The spectacular landscape at Grand Canyon provides a unique view of a deeply dissected aquifer system. Travertines and associated active springs provide key data for understanding the modern hydrology, as well as a record for understanding how this system has evolved and interacted with Quaternary canyon incision. Both the modern and ancient travertine-depositing springs, and the Colorado River itself, record a “tale of two waters”: a mixing of deeply-circulated waters rising along faults with surface- and groundwaters of the plateau. Travertine occurs where major spring-producing horizons (such as the Muav Limestone-Bright Angel Shale contact) intersect Laramide and Precambrian basement-penetrating faults.  We suggest on the basis of spring water chemistry and gas analysis that  large volumes of travertine are produced from deep saline waters that are rich in CO₂ and higher in ⁸⁷Sr/⁸⁶Sr than waters derived from the plateau aquifers. Spring waters in travertine-forming localities record high He/Ar and He/N₂, and ³He/⁴He ratios of 0.15 Ra suggesting contributions of mantle-derived He and CO₂ potentially from nearby coeval volcanic fields. ⁸⁷Sr/⁸⁶Sr in central Grand Canyon springs ranges from 0.710 to 0.728 and we hypothesize that radiogenic Sr contributed by deeply-circulated waters such as these account for previously observed increases in Colorado River ⁸⁷Sr/⁸⁶Sr between Lee’s Ferry and Lake Mead.  We also propose that the radiogenic Hualapai Limestone (~5 Ma) in the Grand Wash trough was formed by spring waters similar to those seen in today’s travertine localities (⁸⁷Sr/⁸⁶Sr of Grand Canyon travertine is comparable to values reported for the Hualapai Ls.). Rapid travertine deposition in selected localities has also caused pervasive cementation and preservation of Quaternary terrace sequences and this allows direct U-series dating of aggradation events and calculation of incision rates.  Dating so far suggests that major pulses of travertine accumulation occurred over the last 350 ka, and seem to correspond with glacial events/ regional wetness (in agreement with previous workers). The travertine record, combined with spring water chemistry and geochemical analysis will provide a quantitative understanding of both the Quaternary history and evolving hydrologic system of the region.