The channel in which this "thermal estuary" exists is only about 2 meters wide. However, the two rivers coexist for a length of 45 meters, creating a long line of thermal "soaking pools".
The thermal water discharge of the Boiling River is the largest in all of Yellowstone. Geologists predict that the underground source of these waters is from the famous Mammoth Hot Springs, traveling a long distance of 3 kilometers underground to supply the Gardner with its thermal adversary.
With amazing geology comes a copious amount of tourists. Today, the site is an extremely popular location for bathing in the mix of two polar ends of the temperature scale. However, Yellowstone National Park has done a rather spectacular job of preserving this location's natural beauty. First and foremost, nude swimming is prohibited, which I for one am quite happy about. Furthermore, the NPS has created restrictions to eliminate trash and touristic rowdiness. The digging of the riverbed and moving of rocks is strictly prohibited.
The temperature of the mixing water varies based primarily on season, but also on the depth of the Gardner River. Thus, a "comfortable" temperature is constantly shifting in accordance with the natural shifting of the water depth. Due to the vast temperature difference, it is possible to be sprayed behind with 60 degree C water while being waist deep in ice cold water. In addition, wintertime soaks are described as "magical", similar to bathing in a natural hot tub outside in the cold.
Image Credit: The Roving Ruges Blog
Explorer's Guide to Yellowstone: goo.gl/T6COa