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Stop #5: Hahamongna Watershed Park

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(4550 Oak Grove Drive, Pasadena, CA 91103)

The Arroyo Seco exits out of the San Gabriel Mountains at Millard Canyon, just north of the Jet Propulsion Laboratory (JPL) and the Hahamongna Watershed. As a river exits out of a headland area, such as the San Gabriel Mountains, the river slows down. As it does so, the decrease in the amount of energy causes any sediments carried in the river to be deposited. These deposited sediments are called alluvium, and they form large alluvial fans at the base of the mountains. All of the cities located at the base of the San Gabriel Mountains (Sunland, Tujunga, Pasadena, Sierra Madre, etc.) are all built upon alluvial fans. Once the river is on the flat valley floor it begins to meander, creating a sinuous path as it continues downstream to the Pacific Ocean.

In the 1920s, the area was designated as a prime location for a debris flow basin, and the Devil’s Gate Dam (Stop #5) was built to contain floodwaters. Currently, the Hahamongna Watershed Park is a multi-use outdoor area, which includes trails, active spreading basins which help to recharge the aquifer located beneath the area, a wildlife and bird refuge, and a prime example of the transition from mountains to valleys.

Channelization of Southern California’s rivers began in the 1930s as a response to a series of devastating floods. Unfortunately, this means that sediment is no longer being transported to the coast to replenish our beaches. One solution to this problem is to dredge out sediment from behind debris dams and relocate it to the beaches in need. This is a fine idea for active debris flow basins, where the basin should be cleaned out periodically in order to make room for more debris. However, recent action by the LA County Flood Control District has authorized the removal of sediment from behind Devil’s Gate Dam in the Hahamongna Watershed. This “Big Dig” is controversial as it would destroy fragile habitats and increase truck traffic in normally quiet residential areas.