I spent a lot of time in Canada,some of the most beautiful country in North America,and spent many hours searching rivers,streams,etc before realizing why I was not finding artifacts.Most of that country was under several feet of Ice when cultures were busy making camps here in the warmer climates. Maybe this is why you may find only birdies or later projectiles.
Man Gtown, I am jealous. I love that obsidian flint. I actually bought a few obsidian birdies last year at the Temple show from one of Dwain's buddies that came from the El Paso - New Mexico area. Looks like you have a pretty good spot to hunt. Keep the pics coming, I enjoy seeing them. Good luck on your hunting!