Natural resource scientists routinely need bathymetric maps of inland water bodies to answer environmental questions. This study integrated differential GPS, GIS, and sonar measurements to construct a bathymetric map of Topock Marsh, Arizona, a shallow water impoundment and wetland. Our procedures are summarized as follows: (1) create a georeferenced base-map using orthophoto quads; (2) locate a base station near the marsh; (3) collect bathymetric measurements; and (4) process, convert, analyze and display results. This technique was inexpensive and is ideal for small- to medium-sized lakes and ponds where sub-meter accuracy is desired. Our aim was to calculate the monthly change in storage of the marsh for a water balance calculation (Guay, 1999). These procedures generate resource maps and depth-surface area-volume relationships.