it blocks the water after getting clogged - then the resulting puddle spreads out onto the trail,,
which gets softened and collapses into the stream - so add more rocks and repeat -
the better way is to create a channel to control the water flow across the trail and protect the edges -
bike can easily handle a couple of 9" gaps. when the water comes up, it flows over the rocks,
but most of the time it is between them.
I've seen the bottom armored, some crossings get much deeper when it rains so not great.
I did some work at jungle on Animal Chute a couple weeks back to eliminate dams.
huge difference in depth of crossing. It didn't look like people put the rocks there,
I think they were pushed there from up-stream by heavy rain.