The cohesive forces between liquid molecules are responsible for what is known as "Surface Tension". Molecules at the surface do not have other "Like" molecules on all sides, and so "Cohere" more strongly with each other at the surface. This forms a "Film" which makes it more dificult to move an object through it.
The Hackle on a dry-fly takes advantage of this phenomenon. The feathers are spread out and float the fly. Most nymphs have nothing in the way of flotation as compared with dry's, and therefore do not take advantage of the surface tension. The faster a body of water moves, the less surface tension it has.