Wikipedia says its name derives from the Wampanoag word "muhs-uhtuq", which translates to "big river." In an Algonquian language, "Missi-Tuk" means "a great river whose waters are driven by waves", alluding to the original tidal nature of the river. (The Mystic now has a flood-control dam that segregates its saline, tidal mouth from the fresh, tide-less upper river.)
During the "polar vortex" cold snap, the fresh-water portions of the Mystic froze solid, but with warmer weather, the flowing portions of the river have opened up again, and the local water birds have responded.
In addition to the usual gulls, the Mystic's open water near my apartment has seen an explosion of swans, canada geese, buffleheads, and large numbers of mallards and other common ducks.
A few cell phone snaps of the more unusual winter birds:
A hawk, probably a red tail:
A night heron, standing on the ice of a small tributary of the Mystic:
digital zoom (alas):
Swan on the partially-frozen river: