One of the general benefits of a tailwater is supposed to be that water temperatures are somewhat regulated. The water stays cooler in the summer and warmer in the winter than it would be otherwise, allowing trout and bugs to thrive all year. Seems like Baltimore City (which manages flows on the Gunpowder) is OK at keeping summer stream temps at levels that don’t kill trout, but that’s about it.
Tailwaters work because water passes through the dam from deep in the reservoir (that spout of water 1/4 of the way up the dam in the picture above), where temperatures are relatively constant throughout the year. When water pours over the top of the dam this either partially or completely negates the tailwater effect. That’s what happening on the Gunpowder right now.
I measured 2 tributaries of the Gunpowder yesterday as well as the Gunpowder itself. The tribs were both within 1 degree of the main river. The water is COLD and fishing, consequently, has been slow. I’ve heard isolated tales of people pulling out a big fish or two, but fishing has been super slow for me and just about everyone I’ve talked to. If you do go out, slow moving water is key to target, since the trout’s slower metabolism in cold water makes them seek these low energy intensive habitats.