I think that the Saugus River is the best choice considering the description in the original post. Other reasonable choices are the Merrimack River, Crane Brook plus Crane River, and Mystic River. That doesn't mean that any of these are compatible with canon. In particular, I don't know how far north any of these towns are, or how far west Dunwich is.
I spent enough time staring at maps to bring myself to madness. Here's a list of rivers in Massachusetts, sorted based on where they drain into the sea from north to south.
Merrimack River
Length: 117 mi. I estimate 49 of those are in Massachusetts.
Direction: South through New Hampshire, then Northeast through Massachusetts, with a little southern turn at the end.
Location: From Franklin NH to Nashua NH. Then Tyngsborough, Chelmsford, Lowell, Dracut, Tewksbury, Andover, Methuen, Lawrence, North Andover, Haverhill, Groveland, West Newbury, Merrimac, Amesbury, between Newburyport and Salisbury
Ipswich River
Length: 35 miles. Long enough.
Direction: Northeast. Wrong way.
Location: From Burlington to Ipswich
Essex River
Length: 4.6 mi estimate. Too short.
Direction: North. Wrong way.
Location: Entirely in Essex. Definitely starts in hill country.
Bass River
Length: 3.7 mi estimate. Too short.
Direction: South east. Right way.
Location: Entirely in Beverly
Crane Brook plus Crane River (Two rivers together makes it a stretch)
Length: 5.5 mi estimate. Hard to squeeze the towns we need in there.
Direction: South southeast.
Location: Danvers, then between Beverly and Salem.
North River
Length: 3.1 mi estimate. Too short.
Direction: SE then NE (wrong turn)
Location: Peabody, Salem
Forest River
Length: 3.1 mi estimate. Too short.
Direction: South then east then northeast .(wrong turn).
Location: Entirely in Salem, but ends along Marblehead
Saugus River
Length: 13 mi. Long enough for 3 towns.
Direction: South south east
Location: Wakefield, Lynnfield, Saugus, Lynn
Pines River / Rumney Marsh
Length: 5 mi estimate. Hard to fit the towns we need in there.
Direction: East then north. (Wrong turn.)
Location: Malden, between Revere and Saugus, ending at Lynn border
Mystic River
Length: 7 mi. Long enough for 3 towns.
Direction: Southeast
Location: Arlington, Medford, Somerville, Everett, Chelsea, Boston (Charlestown neighborhood)