A good drift is mostly a number. Watch a #16 Sulphur dun ride a Penns Creek seam without dragging and you’re looking at roughly two feet of usable slack absorbing about a foot per second of velocity differential — call it two seconds of dead drift before the leader catches the faster current and the fly skates. That ratio is what mending is really about, and once you start counting feet and seconds, the June Sulphur hatch gets a lot easier to plan for.

The Drift Equation

The working version of the equation is this: usable drift time (in seconds) equals usable slack (in feet) divided by the velocity differential (in feet per second) between the fastest seam your fly line or leader crosses and the seam containing the fly. A #16 Sulphur sitting in a 1 ft/s slot, with the leader cutting through 2 ft/s water and 18 inches of in-line slack, drags in roughly 1.5 seconds. Double the slack and you roughly double the drift.