Experiments were performed to determine the physical mechanisms for the local heat transfer enhancement caused by short fins of square planform cross section, mounted on one wall of a rectangular water channel. Local heat transfer coefficients and pressure drops were measured in an array of thirty fins. Inline as well as staggered arrays were investigated. The geometry of the individual fins was held constant, while the spacing between fins, the height of the flow channel, and the Reynolds number of the flow were varied, and their effect on heat transfer enhancement determined. Flow visualization was performed using laser sheet-illuminated hydrogen bubbles, and velocity measurements were obtained using laser-Doppler velocimetry. The hydrodynamic mechanisms that enhance mixing in the flow, and hence heat transfer from the fins, were identified. Separation and reattachment of the flow, interactions between the wakes generated by the fins, and horseshoe vortices formed at the upstream faces of fins were investigated. Reynolds numbers for the onset of transition in the complex three-dimensional flow through the array of fins were determined.