This paper shows how uncertainty undermines collaborative transboundary groundwater management. Focusing on the Santa Cruz Aquifer, spanning the United States-Mexico border between Arizona and Sonora, the authors describe the uncertainties within the aquifer using interviews and hydrologic studies.We discuss how data requirements and ambiguous interpretations exacerbate these uncertainties, and explain how each country’s water-management culture combines with this uncertainty to create contrasting views on groundwater availability and abstraction impacts. As a result, water managers in both countries predict different impacts from pumping and recharge, and each uses that information discursively to support unilateral policies rather than to promote collaborative management.