What did the monks of Bolton Abbey think they were doing?
Surely this can't be a serious attempt at crossing the river. If the water lever drops by four inches then you don't need this at all. If it rises by four inches these stones will be useless.
Is it possible these suspiciously rectangular stones were put there by English Heritage to create a visual effect for landscape photographers...?
Maybe it is just a large scale experiment in turbulent flow and fluid dynamics.