Army Corps Arroyo Seco Ecosystem Study

Lower Arroyo Seco Park

Pasadena's Lower Arroyo Seco (1.5 miles in length): This reach, which extends from the Colorado Street Bridge to the South Pasadena boundary, has been described as one of the best candidates for urban stream restoration in the United States. At the Colorado Street Bridge, the natural Arroyo Seco stream channel flows over a spillway back into a massive concrete channel. The channel is located in a narrow, deeply incised canyon surrounded by quiet residential neighborhoods.

Lower Arroyo Stream Restoration

Project Elements

  • Remove the concrete flood channel and restore streamzone habitat from just downstream of the Colorado Street Bridge to the Pasadena city limit

This project would restore a fully functioning stream and ecosystem influenced by natural hydrology, including periodic flooding, without increasing the risk of flood damage to private or public property. This project will enhance public access to rivers and the great outdoors, connect wildlife from the San Gabriel Mountains to the Santa Monica Mountains, and improve water quality in the Arroyo Seco and Los Angeles River Watershed.

The Lower Arroyo Seco is one of the most beloved and cherished landmarks in Pasadena because of its open space, recreation and access to nature. This area is known throughout the region for its natural setting, peacefulness and abundant wildlife. The potential for a complete restoration of historical ecosystem function is very high, and Pasadena policy documents have backed the restoration for more than thirty years. Unlike many sites along the channelized Los Angeles River Watershed, there are few manmade structures in this stretch so removing the concrete from the stream in the Lower Arroyo would have minimal impact on flood protection and could actually improve it. The Post-WWII concrete has handled flood flows in the area, but it is now aging and crying out for a more sustainable approach to stream and flood management. There is little risk of from even the highest of potential floods. The Lower Arroyo Seco Channel Removal Project is a culmination of years of grassroots planning efforts and analysis focusing on southern California ecosystems. This area is very valuable for habitat and wildlife connectivity, including aquatic habit. The Lower Arroyo has had its share of anthropogenic stressors on its ecosystem, but has endured and thrived with the help of many decades of conservation and restoration activities.

Project components should be designed to:

  • Improve flood water, sediment and debris conveyance and storage through reestablishment of a natural channel/floodplain system
  • Improve water quality due to floodplain and groundwater/surface water interactions that remove sediment and filter flows
  • Restore endangered aquatic and riparian ecosystem communities
  • Reestablish fish passage through the elimination of migration barriers including the concrete channel and the spillway from the newly restored Central Arroyo Seco.
  • Provide active and passive recreational areas that are integrated into native ecosystem habitats
  • Provide an awareness of the impacts of river channelization, water quality issues, invasive habitat and the benefits of an ecosystem influenced by natural hydrology.

San Rafael Creek

Just opposite the foot of West Columbia Street, San Rafael Creek enters the Arroyo. This creek formed the principle drainage of the lower San Rafael area. The stream flows from Johnson Lake down into the Arroyo. Only about 250 feet of the stream have been channelized. This site is an excellent candidate for an inexpensive but very productive stream restoration that could restore the natural character of the stream and a 2 acre wetlands zone right where Pasadena, South Pasadena and Los Angeles city limits all come together.