Life in the winter trout streams of the Driftless. Third in a series.

An Agate Original

After decades of trout fishing, I’m still awestruck by the spawning trout that lay their eggs at the start of winter. The eggs need silt-free water to hatch, and the fry must gain enough size and strength during the winter to survive the spring melt and the inevitable summer floods. They bulk up on some of the most fascinating critters around, the insects that live an improbable life in this watery winter landscape.  

Wait: insects in winter?

It’s a uniquely Driftless story that benefits from a little background. On my Elba farm, almost all the rain and snowmelt soaks quickly into the ground. As it percolates through soil and fractured bedrock, it takes on the earth’s warmer temperature. Gravity draws the water down and through mysterious underground warrens. Before long, the water flows to the surface again though seeps in the hillsides and springs in the valleys.

At our latitude, the earth’s temperature, eight feet below the surface, is a constant 48°F, sixteen degrees above freezing. The water is warm enough to keep the spring-fed steams from freezing. Small seeps freeze to the rock, creating icicles, stalactites, bulbous globules, and shimmering ice sheets on wooded hillsides, bluffs, and road cuts. Wherever the rock opens to meet the winter cold, the seeps sustain the moss and feed the deep-rooted trees. The ice formations mark the mysterious paths of the water until they disappear in the warmth of daytime sun.

As a result, the streams are colder than many of the state’s surface waters in the summer, but in winter, the flowing water represents a ribbon of warmth cascading toward the Mississippi River, offering a home to trout. While the rest of Minnesota is locked in snow and ice with frozen lakes and rivers, the Driftless Area is renowned for its open water and ice formations on the rocks and streambanks formed by splashing water continuously freezing from above and melting from below.

Almost ten years ago, the Minnesota Department of Natural Resources moved to allow winter catch and release trout fishing throughout the Driftless. It took years of experimental regulations to prove that anglers would not disturb the trout or destroy the trout eggs and fry. For me, winter fishing opened a new window to the outdoors.

Beyond the physical challenges of keeping my fingers warm and my fishing rod free of ice, I was astonished to see insects flying about the stream corridor. As the little flies hatched from the water, the trout went on a feeding frenzy, rushing to the surface to catch them.

Looking closer, I identified five or six different species of midges, stoneflies, and caddisflies. My first impression was that these bugs were off kilter, with a dead-end life cycle where they likely died trying to breed. But I later learned the truth when I met Dr. Len Farrington of the University of Minnesota, an entomologist who studies winter insects.

Dr. Farrington told me that he and his students have identified over 20 species in Driftless Area trout streams, including a dozen that were not previously known to science. Winter survival studies show that these amazing creatures can survive temperatures colder than minus 50°F, and they all have a form of anti-freeze in their blood and a unique cell structure that keeps them from freezing. The insects benefit from the 40-48°F water and their jet-black coloring on the white snow rewards them with warmth from the winter sun.

In many streams, the midges and caddisflies reach concentrations of 9,000 insects/square meter. The stoneflies and winter mayflies are comparatively rare and only inhabit the most pristine water.

I learned that these hardy bugs are right at home in the fantastic ice structures that form in these streams. Formations that I call “ice jewels” grow, change, and disappear with changing air temperature and sunlight on the water. Sometimes I find thin lace-like sheets of ice, patterned with dimples and eyes.

At other times, pools are fringed with thicker ice anchored to the bank and suspended over the water, dripping with frozen droplets, goblets, and dendritic lace.

These jewels turn out to be an essential thermal refuge for many of the bugs. The water is always warming the ice’s underside, and the insects fly and crawl into the lacy dendrites and bulbous cavities.

Dr. Farrington recently received his second grant from Minnesota’s Environment and Natural Resources Trust Fund to study the winter insects. His group started a new Citizen Science Project, Bugs Below Zero (https://www.bugsbelowzero.com/). Anyone with an interest can help with the research by finding, photographing, and recording the location and stream conditions of the Driftless winter insects. 

Nowadays, winter catch and release trout anglers are learning to “match the hatch.”  Some are now tying special winter flies, and Orvis and other trout outfitters sell a particular line of winter flies for the Driftless. In my view, there are few things as quiet or as beautiful as winter trout fishing; few things as pretty to hold as a native brook trout above a snow-covered stream; and few things more memorable than the winter insects that pepper the snow.

Those of us who love to spend our days outdoors in the Driftless winter keep track of the countless forms of water and ice, sense the frost depth, and anticipate the bite of a northern gale. We all understand the significance of February’s Hunger Moon and share the same unsettled feelings of the long nights before the hard freeze, along with the comfort of the rising sun that warms our faces. The nearly endless diversity of the land, water, and local inhabitants is the Driftless’ gift for everyone willing to slow down and bathe in the wonder.

It’s funny because every year, after every successive winter, spring, summer and fall, I say the same thing: “This season is the best.”   

I’ve now survived my 66th Hunger Moon and am enjoying the Full Sap Moon of March. But I know I’ll miss the winter, the ice jewels, and the resilient and brilliantly adapted insects.

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Jeff Broberg is a Minnesota Licensed Professional Geologist who grew up in Minneapolis and graduated from the Department of Geology and Geophysics at the University of Minnesota in 1977. He retired from corporate life as an Environmental Consultant in 2017 when he returned to college to earn a Masters Degree in Philanthropy and Development from Saint Mary’s University of Minnesota in Winona. He served on the Legislative Citizen Commission of Minnesota Resources for ten years, has been the President of the Minnesota Trout Association, the National Trout Center in Preston, MN and serves as a Director for Fresh Energy. Jeff currently runs the Minnesota Well Owners Organization (MNWOO), a non-profit serving the interests of people who rely on private wells for their drinking water.