Most days it shows up as a run of refusals. You’re getting drifts you’d swear are good over steadily rising fish, and nothing eats, and the reflex is to go smaller and lighter. Sometimes that’s right. But on cool fall water it’s often a different problem: there are two mayflies out there, a small olive one and a larger reddish-brown one, and a fly that matches the wrong one gets looked at and left.

The two hatch during the same conditions. Cool, overcast days with a little drizzle, or wet snow later in the season, are when you get the heaviest of each, and a fish keyed on one will refuse a good drift of the other. What separates them in hand is tails, size, and where on the river each one hatches.