Arroyo del Valle or Arroyo Valle is a 36.4-mile-long (58.6 km) westward-flowing stream that begins in northeastern Santa Clara County, California, and flows northwesterly into Alameda County where it is dammed to form Lake Del Valle. After that Arroyo del Valle is tributary to Arroyo de la Laguna which in turn flows into Alameda Creek and thence to San Francisco Bay. In previous times the Arroyo del Valle had a significant steelhead migration; however, degradation of the stream in the latter half of the 20th century has decimated this anadromous fish population.