Uppermost Miocene lacustrine deposits of the Ridge basin, California, contain benthic microbial carbonates as fragmentary crusts on clasts in resedimented conglomerates of the Violin Breccia, deposited adjacent to the fault-controlled southwestern margin of the basin, and as fragments and more extensive, in situ stromatolitic units within the Ridge Route Formation around the northeastern part of the basin. Ridge Route microbialites formed in the open waters of the Ridge basin paleolake and have δ18O values that are significantly higher than those from the Violin Breccia. The latter may have formed in warm, littoral pools or in streams maintained by local runoff. Their δ18O compositions are consistent with carbonate precipitation from meteoric waters supplied by rainfall dominated by 67%-75% winter precipitation at temperatures of 16-17 °C. Ridge Route Formation microbialites have a wide range of δ13C and δ18O values and indicate that the lake fluctuated between hydrologically open and closed conditions. Light carbon isotopic compositions for carbonate that precipitated during periods of open-basin conditions suggest that inflow waters came from a fluvial system with a well-vegetated flood or delta plain. Relatively high δ18O values imply that the lake was not maintained by local rainfall but depended on inflow that came either from an upstream lake or from a region to the east or northeast where the rainfall regime was dominated by isotopically heavy summer precipitation.