Henry David Thoreau wrote his first book, "A Week on the Concord and Merrimack Rivers", in 1849 while living at nearby Walden Pond. His book recounted a seven-day boat trip on the rivers with his brother John, who had since died. Thoreau recounted his exploration of the natural beauty of the river, and his accompanied thoughts on such eternal themes as truth, poetry, travel, and friendship. Despite the growth of the suburbs in the vicinity of the river, it remains a popular canoeing destination today. The last mile of the river in Lowell is serious class 3+/4 whitewater.