This activity is designed to illustrate an application of resistive forces in the introductory physics curriculum with an interdisciplinary twist. Students are asked to examine images of riverbed boulders after a flood and estimate the water flow that was needed to push the boulders downstream. The activity provides an opportunity for students to gain an understanding of the physics of natural phenomena with applications to environmental science and environmental engineering, to gain a stronger appreciation for physical modeling and estimation, and to become "environmental detectives"!