One morning, while dry-fly fishing a Trico hatch on Montana's Missouri River, guide Pete Cardinal told me to stop casting and watch the water. We were working a seam where fast water, colliding with a slow pool, was creating a foamy bubble line. At times, that bubble line would disperse and spread out in wide fronds of white, wispy water. At other times, the currents converged and collected hatching insects in a tight, white highway that ran straight through the run. Until Pete pointed it out, I hadn't noticed that the fish were keyed into this system. When the currents dispersed or collapsed, the fish didn't rise. When the foam line formed a hard seam and collected those mayflies, the trout began slurping away at the surface. The lesson? Follow the bubble line. When you see a foam or bubble line on the surface, there's a good chance that feeding trout will be underneath it.—K.D.