For students who have difficulty envisioning how the mathematics that they are learning is used outside the classroom, an adventure as field mathematicians can be enlightening. Measuring stream discharge is a field experience that allows students to engage in a hands-on (and boots-on) real-world, problem-solving activity and that integrates mathematics with physical geography. The term "stream discharge" refers to the volume of water per second that passes a cross-sectional plane on a stream. It is perhaps the most important way that a scientist can describe the size of a stream. The overarching goal of this activity is to have students connect familiar fundamental mathematical concepts and develop a method to solve a sophisticated, real-world problem. The mathematical content includes the important topic of modeling real-world phenomena; flexibility in understanding how the dimensions of width, depth, distance, and time can be combined to express velocity, area, and stream discharge; and a practical application of summation notation to find the area of the irregularly shaped cross section of the streambed by partitioning it into smaller rectangles. Students must first understand what it means to measure the discharge of a stream, devise a method to measure the discharge, and then implement their plan. Finally, they reflect on their result and interpret it in a concrete manner. (Contains 5 figures.)