Drag is the single biggest reason a trout refuses a fly that looks perfect. The fish does not see a fly and a line — it sees a small object behaving impossibly. A natural insect drifts with the current; a dragged fly skates, sinks unnaturally or tips sideways. On a clear pool, a trout will often move away rather than take.

The causes

• Fast water between you and the fly. The line is in a quicker seam than the fly, so it tows it.
• A bad casting angle. Casting straight across a current makes the line cross several speeds.
• Wind on the line. Even on a dead-calm surface, wind can push a floating line faster than the drift.
• Too little slack. A straight line from rod tip to fly is a lever: the moment the current tightens it, the fly moves.

Four ways to beat it

1. Change your position. Fish from below, from above, or from the bank so the line lies in water moving at the same speed as the fly.
2. Mend immediately after the cast. An upstream or reach mend buys seconds of drag-free drift.
3. Use more slack. A slack-line or S-cast lays line in curves; the drift consumes the curves before it moves the fly.
4. Go under the surface. A nymph below a fast surface layer is far less affected: the leader can drag without moving the fly out of the zone. That is one reason nymphing is so effective in broken water.

Reading it

Watch the line, not the fly. The moment the line straightens or starts to accelerate, the drift is over — pick up and cast again rather than fishing out a dragging drift. A drift measured in seconds is normal; a drift measured in tens of seconds is a fish.