LARGELY UNNOTICED in what Californians call "Back East" was the rejection, by an unexpectedly large margin, of the Peripheral Canal by California voters in their June primary. This result tells us more about how Californians and, increasingly, most Americans feel about basic issues of economic growth, technology and the environment than any other election this year is likely to; and the message is not altogether reassuring.

For the rejection of the Peripheral Canal was more than the victory of Northern California, which has plenty of water, over Southern California, which wants it. Southern California favored the proposal by only a 63 percent-to-37 percent margin--a lukewarm result for a sectional issue. In Northern and central California, it was rejected by a near-unanimous 89 percent to 11 percent. The result was in stark contrast to the 1960 referendum in which Californians approved the monumental water plan of which the Peripheral Canal was the latest installment.

True, there were a number of good reasons to vote against the Peripheral Canal. No one can be sure how much the project, scheduled to go on until 2000, would end up costing. The canal, which would have connected the Sacramento River with the California Aqueduct that flows 400 miles south through the Central Valley to Los Angeles, could have a disastrous effect on the ecology of the Sacramento estuary and San Francisco Bay. Although the canal would have nearly doubled the supply of water to the south, one might argue that it already has enough, and the cost of the energy required to pump the water might be uneconomically high.

Those are the arguments voters seem to have heeded. They seem no longer to feel, as California voters in 1960 did, that economic growth is an all- but-unalloyed good. Similar projects were among the proudest accomplishments of the New Deal. Today, we take them for granted. We search, as the voters of California did this year, not to understand how projects like the Peripheral Canal might work and how they might help us, but to see how they will fail or cost more than expected or have negative side effects. Our doubts, on the Peripheral Canal and many other projects, may well be justified. But we may be overlooking the benefits of economic growth when we insist on being blas,e about what Americans 20 and 25 years ago would have regarded as awesome achievements.