Seeing a hatch of mayflies, caddis, or other insects and the resulting rise of feeding trout fills in one very large piece of the eternal puzzle of trout and water; ie: where the fish are and why. They are feeding, and have taken up stations on the pool or run with preference to pecking order, size of the trout, cover, ease of feeding, shyness, shadows, or several of dozens of other variables that anglers have spent thousands of pages debating, investigating, and describing. This problem alone solved, the angler can go on to the problem of how to present a fly to the fish. On low-gradient chalk streams, this is the preferred method. Spot the fish, work out the cast, present the fly, try not to put the fish down, etc. An angler by choice and by circumstance may spend hours just working one little spot. The fish are spooky in the gin-clear water and are hiding under weeds. Walking and casting are about the worst things one can do. Prospecting with the dry fly would be like casting a big bobber into a quiet pool.
Prospecting with the dry can be either fast or slow, entirely at one’s whim. How long one spends on one piece of water is up to the angler. Often using a delicate but faster approach, in effect ‘shotgunning’ the runs makes the best use of time and energy. In doing this one is looking for the exact opposite of the ‘classic’ dry fly presentation. Instead of slack line leaders, drag-free drifts, and complicated mending, the angler is looking to produce a rise when the quarry does not necessarily want to rise to the fly. Try dropping the fly with a ‘splat!’ Step up a size or two in pattern, or twitch the fly invitingly as it drifts back to you. Skitter the fly down and across, or cast the leader over a low hanging branch and let the fly be pulled up and over the branch to alight on the water again. All these are methods I have used to catch trout, often learning by accident, and then retaining the moment in the ‘ol rusty file cabinet brain.