A new Δ18O map of southern Nevada groundwater shows a systematic decrease in Δ18O of ~5? from 36¿ to 39 ¿N latitude. The variation is consistent with higher-latitude recharge following continuous flow paths along north-south trending graben valleys and systematically increasing in Δ18O due to mixing with lower-latitude, higher-Δ18O recharge. The data do not suggest that large masses of groundwater with unusually low Δ18O values were recharged during the last pluvial period as suggested by previous workers. Many Δ18O-ΔD pairs in groundwater indicate variable amounts of evaporation relative to the global meteoric water line. The precipitation rate and type (rain versus snow) for a given geographic area controls the extent of evaporation. A model Δ18O value was calculated from evaporated groundwaters by subtracting postcloud evaporation. The distribution of these model Δ18O values suggest new regional groundwater flow paths previously undocumented. Dissolved inorganic carbon in groundwaters collected from deep regional flow systems typically has low 14C concentrations (≤12% modern carbon). In contrast, groundwaters collected from the carbonate rock of the Spring Mountains and alluvium of Forty Mile Canyon have higher 14C contents, indicating more recent recharge. Because of nonlinear mixing of 14C, groundwater in the regional flow system likely acquired most of its observable 14C from mixtures of young, locally recharged groundwater.