A green heron fled before us, taking off each time we approached. A big double family of mergansers – single mothers, both of them, with probably ten kids between them – squawked away in fright half a dozen times before both they and we had an inspiration: We went to the left bank, they to the right, and there was an end of it; they splattered noisily back upstream. In the silence afterward – the Dead Diamond in the summer barely gurgles, except where it drops down over Halfmile Falls – we could hear a white-throated sparrow calling for Bishop Peabody and a hermit thrush imitating a squeaky screen door. Very peaceful.

The big birds, too, have returned to the North Country. When I first came here in the 1960s, there were none; they had nested occasionally, but unsuccessfully, and had disappeared After the publication of Rachel Carson’s The Silent Spring in 1962 and the banning of DDT in 1972, the raptors began coming back. I passed three active osprey nests on the way up here yesterday, ravens squawk past overhead, and just a few minutes ago Jack and I watched a magnificent bald eagle riding the updrafts and glistening in the sunlight above the Diamond Peaks.

The Dead and Swift Diamond rivers, which join to form the Diamond at the head of a spectacular gorge, run for most of their length through private land, the 27,000-acre Dartmouth College Grant. Anyone is welcome to hike, fish, or hunt on the property, but motorized vehicle access is restricted to contract loggers and members of “the Dartmouth family.” This has provided considerable protection to these two relatively pristine rivers and their historic brook trout populations. Nevertheless, the rivers – especially the Dead – are in trouble.

Indiscriminate clear-cutting in the headwaters – off the College’s property – has removed the forest canopy that once kept the water cool, allowed the absorbent duff on the forest floor to wash away, and caused extensive siltation of what once were rocky, fertile spawning beds. Jack has documented the rise in water temperature in his 2009 book, Wintering with Amasa Ward. Summertime temperatures in both rivers now routinely approach or surpass toxic levels for brook trout. The little feeder streams in pockets where logging has not occurred are, however, still cool and full of tiny, active native trout.

Those of us who’ve long loved this river for its beauty, seclusion, and (former) fishing have often lamented the changes. Years ago our worst fears were buttressed by the visit of the original Gloomy Gus, a New Hampshire Fish & Game biologist, who told our committee that the river was such an acidic, borderline-barren habitat, anyway, that it was probably a lost cause. Given the drop in oxygen levels that accompanied the annual rise in temperature, we might as well forget it.

If there is one bright spot in that sad picture, it’s that over 40 years ago, when Jack and I first started coming here, a summer bath in the river was an occasion of much thrashing and gasping. This morning, when I took a bath next to a handy concrete logging bridge abutment, I found it a delightfully cooling experience, interrupted only when Jack called from across the river, “Will, there’s about a seven-inch-long leech headed your way.”

Recently another bright spot has shone through the drapes of doom hanging over the river. An experimental fish-tagging program supervised by the Fish & Game Department has revealed that the fish in the river aren’t dying from the heat; they’re simply leaving. Dianne Timmins (easily the most engaging biologist I’ve ever met; you can Google her at New Hampshire Fish & Game’s Wildlife Journal) has been working in the Diamond watershed for several years, trying to determine why, when the feeder streams are so prolific, electroshocking the main river during the summer produces almost no trout over 9 inches.

Working with a gang of volunteers and a nearby Trout Unlimited chapter, Dianne has been surgically implanting coded transmitters in any brook trout large enough to sustain the initial procedure and carry the devices without strain. A much simplified version of her results is that when Diamond River temperatures rise to a discomfort or danger level, the Dead Diamond trout migrate downstream into the Magalloway River watershed, which is kept cold by releases from the bottom of Aziscohos Dam – “hypolimnetic draw.” Some have migrated as far as 50 miles in the course of a year. The Swift Diamond trout move upstream to undisturbed tributaries.

Our obituaries for the Diamond rivers have been exaggerated; the trout are still alive – if no longer with us except during cool summers. Dianne recently had the incomparable experience of watching a very large male brook trout chase away smaller males in order to fertilize the eggs of a spawning female in the Dead Diamond. Still, it would be fatuous, in the face of habitat degradation and global warming, to suppose that all is well with the brook trout of our nearly pristine northern fisheries. About all we can do is try to mitigate the negative effects of development, and perhaps gaze wistfully at the possibility of gaining control of their ravaged headwaters.