The fish stations a few centimeter from the surface, hence it sees the drifting flies at the very last moment, for the vision window is very narrow in proximity of the surface. This means that it is possible to drop the artificial fly a 20 cm. upstream for the fish to consider it as transported by the current and not as if it was falling from the sky. One only cast, through a narrow channel, with the risk that each false cast may snag the protruding root.