A quick Internet search didn't turn up much about the water flows in Antelope Wash. I probably should have gone to the Visitor's Center in Page. Be that as it may, my understanding is that the water flows down the wash, sometimes alot of water, meets the barrier of rock, cuts through the slot canyon (about a quarter of a mile long) then continues flowing down the wash. An interesting thing is that the depth of sand in the bottom of the slot canyon changes from year to year or flood to flood.
I'm really glad I saw this slot canyon, but not sure I'll go back. Too many people. No opportunity to absorb the awesomeness of the canyon. There are special 'photography' tours for 'experts'. Those might be smaller (and probably include mostly people who don't forget to put their memory cards back in their cameras!).
This shot was taken with my iPhone. Wish I had something better for you. Next time I'll visit the Lower Canyon. It has some ladders and stairs, maybe that discourages a few folks. Sorry to be so negative! It really is awesome!!!!