1. Research Geologist, U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, Flagstaff, AZ 86001 (corresponding author).
2. Research Hydrologist, U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, Flagstaff, AZ 86001.
3. Assistant Professor, School of Earth and Climate Sciences, Univ. of Maine, Orono, ME 04469.