Evocative, companionable voyages down Massachusetts' Assabet and Sudbury Rivers, sources of Thoreau's Concord (but not, like Thoreau in the Mountains, above, travels-in-the-footsteps-of-the-Master). After a survey of the terrain by air, Zwinger, a Colorado river buff, and the late Edwin Way Teale, then 80+, alternate narrations on the guided tour. As they skim down the rivers (or maneuver their way around natural or man-made obstacles), they lovingly record the presence of swamps and natural flood-control phenomena (mud, ooze, tannin-filled water); polluted sections, resembling the Styx (""yet the river can become itself again in a: matter of rods""); the march of loose-strife and other opportunistic plants; some handsome bullfrogs, some painted turtles and assorted insects; and always, in Teale's words, ""those times we would gladly live again""--""willows set amid the wide meadows under a windy sky,"" ""the shimmering verdant silence"" of a pond. But these are peopled rivers, which run deeply through New England history. Artifacts--from Indian and Colonial times--tell diverse tales; sites (old stone bridges, ruins, of iron works, abandoned mills) speak of economic and population growth and decline, past and present conflicts. A remnant of wild hay on the Sudbury leads to a brief history of the farmer/town battles over damming and the water level. (Today, environmental groups are working to save swamps and control pollution.) With many mini-biographies (of tycoons, pioneers, inventors) and remarks culled from Thoreau: pleasant reading for other river rats--and a superb guide for canoeists and hikers tempted to retrace those inviting routes.