![]() Natural flows peaked in winter and again with spring snowmelt, and often fell to very low levels in fall. In each twenty-year period since 1930, average annual flow toward the Golden Gate has declined: from 81% of “unimpaired” flows in 1930-1949 to 48% or less in the first two decades of our century. ![]() 6ĭelta outflows have dropped correspondingly. 5 The proportion exported via the pumps grew from 15% in the 1970s and 1980s to 17% in the years leading up to 2010. It was zero in 1950 and reached 6.4 MAF in 2006, a level unlikely to be reached again. The amount taken directly from the Delta for export-the amount that passes through “Delta conveyance”-is not the largest slice of the pie chart, but it is the slice that has shown the greatest growth. In very wet years, that leaves a great deal of water to flow toward the Golden Gate in very dry years, outflows drop to mandated minimum levels of about 10 MAF. The combined draw has recently fluctuated around 12 MAF. It includes water that people consume in the watershed before it can reach the Delta water the Bay Area pipes in directly from the Mokelumne and Tuolumne Rivers, bypassing the Delta water used in the Delta itself and Sacramento River water drawn through and out of the Delta for shipment mostly southward (but serving additional swathes of the Bay Area). The human draw varies too, but within a much smaller range. In very dry ones, it could be under 10 MAF. In very wet years, what is called the “unimpaired” flow from the Delta into the Bay would exceed 50 million acre-feet (MAF). Reservoirs are time machines, shifting supply from wet seasons to dry and from soggy years to arid ones. That is one reason we have our elaborate water systems. In our variable climate (and becoming more so), the amount of runoff to the Delta and the Bay fluctuates wildly from year to year. A second paper will focus on the vexed question of Delta outflows, but here is a very brief sketch. This year the state offers a new, scaled-down plan.Ĭross-Delta conveyance has to be seen in the context of overall water movements. 2 Efforts to alter this Rube Goldberg system have, one after another, died in controversy, due in essence to fears that improved engineering would lead to greater exports and lower outflows. The ill effects on salmon and other species-on an entire ecosystem-are well known, compounding the damage done by the sheer volume withdrawn. Accelerated currents erode banks and hurry nutrients out of useful reach. Plankton, eggs, and small fish vanish into the intakes many larger fish die in the works despite inadequate fish screens. These flows cut across natural seaward ones, causing water in many channels to run backwards at times. Water entering the Delta at the north exits at the southwest corner, drawn that way by three great arrays of pumps near Tracy, two serving the federal Central Valley Project and the third belonging to the State Water Project. A rare point of agreement is that the present arrangement is not satisfactory. The second question is by what route the water sent southward should travel. ![]() ![]() But almost one hundred years after it was first asked, this question is still in search of an answer. It is now seen much more in environmental terms. This issue nests inside a larger one: how much of the natural flow from the Central Valley watershed should still be allowed to follow its old route down various channels in the Sacramento-San Joaquin River Delta to Suisun Bay, San Pablo Bay, and San Francisco Bay? The debate used to be framed as an economic contest between the thirsts of a single Bay Area County, Contra Costa, and those of San Joaquin Valley and metropolitan southern California. Two great, intertwined, and unsolved questions haunt this north-south transfer. 1 The cross-Delta transfer still dwarfs any other single artificial movement of water in the state, and it is the link that binds the vast majority of the state’s water systems into an interconnected, though imperfectly integrated, whole. ![]() Feeling our way through a thicket of numbers, we can say that the Bay Area, the San Joaquin Valley, and metropolitan Southern California each get as much as a third of their supply via pumps at the Delta’s southern edge. Some regions do depend quite heavily on this water. Yet there are reasons why we can’t take our eyes off the Delta. Water moved in this manner makes up, on average, no more than a tenth of California’s developed water supply, and this proportion is decreasing. Sometimes it seems that California water debates revolve around a single question: how best to shift water from the Sacramento River, across, around or under the Sacramento-San Joaquin Delta, to farms and cities to the south and west. ![]()
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