The Upper Arroyo Seco provides a transition from the urban land uses  in Pasadena, the other neighboring foothill communities and the vast open space areas of the Angeles National Forest.  The Hahamongna Watershed Park(HWP) Master Plan is one of the three park master plans that make-up the Arroyo Seco Master Plans and addresses the southern-most 300 acres of Hahamongna Watershed Park which lie within the Upper Arroyo Seco.  Thirty new acres were recently purchased and added to the park.  The Hahamongna Annex, as it is tentatively being called, will bring a variety of equestrian facilities to the City along with a future environmental education center.  The future of the Hahamongna Annex is currently being planned and an update to the HWP Master Plan to incorporate the vision for this area of the park is also underway.

As a flood basin, storm waters make their way to this basin and are collected and managed behind Devil’s Gate Dam before making their way downstream, along the the Arroyo Seco tributary.  Sediment and debris collect in this basin due to the topography of the area and its proximity to the highly erosive San Gabriel Mountain range. 

The eastern edge of the park also contains Johnson Field, an inactive spreading basin, which is used for passive recreational uses, an interconnected system of hiking trails, a series of active spreading basins that greatly contribute to the city’s water conservation mission, as well as the Jet Propulsion Laboratory (JPL) parking lots.  Bicycle riding in the park is permitted on the various paved routes throughout the park but not on the hiking trails. 

The western portion of the Hahamongna Watershed Park Master Plan area contains the Oak Grove area (formerly Oak Grove Park).  The Oak Grove area is divided into two areas, the Upper Oak Grove and the Lower Oak Grove.  The Upper Oak Grove is characterized by prime oak woodland, picnic facilities, an extensive system of hiking trails, restrooms, a maintenance building and a group picnic and equestrian gathering area.   The Lower Oak Grove is also characterized by beautiful oak woodland, a hiking trail system, various picnic areas, a multi-purpose playing field, rest rooms and the world’s first disc golf course.   It can be summarized that Hahamongna Watershed Park area contains a blend of passive recreation uses, water conservation, and flood management activities blended with diverse plant and animal habitat.  

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