that has puzzled visitors for decades. One branch thunders over a 50-foot waterfall into a pool below. The other falls into a dark, rounded opening called the kettle, and from the overlook that second stream seems to vanish. No object tossed in has ever been seen resurfacing downstream.

The falls sit about a mile inland, high above Lake Superior, within Judge C.R. Magney State Park on Minnesota’s North Shore. One North Shore guide gives the drive from Grand Marais as about 14 miles northeast on Highway 61, but elsewhere puts the park 20 miles north of town. The Devil’s Kettle is the most visited attraction in the park, and the main hike to it is a strenuous 1.1-mile walk that goes down many stairs to the falls. Counts of the stairs differ, from nearly 200 on Wikipedia to over 200 in a North Shore guide.

The most common theory was that the water ran underground and came out somewhere under Lake Superior. Park manager Peter Mott heard stranger ones. “I’ve heard locals describe the possibility that this water splits at the waterfall and some of it flows into Canada,” he told MPR News. “I’ve actually heard people suggest that it may flow somehow back into the Mississippi River.”

Geologists had long doubted the idea of hidden tunnels. The North Shore is underlain by rhyolite and basalt from the Midcontinent Rift System, which erupted about 1.1 billion years ago. Underground waterways like those that carve limestone caverns simply do not form in this rock type. Lava tubes can appear in certain volcanic formations, but not in rhyolite, and the basalt beneath the riverbed is too deep to be relevant.

In late fall 2016, a team of Minnesota Department of Natural Resources hydrologists tested the mystery with a different approach: they measured the river itself. DNR springshed mapping hydrologist Jeff Green proposed comparing the volume of water entering Devil’s Kettle with the volume emerging downstream. Two DNR experts took the readings, recording 123 cubic feet per second above the falls and 121 cubic feet per second several hundred feet below.

The two numbers were virtually identical. “In the world of stream gauging, those two numbers are essentially the same and are within the tolerances of the equipment,” Green told the Minnesota Conservation Volunteer.

The evidence pointed to a straightforward answer: all the water that disappears into the kettle resurges into the Brule River just downstream, where the rugged terrain and turbulent flow keep it hidden from the overlook.

But why do sticks, ping-pong balls, and other tossed objects never reappear? Calvin Alexander, a University of Minnesota geologist working on the project, offered an explanation rooted in fluid dynamics. “The plunge pool below the kettle is an unbelievably powerful system of recirculating currents,” he said, “capable of disintegrating material and holding it under water until it resurfaces at some point downstream.”

When the DNR announced its findings in February 2017, Mott allowed that “a few people may be disappointed.” A follow-up dye trace, using fluorescent, biodegradable tracer visible at ten parts per billion, had been planned for fall 2017 to visually confirm the exact point where the water re-enters the river. But park management discouraged the experiment, and Green ultimately decided it was not necessary given the strength of the flow data. WTIP reported in December 2017 that the dye study did not take place.