The Green River lake system has a complex history throughout a series of sedimentary basins in Wyoming, Colorado and Utah. Within the Piceance Creek Basin (PCB) of northwestern Colorado, in the early Eocene, that history consists broadly of a lake within the center of the depositional basin and a rapid expansion southward and eastward to cover the entire basin through the middle Eocene. This expansion occurs in part due to the addition of water to the PCB from Lake Uinta across the Douglas Creek Arch. The timing of this expansion is poorly constrained within the PCB because of the relatively poor fossil record within the lake sediments. However, marginal sediments with mammalian fossils can provide some information to help constrain the timing of this expansion. The discovery in the DeBeque Fm. of a early Bridgerian fauna superposed over a sequence of middle and late Wasatchian faunas from the Mamm Peaks area near Rifle, Colorado, and comparison of that section with the coeval section at Sherrard Park below Anvil Points, allows us to place the lake margin during that period of time (approximately 52 mya) between these localities what is now Battlement Mesa and Anvil Points. This places constraints on the size of Lake Uinta within the PCB at that time and on the timing of the expansion of the lake during which much of the oil-shale was deposited, thus illustrating the utility of mammalian biostratigraphy to the fluvial Eocene deposits Green River lake system.