According to Wikipedia, The John Dunn Bridge
[It] is located about three miles west of Arroyo Hondo on a gravel road that parallels Rio Hondo. The road, off of NM 522, runs through Bureau of Land Management property, [and] is known as John Dunn Bridge Road and County Road B-007.
Black Rock Hot Springs are located off of a dirt road on the western side of the Rio Grande after crossing the bridge.
Few of northern New Mexico’s bounty of hot springs have escaped the hands of developers and remain in a primitive condition. Blackrock Hot Spring has two characteristics that kept it from development: It has low flow, and it is located on the west bank of the rugged Rio Grande Gorge.
Far from isolated today, Blackrock Hot Spring is New Mexico’s most accessible primitive mineral spring. From the parking area at the hairpin turn above Dunn’s bridge, a well-developed trail dives from the road and heads downstream. The trail descends quickly to the river, reaching the spring in less than a quarter-mile.
The pool is small, and the volume of hot water is low. The mineral water issues from the base of the thick pile of black lava in a narrow drainage in the wall of the gorge. Boulders that have tumbled down the watercourse have completely covered the spot where the water bubbles up from the surface.
I’ve visited the John Dunn Bridge and Blackrock Hot Spring many times. In fact, Blackrock Hot Spring was the first natural hot spring I ever soaked in.
In the summer, visitors and locals alike enjoy swimming or floating in inner tubes in the Rio Grande just below the John Dunn Bridge. Blackrock Hot Spring is popular for soaking year round.
There are actually two pools at Blackrock Hot Spring, one warmer than the other. The cooler one is right next to the Rio Grande, and I’ve seen strong swimmers jump right into the river for a quick cool-down. The less adventuress can achieve a similar effect at a slower rate by taking a dip in the cool pool.
There’s no closing time on the hot spring pools, and although folks are not supposed to stay overnight in the small parking area, I’ve done it with friends a time or two. Because I prefer to soak in the hot water when the air is cool, my favorite time to utilize the pools–especially in the summer–is around 2am. It’s usually (although not always) empty then, and other middle-of-the-night soakers (if they’re not drunk partiers), tend to be quiet and respectful of the sacredness of the spring. A friend and I once sat in the hot water with no one else around and watched a meteor shower. That was a sweet night.
Blackrock is one of my favorite undeveloped soaking spots. Clothing is optional as far as the hippies who soak there are concerned, and I’ve never heard of a ranger hassling anyone for being naked there. The water is full of lithium (so the locals say), and it’s not only relaxing, but mood-lifting as well. The view is fantastic, and if there are no stupid rich people around talking about their real estate investments, it’s a wonderful place to rest and rejuvenate.