The glassy, cold Carmel River surged through a little valley in the Santa Lucia Mountains, cascading in front of a half-dozen workers and observers one day last week down a series of rock outcroppings, as if its sinuous path had been designed by nature. Nature, of course, had nothing to do with it. The half-mile section of river, with steppingstones, pools and a tableau of freshly planted trees and bushes along the bank, flows through what was once the flooded plain of the San Clemente Dam, a Monterey County landmark for 94 years until it was removed last year.