A pretreatment swale is a gradually sloped channel that increases travel time, reduces runoff velocity, and utilizes vegetation to filter coarse sediment and debris from runoff. Pretreatment swales provide both conveyance and pretreatment for downstream stormwater BMPs. Check dams may be utilized to increase pretreatment capacity by temporarily storing runoff, further reducing the runoff velocity in the swale. Pretreatment swales can be incorporated into highway and road drainage systems but can also be used in place of traditional curb and gutter drainage systems.

Unlike the Water Quality Conveyance BMPs (Wet Water Quality Swale and Dry Water Quality Swale), which are suitable for providing stormwater retention and treatment credit, pretreatment swales are not stand-alone treatment practices due to their limited pollutant removal, runoff volume reduction, and groundwater recharge. Pretreatment swales should only be used upgradient of another structural stormwater BMP.