They've thinned out by now, but there was still one actively diving on the water.
There are two species of Grebe in this photo - 1 Clark's and 2 Western. I'd been trying to figure out how to identify them in the field and this helped a lot. They were everywhere that day!
The Western Grebe, (Aechmophorus occidentalis), is a species in the grebe family of water birds. Folk names include "dabchick", "swan grebe" and "swan-necked grebe".