A mend changes the fly line’s layout through rod movement after the stop on a delivery cast. It is a line-shaping move, not another cast.

That timing is what separates a mend from a cast. The cast forms the loop and sends the fly; the mend comes after the stop and rearranges how the line lies. Its aim is the line’s layout — the shape of the line between you and the fly — though a mend can move the fly a little, which is why you keep them quiet around spooky fish.

You mend so moving water does not grab the line and tow the fly. A mend puts a curve or slack into the layout, so the current has something to feed on before the fly starts dragging.

Mends come in two timings:

• Aerial mends — mending before the fly line and leader land on the water. Most often used in moving water to produce a drag-free drift.
• Water mends — mending after the line and leader have landed.

Source

From the CI Study Guide (2025) (PDF), the Task 4 reach mend definitions; terms from the FFI Fly Casting Definitions.