Biofilm fouling is ubiquitous on ship hulls and other aquatic surfaces, such as corals and soft sediments. Biofilms increase drag on ships, reducing performance and increasing fuel use. Algal biofilms can impact ecosystem functioning as well, altering flow dynamics over coral reefs, for example. However, little is know about the effects of biofilm on boundary layer hydrodynamics. Because biofilms are compliant and have flexible protrusions from the surface (called streamers), interactions between biofilms and fluid flow are complex. I use high resolution Particle Image Velocimetry (PIV) to image and quantify the flow over biofilms in a high speed flow tunnel.