A generation ago the Sacramento-San Joaquin Delta tunnel project might have made a certain kind of sense. California’s lakes and rivers had been so thoroughly replumbed by dams, drains, pumps, canals and aqueducts that the state already contained the world’s most engineered water system — so why not add one more megaproject to the labyrinth? Water from the Sacramento and San Joaquin rivers flows into the delta, where some of it is directed into pumps that send it south to farmers on the San Joaquin Valley’s west side and to municipal users in Southern California.