Mike - its 28 miles by car, more by river miles (40?) from the Lawrence Dam to the mouth of the Merrimac. I'd guess that the current flow is an average of 1-2 knots, or 20 hrs w/out tidal effects.
I worked 1/2 mile from the Sutton St. Shad/bass area 1/2 mile down from the lawrence dam and never saw/heard of fish rolling/spawning. Which isn't to say they don't spawn in the side channels.
6 inch micro bass - I've caught them for the past 2 years in both Plum island and down on the cape. I seem to recall last year catching a 4" bass in Popponessett bay which certainly could not have made the swim. I wonder what a seine net in Barnstable Marsh or one of the Vineyard ponds would yield in the spring?
L.