Forty-five percent of the water from the river supplying Lake Chapala has been diverted in the past 10 years. The water quality consequences of this change in water quantity are numerous. In this period, the lake volume has declined by 50 percent. Because of the reduced flushing and the corresponding relative increase in outputs via evaporation more than twofold, the lake's total dissolved solids content has increased. A second consequence, and one of great importance to the ecology of the lake, is the increase in water turbidity. With a reduction in lake mean depth from 7.2 m to 4.5 m, wind-driven resuspension of the inorganic sediments has increased. This has caused light limitation of phytoplankton production to the extent that nutrient loading models to predict production of biomass fail when applied to this lake. Reduction in turbidity by increased flocculation could significantly speed up the eutrophication process, because high concentrations of nutrients are present in the system (total inorganic phosphorus = 520 μg/L and total inorganic nitrogen = 512 μg/L). Because of the importance of this lake as a water supply resource and the unique nature of the water quality problems, a special set of water quality criteria appropriate to Lake Chapala is presented. To avoid further reduction in the water supplied to Lake Chapala from the River Lerma, its principal influent, consumptive water use in the River Lerma cannot increase. Further increases in the irrigated areas should come only from other water sources (e.g., wastewater reuse) or more efficent irrigation.