Students use a topographical map featuring a body of water, such as a lake, pond, or river, to figure out how much the level of the water would rise after a one-inch rainfall the previous night. An extension of the problem and a discussion of the underlying mathematical ideas are included. This mathematically rich problem was originally developed for the Project Discovery Mathematics by Inquiry institutes for middle grades teachers taught in 1992 - 1994 at Ohio State University. Project Discovery was co-funded by the Ohio Board of Regents and the Statewide Systemic Initiative (SSI) program of the National Science Foundation. (author/sw)