These are cottonwood seeds. Each bright white speck is a tiny seed surrounded by a nimbus of fluffy stuff that floats on the merest hint of a breeze. Just today they started appearing in quantity so that in places it looks like drifting snow. It's hard to imagine that tiny speck growing into this humongous tree:
You can't really see the trunk, it's back behind that bright dead tree in the foreground, but trust me that it's about four feet in diameter at the base.
The tomecats being tunecats