1 * This paper is under review for publication in Hydrogeology Journal as well as a chapter in my soon to be published 2 master’s thesis. 3 4 Quantifying the base flow of the Colorado River: its importance in sustaining perennial flow in northern Arizona and 5 southern Utah 6 7 Riley K. Swanson1* 8 Abraham E. Springer1 9 David K. Kreamer2 10 Benjamin W. Tobin3 11 Denielle M. Perry1 12 13 1. School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ 86011, US 14 email:
[email protected] 15 2. Department of Geoscience, University of Nevada, Las Vegas, NV 89154, US 16 3. Kentucky Geological Survey, University of Kentucky, Lexington, KY 40506, US 17 *corresponding author 18 19 Abstract 20 Water in the Colorado River is known to be a highly over-allocated resource, yet decision makers fail to consider, in 21 their management efforts, one of the most important contributions to the existing water in the river, groundwater. This 22 failure may result from the contrasting results of base flow studies conducted on the amount of streamflow into the 23 Colorado River sourced from groundwater. Some studies rule out the significance of groundwater contribution, while 24 other studies show groundwater contributing the majority flow to the river. This study uses new and extant 1 25 instrumented data (not indirect methods) to quantify the base flow contribution to surface flow and highlight the 26 overlooked, substantial portion of groundwater. Ten remote sub-basins of the Colorado Plateau in southern Utah and 27 northern Arizona were examined in detail.