WILLIAMSTOWN — According to investigators, there were "tons of fish" in Hopper Brook in July. There's a reason for that. At this time of year, when flows are low and air temperatures are high, fish search out cold water.

Two students from Williams College spent part of their summer internships taking the temperature of a branch of Birch Brook in Hopkins Forest, of the Hoosic between Adams and North Adams and of the Green in Williamstown from its mouth to the Five Corners. Birch Brook is a tributary to the Hemlock Brook, which flows into the Hoosic. The Green is a tributary to the Hoosic. At issue — how will climate change affect rivers and their denizens?

Birch Brook serves as a reference. It flows from high elevations under heavy shade in Hopkins Forest. The entire sub watershed is well-studied and controlled by Williams as part of its forest laboratory. Conditions on the section of the Hoosic between the Adams and North Adams flood chutes are virtually the opposite: low elevation, with little shade. The reach of the Green is relatively low elevation but has considerable shading.

No doubt of it, this is a quick and dirty study, but we hope it will help determine the circumstances under which a river may be able to prolong its cold water characteristics. Trout reproduce in all three, albeit small ones in Birch Brook. The students were looking for a gap between air temperature and water temperature.

Hopkins Forest's ecologists have figures for the last 19 years that that gap in winter grows to 15 degree F., with the water temperature warmer. In the warmest part of the summer, July, on average the air temperature in Hopkins crests at 70 degrees, while the water in Birch Brook remains at 60. At a higher elevation, the students found a differences ranging to 14 degrees. Such conditions provide hope for trout, for which 70 degree water temperatures are lethal.

Can we find a similar gap elsewhere? On the inter-flood-chute reach of the Hoosic on late morning of a sunny July day, with air temperatures to 80 degrees, water temperatures at three sites were about 70 degrees. So, while they were cooler than the air, given the amount of sun hitting the water they were too warm for trout. Fishermen should stay away, in order to avoid stressing the trout further.

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The results on the Green showed water temperatures in the mid-60s. At the Five Corners, both on the East and West branches, temperatures were about about eight degrees cooler than the air; 10 degrees cooler where Hopper Brook entered, eight at Blair Road, only two degrees where Christmas Brook enters in a pipe, and five at the mouth. Thus Hopper's cold water, from high elevations of Mount Greylock, considerably cools the Green, whereas Christmas, which flows exposed over the Taconic Golf Course, barely improves it. By the time the Green reaches the Hoosic, it is approaching 70 degrees.

The studies need to be run through time, as have been ones in Hopkins Forest, and on other tributaries. Still, even with other variables figured in, it is clear that trout are more likely to find heat relief in the tribs, especially those forming at higher elevations and shaded on the way down. That, in turn, sharpens a focus for activities to preserve a cold water fishery.

At least, that's how it looks from the White Oaks.

A writer and environmentalist, Lauren R. Stevens is a regular Eagle contributor.