The authorities noted that a number of other dead whales have beached in the area recently, so it might not necessarily be our directionally-challenged cetacean friends -- and searches this morning have turned up nothing so far.
The fishermen were pretty sure they'd seen something, though, so everyone's still looking. The authorities also note that they haven't seen flocks of seagulls ("I Ran.... I ran so far away") circling, which you'd expect if something that large had died. Fingers crossed!
Update: 4:00 p.m. They found the whale, It wasn't Delta or Dawn.
.Picture from the Sacramento River by Sarah Wilkin, from the NOAA Fisheries Service.