Very cool trip, Smallfry. Thanks for sharing the pics. That's around my neck of the woods... my family has a cabin on the other side of the San Luis valley up in the San Juans.
My Wife and I checked out Medano Pass & the Dunes last year, early in the season. That deeper water crossing right before you get to the dunes was DEEP and I didn't try it. The water was about 1.5 feet higher than it was in your pics and muddy so you couldn't see the bottom or judge depth.
Next time you're out that way, check out Zapata Falls. It's a short hike, but a cool sight to see.