Harkins Slough is the largest and most north-reaching slough in the Watsonville system. It receives drainage from Larkin Valley and flows eastward under Highway One, between Buena Vista Road and Airport Boulevard. Here it broadens, often flooding Harkins Slough Road. Gallighan Slough, which is inaccessible, merges into Harkins Slough at a point below the county landfill. Harkins Slough continues south and joins Watsonville Slough east of San Andreas Road. It can be viewed from Harkins Slough Road west of Highway One.