Lesson 5. Current Speed ¿¿Dictates Fly Size
When the fish are focused on a certain insect type, you want to pick a fly pattern that best imitates its size, color, and so forth. It's not rocket science. But when the trout are eating opportunistically, you can and should use larger flies in faster water.

Here's evidence that we gleaned from observing two sections of the exact same run, the Bridge Hole at Boxwood Gulch Ranch in Shawnee, Colo. At the top of the run, the water moved quickly through a riffle and side channel. At the bottom, the water pooled and moved slowly.

In the fast water, we watched via the remote camera as Hyatt hooked several fish on a rig with a No. 12 San Juan worm and a No. 14 Prince nymph. The fish could see these flies well but had less time to scrutinize them as they pulsed through the swifter water; the trout therefore made impulse reactions and ate the flies. At the bottom of the same run, however, in the slow water, the big flies weren't catching any fish. We had to use a No. 20 RS2 to get just one strike (and not coincidentally, we had a harder time positioning the video camera in a way that didn't spook the trout).

Faster currents allow you to get away with more, and sometimes those itty-bitty bugs get lost in the flow.