Arroyo Seco is an 11.7 mi watercourse in Alameda County, California, that traverses through the city of Livermore, emptying into Arroyo Las Positas. Arroyo Seco means "dry stream" in Spanish. Arroyo Seco lies above the Arroyo Seco watershed, which includes the eastern part of the city of Livermore and also the Sandia National Laboratory. The Mocho Subbasin is the largest of the subbasins in the Livermore Valley watershed. This subbasin is bounded to the west by the Livermore Fault Zone and to the east by the Tesla Fault. Some groundwater flow occurs across these fault boundaries, but flows are discontinuous below a depth of 50 ft across the Tesla Fault and south of the Arroyo Mocho channel across the Livermore Fault. A number of threatened and endangered species reside in this watershed.