1. The problem statement, all variables and given/known data Around one of our buildings we have an "ant channel" of roughly square cross section (say 20 cm x 20 cm) with fish in it to control the mosquito larvae. The surface of the water carries particles giving a good indication of any surface movement. Late one afternoon we observed the surface moving relatively quickly (say 3 cm/s). Suspecting a leak we followed the moving surface water and were amazed to find that the moving surface gave way abruptly to a still surface. Watching a little longer we observed that the interface between moving and still areas of the surface was itself moving -- at about half the velocity of the moving surface. It appeared as if there was a discontinuity in the flow of water. 2. Relevant equations None. 3. The attempt at a solution I think I can explain the phenomenon. If no one gets it, I'll give clues.