Greater Boston alternative to the Ipswich

My favorite kayaking is in the area salt marshes. With the assistance of Google Maps, I love getting “lost” in the reeds and hitting dead end creeks and enjoying the wind, gentle waves, tides, and the cool (but not cold!) water. However, we are now in the middle of greenhead fly season and I’m avoiding most of my favorite marshes until the feeding frenzy is over (rumored to be about 7+ days into August).

As an alternative, I am seeking out the cooler rivers. I like to alternate occasionally between kayaking and swimming when it gets hot, or at least enjoy the cool spray of the ocean or a cooler river. Most of the rivers and all of the ponds are now too warm to be enjoyable on a hot summer day. The one exception I have seen so far is the Ipswich River, probably due to its smaller size and greater shade. Over the past two years though, I have done the Ipswich to death, although I haven’t done the Middleton section yet.

What are some good, shady river alternatives to the Ipswich in Greater Boston? Perhaps the upper parts of some of the bigger area rivers are good (Charles, Assabet?) What are some specific put-ins on some of the area narrower, shadier rivers? I live in Lynn so I prefer not to go down I-93 through Boston to get to places on the South Shore on my days off.