I found these markers hanging on wires strung between the trees on either side of the river.   There are 3 red and white "gates" and one green and white "gate".   All 4 gates are all hanging over the water at just about the same elevation.   During the spring the higher river water runs up several feet on the markers.   In this image taken in August 2011, the markers are about 12" above the surface of the water.   The gates do not appear to be used for sport - anyone know what these markers are intended to do?