I covered Powell in some detail during a meeting of the Geological Society of America section meeting in Lawrence, Kansas. Abstract available here:
http://gsa.confex.com/gsa/2007SC/finalprogram/abstract_120189.htm
A University of Kansas Professor did an outstanding biography of Powell, that you might be interested in: Donald Worster, 2001. A River Running West. Oxford University Press, 673pp. I've visited many of the major sites associated with Powell, including Expedition Island in Green River, WY; the Grand Canyon of the Colorado River in Arizona, Shiloh, Vicksburg, and Cape Girardeau. Missing from this list that I intend to rectify at some future date are Franklin, TN; and his grave site in Arlington National Cemetary. The USGS not only has a research vessel named after him; in addition the USGS Headquarters in Reston, VA is located in the John Wesley Powell Federal Building. He is truly a great American hero for his contributions to this country, let alone his contribution to the Union effort during the war.