Mending is what you do after the cast lands. The fly is in the right place, but the line between you and the fly is lying across water that moves at a different speed. Left alone, the faster water grabs the line and tows the fly — that is drag, and it kills the presentation.

A mend puts a curve or a stack in the line, so there is slack to pay out before the fly starts moving.

The three mends you actually need

• Upstream mend. From a downstream position, flick the rod tip upstream so the line lands above the fly. The belly of line then feeds the drift before it tightens.
• Reach mend. Extend the rod upstream during the cast so the line lands in a curve without an extra motion. Quietest of the three, and the one to use when fish are spooky.
• Slack-line / S-cast. Lay the line in an S across the current so different seams feed line at different times.

When to mend, and when to walk

Mend when the current between you and the fly is faster or slower than the current the fly is in. If the fish is close or the water is uniform, mending can do more harm than good: every mend moves the fly a little, and on flat water that is enough to spook a fish.

The alternative to mending is repositioning — move your feet so the line lies in friendlier water. It takes longer, but it is always cleaner.

Mending, drag, dead drift and slack line are all terms in the FFI Fly Casting Definitions; the mechanics of the reach mend are covered in the CI exam tasks on reach mends in the exam section.