Interference fringes at Fickle Steps

This is a photograph of the water flowing between two of the stones on the Fickle Steps in the Duddon Valley. The stripey pattern is a permanent feature - so must be some kind of stationary wave. The lines on the water look like double slit interference fringes. I've been thinking about how they form. The flow above the stones is laminar. There were no surface waves visible. The water is squeezed between the rocks. Thus some of the laminar flow is directly into a rock. There will be reflection due to Newton's Third Law: the rock stops the flow of water by applying a force. The reaction force pushed the water back, hence ripples. But why the permanent feature? It looks interference but what are the reflected waves interfering with?