We used water δ₂H and δ18O from ca. 1000 lakes sampled in the 2007 U.S. Environmental Protection Agency’s National Lakes Assessment (NLA) to assess two hydrological variables—evaporation as a percentage of inflow (E : I) and water residence time (τ) for summer 2007. Using a population survey design, sampled lakes were distributed across the conterminous U.S., and results were scaled to the inference population (~50,000 U.S. lakes). These hydrologic variables were related to lake nutrients and biological condition to illustrate their usefulness in national water quality monitoring efforts. For 50% of lakes, evaporation was ~ 25% of inflow, with values ranging up to 113% during the 2007 summer. Residence time was ~ 0.52 yr for half of the lakes and ~ 1.12 yr for 75% of lakes. Categorizing lakes by flow regime, 66.1% of lakes were flow-though lakes (60% or more of the water flows through the lake, E : I < 0.4), 33.6% were restricted-basin lakes (40% or more of the lake inflow evaporates, 0.4 E: I < 1), and , 0.3% were closed basin (all water entering the lake leaves through evaporation, E : I > 1). While climate patterns drove some of the spatial patterns of E : I and τ, variation in lake depth and watershed size (influencing precipitation volume) were also significant drivers. Lake hydrochemistry was strongly correlated to E: I and more weakly related to τ. Lakes in poor biological condition (based on a predictive model of planktonic taxa) were significantly more evaporated than lakes in good biological condition.