Extensive travertine deposits occur over 50 - 100 square mile areas near the Green River in central Utah, and the Little Colorado River between Springerville and St. Johns in eastern Arizona. Both areas occur adjacent to fault zones with significant differential vertical displacement of Colorado Plateau strata. Analysis of drill stem pressure measurements from deep exploration wells, and potentiometric data from groundwater, springs and CO₂ geysers, suggests that these areas are outflow zones of deep basin fluids saturated in CO₂ originating from aquifers up to 1000 square miles in area. Older travertine caps terraces and forms domes that are up to 200 feet above the presently active seepage areas. Based on erosion rate estimates of less than one foot per thousand years for the Colorado River system in Utah, the fluid outflow has been active for at least several hundred thousand years. These areas may be natural analogues for some of the potential effects of CO₂ leakage from subsurface reservoirs with imperfect seals.