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~ 21.03[1] MINERAL LAW INSTITUTE 21-20 Ferry was 13.5 m.aJ. per annum between 1564 and 1960.69 The lowest ten-year flow during those four centuries was 9.7 m.aJ. per annum between 1584 and 1593/0 and it is to be ex- pected that at some point in the future the flow will again subside to that level or less. Indeed, the worst drought of which researchers are now aware occurred before 1564. Tree- ring studies completed in 1979 indicate that the driest period in the Colorado Basin was from 1130 to 1180 and was proba- bly what drove the Anasazi off the Colorado plateau.71 Fur- thermore, the "greenhouse effect" may diminish River flows even below projections based on these historical studies. In 1983 a report by the Carbon Dioxide Assessment Committee of the National Academy of Science warned that increased levels of carbon dioxide in the atmosphere could reduce the water supply on the Upper Colorado by 39.6% and on the Lower Colorado by 56.5%.72 There may be other stresses on the usable supply of River water independent of future meteorological conditions. Sa- linity controls may at some point effect a reduction in the amount of available water. Saline water emitted by natural springs might be impounded.73 The Colorado Basin occupies the bed of a vanished ocean and is underlain by highly saline shale formations. Irrigation water percolating through the ground tends to leach those salts into the River. Moreover, evaporation from reservoirs serves to concentrate the salt in the remaining water.74 Limitations on irrigation and storage might conceivably be imposed to lower salt levels. Further- more, the Mexican Treaty deprives the Basin states of water in excess of its stated minimum delivery requirement be- cause of evaporation and channel losses which the United 69 C. Stockton & G. Jacoby, Long-Term Surface-Water Supply and Streamflow Trends in the Upper Colorado River Basin (Lake Powell Research Project Bulletin No. 18. 1976). 70 [d. 71 A. Kneese & G. Bonem, "Hypothetical Shocks to Water Allocation Institutions in the Colorado Basin" in New Courses for the Colorado River, supra note 5, at 106. 72 National Academy of Science, Carbon Dioxide Assessment Committee, Chang. ing Climate 423 (Washington, D,C. 1983). 73 G See, e.g" etches, supra note 7, at 463. 74 See generally Environmental Defense Fund v. Costle, supra note 63.