For the past decade, WSDOT has been designing water quality studies at various Bioswales. These studies have been plagued with difficulties and provided many lessons learned. WSDOT has now used these lessons to create a study methodology and site selection strategy that can be applied to bioswales and potentially other ephemeral stormwater BMPs. The unique hydraulics of bioswales caused challenges in collecting scientifically sound, single storm event, composite samples. The slow-flowing, flat profiles of swales pushed the capabilities of volume and flow measuring technologies (WSDOT utilized flumes over weirs to minimize back watering). Using past failures, WSDOT was able to develop a site selection process and sampling methodology that ensures accurate hydrological data collection is possible at any study site.