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N ~ 0') C:) Arizona's Growth - Eldridge p. 2 magnesium and sodium, and the negative atoms like sulfate, chloride and bi- carbonate. I pointed out that most water chanists prefer to call it SMC, or soluble mineral content, but it means the same thing as TDS. I explained that ppm, parts per million, and mg/L, milligrams per liter, mean the same thing too, at least up to concentrations of 7,000 mg/L. .I said that the threshhold for acceptable drinking water is 500 ppm, and for irrigation water 800-1,000 ppm, and that sea water averages 35,000 ppm. That led me to describe the pre- historic sea that covered mid-America and that left behind salty shale deposits as it evaporated and retreated when the mountains we know as the Rockies rose into place. Davi d fi na lly cut me off and s ai d, "You mean it's sa 1t in the water that makes it taste yucky and hurts plants ," and ran out to play. Si nce then, questions I get around the dinner table are quickly fol1C1i1ed with, "Give the short answer, Dad!" So here are the short answers to the question of what we don't know about what li es ahead for water managers and users in the Colorado River Basin that we share. 1. We don't know how the 1944 Treaty with Mexico will work out when we have a drought. The treaty establishes Mexico's share of the river flow at 1,500,000 acre-feet per year, but in the event of "extraordinary drought or serious acci- dent" in the United States, the amount flowing into Mexico may be reduced in the same proportion as are consumptive uses in the U.S. The treaty does not tell us how to distinguish between a run of the mi 11 drought and an "extraordinary" one, nor whether the drought must affect the entire basin or just one of the sub-basins. How do we factor in U.S. storage reservoirs that can cushion the local effects of drought for two or three yea~s? Do we count transportation and evaporation 'losses on what the up-stream states "deliver" to the Mexican claim at the border? We not only do not have answers to these basic questions, we have no system in place to administer the answers. Who manages drought re- sponses in the Basin? Let's hope that we don't have another 'drought like the one that drove out the Anasazi in the 1300's. 2. We don't know how long the federal government will continue to pay for salinity control projects in the Colorado River Basin. Both Upper and Lower Basin states are united in expecting total federal subsidies for salinity con- trol, despite the contemporary evidence that federal subsidies for water pro- jects are becoming a thing of the past, or at best only available on a cost- sharing basis. It is very significant that the larger portion of the present