Date of this Version
Chloride flux, a surrogate for heat flow, was determined for the four rivers draining Yellowstone National Park (the Park) for the water years (October 1 through September 30) 1983 through 2003, with the exception of 1995 and 1996. The chloride emitted by the geothermal system underlying Yellowstone Park is designated “thermal chloride,” and it constitutes 94 percent of the total chloride exiting the Park. The remainder of the chloride is contributed by rainfall, rock weathering, and a minor amount due to human impact.
Of this 94 percent, the Fall, Madison, Snake, and Yellowstone Rivers have been determined to discharge 93 percent of the chloride leaving the Park; the remaining 7 percent exits along the west boundary into the Henrys Fork River. The chloride flux for each river varied seasonally and annually, and we postulate that it depended primarily on the flow of hot springs. This flow, in turn, depended on the height of the local water table, which increased during spring runoff and varied annually in synchronism with changes in precipitation.
The sum of the annual chloride fluxes for the four rivers varies as much as 20 percent year-to-year. This sum, when corrected for the climatic factors, shows a decline of 10 percent during the past 20 years. A lengthening in the period between eruptions of Old Faithful Geyser has also been observed. We believe that these changes may be related to deflation of the Yellowstone caldera documented by changes in ground levels surrounding Yellowstone Lake.