Sacramento Delta Three Mile Slough bridge / ?? (#1207)

Another bridge that makes you wonder ‘why?’. This is the Three Mile Slough bridge (build in 1949), south of Rio Vista. Three Mile Slough at this point connects to the Sacramento River/Ship Channel (the opposite side of the bridge), but on this side the Three Mile Slough is a short stretch of water (three miles long, apparently), connecting the Sacramento River with the San Joaquin River. While the San Joaquin has ocean-going ships going to the port of Stockton (another inland port), they don’t use this slough. So…. why an elaborate lift bridge which probably only raises for recreational boats?