OBSERVATIONS FROM STREAMS AND SPRINGS PROVIDE INSIGHT INTO GROUNDWATER IN YELLOWSTONE

Observations from stream geomorphology and stream hydrographs and hydrograph separation provide evidence for large scale, rapid groundwater circulation in Yellowstone. Ratios of annual maximum to minimum mean-daily discharge, baseflow index, and mean annual basin recharge were calculated from streamflow data for the period of record at 32 U.S. Geological Survey gaging stations in the Yellowstone area. Ground-water ages were determined from tritium and CFC analyses of water from 30 low-temperature springs discharging from Quaternary volcanics in the Norris area of Yellowstone National Park. The ratio of annual maximum to minimum mean daily discharge varied from 1.9 to 226. Streams with basins underlain by Yellowstone Quaternary rhyolites and tuffs had ratios ranging from 2 to 7 indicating that these streams were dominated by ground-water inflows. Streams in basins with high ratios (30-226) generally were underlain by Precambrian crystalline, Paleozoic sedimentary, and Eocene volcanic rocks and streamflow was dominated by runoff. Recession and baseflow indices have similar characteristics and correlation with geologic units. Water from all springs sampled had measurable CFC and tritium and a significant fraction of modern water indicating rapid flow rates through Quaternary volcanics. Ground-water recharge calculated from baseflow hydrographs indicate 50 percent of precipitation becomes recharge providing further evidence that Yellowstone Quaternary volcanics are highly permeable. The ground-water flow system in the Quaternary volcanics, because of its high permeability, is dynamic and is likely an important component in the surficial expression of the Yellowstone geothermal system.