I got a good look at a Sage Thrasher at Harriman Lake Park on Saturday July 19. (Jefferson County). Similar surroundings to what you have described -- small picnic shelter, grove of cottonwood trees, gravel walking path with frequents passersby (dog walkers, joggers, etc), and nearby weedy fields. Also unusual that day were several small flocks of Chipping Sparrows, and a beautiful Snowy Egret.