A context for recent hydroclimatic extremes and variability is provided by a ∼10 k.y. sediment carbonate oxygen isotope (δ18O) record at 5–100 yr resolution from Bison Lake, 3255 m above sea level, in northwestern Colorado (United States). Winter precipitation is the primary water source for the alpine headwater lake in the Upper Colorado River Basin and lake water δ18O measurements reflect seasonal variations in precipitation δ18O. Holocene lake water δ18O variations are inferred from endogenic sedimentary calcite δ18O based on comparisons with historic watershed discharge records and tree-ring reconstructions. Drought periods (i.e., drier winters and/or a more rain-dominated seasonal precipitation balance) generally correspond with higher calcite δ18O values, and vice-versa. Early to middle Holocene δ18O values are higher, implying a rain-dominated seasonal precipitation balance. Lower, more variable δ18O values after ca. 3500 yr ago indicate a snow-dominated but more seasonally variable precipitation balance. The middle to late Holocene δ18O record corresponds with records of El Niño Southern Oscillation intensification that supports a teleconnection between Rocky Mountain climate and North Pacific sea-surface temperatures at decade to century time scales.