Use a reach mend when the line would otherwise land downstream of the fly — most often in moving water, where you want a drag-free drift. Its purpose is to position the fly line and leader upstream of the fly, and the guide lists the benefits:

1. It creates a longer drag-free drift, because the line and leader sit upstream of the fly.
2. It positions the fly in the fish’s feeding lane while the fly is still in the air.
3. It does not spook the fish — the line is mended before it lands, so there is no water disturbance.
4. The line is mended before water currents have affected it.
5. You can cast past the target to aim, then let the reach pull the fly back into the lane upstream of the fish, to float down to it.

Without slipping line the reach shortens the cast, which is what makes it excellent for dry fly presentation at short distances.

Source

From the CI Study Guide (2025) (PDF), Task 4 — reach mend with and without slip and the reach-mend lesson plan (Appendix C); terms from the FFI Fly Casting Definitions (Aerial Mends, Slack Line Presentations).