This paper presents a study whose purpose is to help in the understanding of the characteristics of the flow through and downstream of a series of abrupt 3D contractions/expansions, seeing them more from the fluid mechanics point of view, rather than from the hydraulic perspective. These contractions/expansions comprise cross-sectional area changes in plan and side views in an open channel. An existing model, built to optimize the design of canoe chutes, was considered as suitable for this endeavor. The device included up to five "drop" structures, and flow variables were observed in one of them. Measurements of the three-dimensional velocity field and of the geometry of the free surface were performed. This paper describes and analyzes those measurements, as well as the feasibility of using acoustic sensors for measuring the location of the free surface.