The Arroyo Seco, meaning "dry stream" in Spanish, is a 24.9-mile-long (40.1 km)[4] seasonal river, canyon, watershed, and cultural area in Los Angeles County, California.

The watershed begins at Red Box Saddle in the Angeles National Forest near Mount Wilson in the San Gabriel Mountains.

As it enters the urbanized area of the watershed, the Arroyo Seco stream flows between La Cañada Flintridge on the west and Altadena on the east.

[5] The Arroyo Seco stream, which is fed by a watershed of 46.7 square miles (121.0 km2), helps to replenish the Raymond Basin, an aquifer underlying Pasadena that provides about half of the local water supply.

This arroyo is one of two major streams that capture rainfall and storm water in Pasadena, the other being Eaton Wash on the eastern side of the city, which is a tributary of the Rio Hondo watershed.

[6] The Arroyo Seco then passes under the Ventura Freeway and the Colorado Street Bridge, and it crosses the Raymond Fault at the southern boundary of Pasadena at the San Rafael Hills.

The channel continues along the western boundary of South Pasadena, then into northeast Los Angeles flowing southeast of the Verdugo Mountains and Mount Washington.

After the 1820s secularization of the Missions, the broad area to the east of the Arroyo was the Mexican land grant of Rancho San Pascual, present-day Pasadena, California.

With the 1874 establishment of the community of the Indiana Colony, the new residents built their homes along today's Orange Grove Boulevard, the major north–south avenue paralleling the Arroyo on the east.

However, the deep and seasonally flooded Arroyo presented a barrier to easy travel and transportation between renamed Pasadena and Los Angeles.

Dobbins was only able to build a two-mile portion of the cycleway from the Green Hotel to Raymond Hill before competition from the railroads and the growing popularity of the horseless carriage undermined the project.

The Arroyo Seco generally has a flow of several cubic feet per second,[13] but periodically it is inundated by torrential floods from its steep, erosion-prone mountain watershed.

Devil's Gate Dam in the Arroyo Seco in northern Pasadena between La Cañada Flintridge and Altadena is managed by the Los Angeles County Department of Public Works.

The County of Los Angeles conducted public hearings on the Environmental impact statement before selecting a method to remove debris from the Hahamongna area.

In 2014, the Board of Supervisors approved a five-year project to remove 2.4 million cubic yards of sediment (1,800,000 m3) from the basin despite strong opposition from neighbors and recreational enthusiasts.

For more than one hundred years, the natural environment of the Arroyo Seco and its proximity to a large urban population have inspired efforts to protect, manage and preserve it.

[citation needed] L.A. County flood control was first founded in an effort to control the wild waters of the Arroyo Seco and Charles Lummis founded the Arroyo Seco Foundation in 1905 for the purpose of preserving recreational use and habitat, residents in Los Angeles and Pasadena contributed generously to efforts to buy up open space in the great canyon to protect for future generations.

JPL has been criticized by regional environmentalists and the community for its water pollution of the local groundwater with toxic chemicals, such as solvents and perchlorate rocket fuel accelerants.

A monumental cleanup project by NASA has begun, which includes a multimillion-dollar pumping and water filtration system to treat the groundwater, removing toxins until the aquifer contamination level has been reduced below their federally-specified limits.

Millard Canyon was a major pedestrian thoroughfare for Tongva, Serrano people, and other Californian Native Americans traveling between the Los Angeles Basin coastal plain, the San Gabriel Mountains, and the Mojave Desert regions of Southern California.