A mend is manipulating the fly line layout by rod movement after loop formation on a delivery cast. It affects the rod leg of the loop, not the fly leg.

That distinction is what separates a mend from a cast. The cast forms the loop and sends the fly; the mend happens after the stop, when the rod is moved to change how the line lies on the water. A mend does not change where the fly lands — it changes the shape of the line between you and the fly.

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 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.