On a fall day that still looked like summer in southeastern Minnesota, my friend Justin and I were fishing East Indian Creek near Weaver. We weren't having much luck, though, and overhanging branches made casting tough.

Finally, Justin caught a trout—a brookie of about four inches. Painted intricacies of gray and green camouflaged its back, while white leading fin-edges set off the deep orange that would soon match the maple leaves overhead. As we watched, it wriggled through the mesh of our rubber net, designed for larger fish, and returned to its undercut bank home.

Which was fine: We weren't going to eat it anyway. We just wanted to see one—Minnesota's only native stream trout, hanging on despite a changed landscape. And in East Indian, even doing a little better than just hanging on.

The Department of Natural Resources has been monitoring the stream's flow, temperature, and trout abundance since 1981. A recent DNR study of this data found that the native brook trout have been displacing the nonnative brown trout for almost four decades without direct intervention like stocking or brown trout removal. As brook trout production rose, brown trout production ground to a halt.

It's a good sign, says DNR fisheries scientist John Hoxmeier, one of the study authors. "If there's naturally reproducing brook trout in a stream," he told me, "that stream's doing well."

Brookies are often out-competed by brown trout, he said. In the late 1800s and early 1900s, farming and logging practices in the region led to warmer, degraded streams. Brookies declined and the introduced browns thrived. Since then, better land use practices, such as contour farming and reforestation, have helped some streams recover by reducing runoff. Rain can soak into the aquifers that feed cold, clean water into the streams.

And now, in East Indian Creek, the brookies have turned the tables on the browns. As summer stream temperatures dropped below 60 degrees, said Hoxmeier, brook trout numbers rose and brown trout numbers fell.

Similar conditions are showing up regionwide, he noted. But it's taken many decades of careful conservation work, and it could go back the other way if we're not careful, he said.

In the face of a warming climate, the southeastern Driftless region, with its cold ground water, may become a critical refuge for native trout. "As long as we protect the groundwater resources," Hoxmeier said, "this looks like an area where brook trout can survive. They've shown they can take care of themselves as long as you give them the right conditions."

On our hike back to the truck, the midday sun illuminated more trout in the bends and pools of East Indian Creek. They finned in the current, taking large gulps of the clear, cold ground water. We didn't need to catch them. They looked profoundly at home.