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A 35-year monitoring record for the water column of Lake Dillon, a reservoir of the southern Rocky Mountains, shows near-surface warming of 0.76 degrees C/decade and warming at all greater depths (55 m); warming was progressively smaller with depth. Annual heat budget of the lake increased (67 cal.cm(-2).year(-1); 0.089 W/m(2)) as did Schmidt stability (41%). The mixed layer was affected by climatic conditions at the high elevation of the lake (2,750 m above mean sea level); heat fluxes were high during both the seasonal warming and cooling. Density gradients below the mixed layer were weak because of low water temperatures associated with high elevation. Annual cooling of the mixed layer was rapid following a brief initial stabilization and showed high interannual variability across years for a given month, which obscured any trend in mixed layer thickness that might have been caused by heat accumulation. The hypolimnion was warmed by advective heat exchange from tributary inflow and deep water withdrawal, not by carryover of fall or spring warming; advective warming by tributaries can be expected in many reservoirs.

Plain Language Summary Lake Dillon, a mountain reservoir in Colorado, showed a high degree of surface warming (2.5 degrees C) over a 35-year interval as a result of climate change. Reservoirs have not been studied for response to climatic warming. Release of water from the bottom of the lake, which is common for reservoirs, caused Lake Dillon to show warming of deep water caused by replacement of cool water withdrawn through the outlet by inflowing river water, which showed climatic warming over the 35 years. Lake Dillon, which has higher elevation (2,750 m above mean sea level) than other lakes that have been studied for response to warming, did not show change in thickness of its surface layer (epilimnion) in response to warming, as expected at lower elevation, because the low surface temperatures of Lake Dillon cause irregularity in thickness of the mixed layer that overwhelms any tendency for the epilimnetic thickness to change in response to climate warming.