A strange ball that wouldn't move

 

At the foot of the right hand outflow pipe, there is an inflatable beach ball. The water was flowing fast from behind it and you'd think it should push the ball forwards but in the minutes that I watched it, the ball stayed where it was. This must mean that at the surface of the water, there must be a force backwards on the ball. It's a long time since I studied fluid dynamics. I wonder if there is something to do with Newton's Third Law going on, that when the incoming water pushes forwards on the main stream, the main stream pushes backwards on the incoming flow.