How does the author describe the Concord and Merrimack Rivers in the biography, Thoreau of Walden Pond?

John and Henry Thoreau sail down these two connected rivers in a memorable trip on a boat which they make with their own hands. The Concord River is placid, with slow, predictable currents. It drifts past woodland meadows and picturesque, well-developed farm land. To cross into the Merrimack River, they must get around waterfalls by pulling their boat through manmade canals. The Merrimack River is swifter, and riddled with waterfalls which the young men cross with the help of locks, a series of cement chambers that can be filled with water to lift boats to a new river level.