Why is the Farmington River so warm? Or: when tailwaters don’t work

I’ve been getting a lot of questions about Farmington River water temps. The basic outline is, “I thought the Farmington is a tailwater. Why is the river so warm?” The answer is pretty simple.

In case you don’t know, a tailwater is a river with a dam release. The flow of the Farmington River West Branch is controlled by water that comes out of gates from the base of a reservoir dam (Colebrook Reservoir and Hogback Dam). If you’ve ever gone swimming in a pond or lake, you know that the water is warmer near the surface and colder near the bottom. This is called thermal stratification. In a larger reservoir, you have three different thermal layers: the epilimnion (the warmer upper layer; the metalimnion (or thermocline, the middle layer); and the hypolimnion, the coldest bottom layer. But I said this was simple, and there I go throwing all kinds of hifalutin hydro-science terms at you.

Ideally, there’s a happy medium of bottom release flows on a tailwater: cold water is never released in amounts that cannot be quickly replenished. But there are two situations that screw everything up.

The first is severe drought. Water that flows into the Colebrook Reservoir is reduced to a warm trickle. The volume of the reservoir shrinks from evaporation. The temperature of the water rises, especially if it’s hot, which decreases thermal stratification. As a result, what comes out of the bottom of the dam is warmer than ideal. (By agreement with the CT DEEP, the MDC must maintain a minimum release of 50cfs.) Because the flow is so low, water downstream warms quickly, trout get stressed, and it’s a bad scene all around. (This was the case in 2016. It was so bad that the DEEP took the unprecedented step of declaring thermal refuges on the Farmington.)

But that’s not what happened this year. This year, we had too much water. All that rainfall in July meant that the MDC needed to bleed water, and lots of it — so much, in fact, that it wiped out any meaningful thermal stratification in the reservoir. They’re still running such a high volume of water that there’s no chance for the stratification to re-establish — at least not in the heat of August. So that’s why at noon today the water was coming out of the dam at a very trout-unfriendly 69 degrees.

Eventually, it will get better. But right now, the best thing you can do for Farmington River trout is play the waiting game.

Taken this winter when the water was very, very, cold. Hang in there, buddy!