I didn't write earlier about the Souhegan River because I don't know it so well, but I found a site that describes it, with canoeing in mind. Here 'tis.

You can get realtime river flow information on most of the state's bigger rivers online, courtesy of the USGS.

I just stumbled on a new (to me) resource. It's a trail guide for all the towns around this area, courtesy of the Souhegan Valley Land Trust. More to explore!