Yet if I'd never discovered the Eagle on my rambles through John Gierach's flyfishing classics, perhaps that wasn't his fault. Take a look at any map of Colorado, and it's suddenly clear how deeply this landscape has been scarred by the mining that opened it up to the outside world. With Gypsum, Radium, Leadville, Carbondale and Basalt all dug in within a hundred miles of each other, very few streams were lucky enough to escape unscathed, and the Eagle wasn't one of them.

Almost overnight, in the late 1880's, a seam of zinc made the little town of Gilman rich. But toxic tailings from the Eagle Mine slowly suffocated the life of its namesake, for many years and many miles downstream - and it wasn't until the 1980's, when the mine went bankrupt, flooded catastrophically, and threatened to debouch a really massive slug of pollution that anyone seems to have taken serious notice.

Even at that point, when funky ideas about river regeneration were still in their infancy, this must have seemed the decisive moment in the Eagle River's fortunes. Suddenly the film colossus Viacom found it owned the redundant workings, and the Environmental Protection Agency stepped in to declare them a Superfund site. At vast expense, public and private, the worst of the problem was pumped out of the old shafts for treatment, with a chemical plant installed to treat future seepage.

And the plan worked. By the late 1990's, fish and insect life were re-seeding from the tributaries and reaches above Gilman, and a vigorous population of rainbows, browns, cutthroats, and even brook trout - as well as all those caddis - now count once-dead water as home.

So, as I line up my next cast, I'm aware I'm wading where something wonderful has happened, very recently - and it's not just knowing that Steven Seagal shot "Under Siege 2" somewhere upstream in that old mine. This run being deeper, I'm rigged with a tungsten-headed nymph to take the midge down - it seems we're into brownie territory, and I hook up with a good 12-incher on the bigger, heavier pattern almost immediately.

These browns are hungry, well recovered from their spawn, keen to bite on anything that looks like food and drifts convincingly.

And yet, with an air temperature hovering around -4 degrees C, it's all so delicately balanced. Two pools up, not half an hour later, the sun has slipped imperceptibly down the sky, and the action's slipping too. Ice forms just a little faster in the rod rings, and there's a skim of frost round my knees.

I miss a couple of half-seen strikes, take one solidly on the dangle, straight downstream, and then it's as good as over for the day.

"The bugs stop moving," Alex confirms. "They switch off, just like that".

A dark little water ouzel darts past; a search-and-rescue helicopter powers over, dramatically backlit, thudding through the mountains.

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