Arroyo Seco (Los Angeles County) - Floods and Controls

Floods and Controls

Despite its name, the Arroyo Seco generally has a flow of several cubic feet per second, but periodically it is inundated by torrential floods from its steep, erosion-prone mountain watershed. The reputation of Arroyo Seco floods led the Spanish to site the original Pueblo de Los Angeles away from the confluence of the Arroyo Seco and the Los Angeles River. Historically, these floods would race down the stream bed and overflow through adjacent communities all the way to the Los Angeles River. As Los Angeles developed into a city and grew outwards, the damage from these floods was particularly severe in 1914 and 1916.

In 1920 flood engineers from the Los Angeles County Flood Control District built Devil's Gate Dam in the Arroyo Seco, the first flood control dam in Los Angeles County. Named for a rock outcropping which resembles the face of a devil, Devil's Gate gorge, located in Pasadena between La Cañada Flintridge and Altadena, is the narrowest spot in the Arroyo Seco. The construction was by the Bent Brothers Company.

Above the dam is the flood basin which captures the flows of the mountain watershed of the Arroyo Seco. This area is now called "Hahamongna", a phrase meaning "Flowing Waters, Fruitful Valley", a remarkably different designation than the name given the area by the Spanish explorers. Hahamongna also refers to the original Native American tribe of the Tongva Indians who once inhabited the area. Pasadena is developing the Devil's Gate/Hahamongna flood basin into Hahamongna Watershed Park, a 1,400-acre (6 km2) regional park, which emphasizes the unique natural values of the park. By 2011, the flood basin had filled with about 1.5 million cubic yards of mud brought down after the 2009 Station fire, reducing its effectiveness as a flood control measure and leading to public debates on how to restore the basin.

Below the dam, most of the Arroyo Seco stream, with two short exceptions, is contained in a concrete channel that captures stormwater and municipal runoff. This channel and other similar flood control structures throughout the Los Angeles Basin and along the foothills of the San Gabriel Mountains were built following the devastating Los Angeles Flood of 1938.