Doughnut Damming

I was on the banks of a swift flowing and crystal clear stream. It was quite deep and had a rounded gravelly bed.
I knew I had to dam the stream, although can't remember why.
After kneeling down to assess the problem I went off and returned with a giant doughnut(donut?).
I jammed the doughnut vertically on the rounded stream bed.
It worked a treat until the water level rose up to the doughnut hole and, of course, poured through.
Some things it seems, doughnuts cannot do.