I got to visit the USGS gage on the Gallinas River recently. This is near Las Vegas, NM, on the east side of the Sangre de Cristos Mountains. Luckily, I recently purchased and have been perusing the NMGS "Tectonic Development of the Southern Sangre de Cristos Mountains, New Mexico", so I actually know a little bit about what I got to see.
West of Las Vegas, the Gallinas River has cut through the Crestone Anticline to expose some of the Precambrian metamorphic and igneous intrusive rocks in an area between the Crestone upstream and west to around Trout Springs. This exposure, and the bedrock-controlled river substrate (an F in Rosgen parlance), make this a great place for a stream gage.
The Precambrian rx have been well-sheared and folded as observed in their exposures along the faulted frontal zone of the Sangres uplift. In the first photo, you can see the gage equipment box, and the folded Precambrian adjacent to it. The picture barely does it justice.
A shot upriver from the gage station. The canyon is steep, with poorly- to-undeveloped riparian areas, clearly due to the lack of flood terraces. This changes immediately downstream of the gage station. Between the road grade and areas of less-cohesive, more mafic, weathered zones (fault splay, maybe? - I don't do structure for a living), the river has a greater ability to form riparian habitat/flood terraces.
Here's a shot of the near-vertical Precambrian with a boudin of felsic material. This is just downstream of the gage, in the canyon wall/roadcut.
A little further down-canyon (east), a well-weathered zone of mafic material interspersed between the more competent rock. The mafic was easily crushed by hand. It's very similar to the exposure of the Romero fault zone near Mora (post to come), so I wondered if there might be fault splays along this stretch. There aren't any noted in the references I have, but this stretch also has restricted access.
I should note that this location is also occupied by northern leopard frogs, petitioned for listing under the Endangered Species Act. The person accompanying me had surveyed for them in the past, and had, with others, determined that the frog occupied adjacent rock pools and wetter zones; habitat much more favorable during lower water years. During freshet or high water years, this habitat would be inundated, lowering the species' ability to reproduce. When the habitat is well-exposed and well-wetted, the species would thrive.
We had a fine time exploring the canyon and discussing the habitat and hydrology drivers. I was heartily reminded of a friend's comment (Josh Collins, who works for the San Francisco Estuary Institute and whose wife was a grad student of Luna Leopold) that habitat occurs at the intersection of hydrology and geology. The Gallinas at the USGS gage is a fine example.