ON Sunday, thousands of people will participate in ArroyoFest, a community festival and walk and bike ride on the Pasadena (110) Freeway. The theme of this innovative and historic event is connecting communities and focuses specifically on the Arroyo Seco, the stream and canyon that flows from the San Gabriel Mountains down to the Los Angeles River.

Bringing the communities that share the Arroyo Seco together on the freeway is symbolic of the cooperation needed to improve the quality of life for everyone in the area. Two key places to begin are our transportation systems and the need for open space, recreation and a restored urban stream.

Along its 22-mile course, the Arroyo touches the communities of La Canada Flintridge, Altadena, Pasadena and South Pasadena before connecting with the historic neighborhoods of northeast Los Angeles, such as Highland Park and Lincoln Heights.

In addition to being connected by the stream, these communities also share a major transportation corridor centered around the Pasadena (110) Freeway, which opened in 1940 and was initially known as the Arroyo Seco Parkway.

The communities of the Arroyo Seco have a storied history enriched by the diversity of cultures that have always lived, worked and played alongside its grand canyon. In the early 20th century, the Arroyo served as a symbol of the Southern California lifestyle; it provided a connection back to the landscape and represented the rough edges that helped set Southern California apart from life back in the more refined East Coast.

The Arroyo Seco stream, known for its supple beauty, diverse environments and dramatic change in flows was a unique element to throw into the mix of a rapidly urbanizing area. And it was around the Arroyo that Pasadena and other communities in the area shaped a unique identify, which they have fought hard to preserve over the last century.

In many ways, the symbol of this fight has been the Historic Arroyo Seco Parkway. Called the first freeway of the West by Governor Culbert Olson on its opening day, the Parkway was praised for landscaped plantings, aesthetically valued bridges and graceful turns that followed the natural topography. The intent of the Parkway was to provide an uninterrupted and pleasurable drive that allowed the driver to connect and appreciate the communities through which it passed.

From its opening day in 1940, however, this parkway concept quickly gave way to a notion of a high-speed freeway, with its high volume of traffic and the attitude that one had to bust through the communities on the way to somewhere else. The intended sense of place became a blur from the car window. And the Parkway's graceful curves became dangerous stretches as drivers tried to pass through at 60, 65 and 75 mph rather than the original 45 mph that was intended when the road was designed.

In the years since the freeway's on- and offramps have become accident traps, with the Pasadena Freeway becoming the most accident-prone freeway in southern California according to a recent study by researchers from UCLA and Occidental College.

Similarly, the idea of communities connected by a living stream succumbed to the fears of potential flooding. Serious floods in 1914 and 1938 did cause terrible devastation to the homes along the Arroyo Seco, but the solution was in many ways equally devastating encasing a living stream in a concrete channel and fencing it off from the surrounding community.

The stream that was at the root of the community's identity had become beyond reach. But not entirely. The hanging sycamore leaves, curves and bridges along the parkway still signal a sense of home for residents driving up from downtown Los Angeles. The canyon still provides quiet places for walks and inspires passion in its residents.

The Upper Arroyo in the Angeles National Forest is still an environmental and scenic treasure. And even the urban stretches of the stream encased in concrete offer the promise of reclaimed open space and parkland.

A study last year of the Arroyo Seco watershed by the Arroyo Seco Foundation and North East Trees provided some specific recommendations on how to revive the Arroyo Seco and make the stream a natural asset rather than concrete ditch, disfiguring communities and landscapes alike.

This spring these two organizations have sponsored an exciting six-week symposium on the Arroyo Seco. Entitled Watershed U, the program has allowed stream advocates to become watershed experts and dialogue with engineers and various public agency officials about to bring nature back into the city. Far from indulging a romantic yearning for a pre- channelized stream, the program has equipped residents with the skills and knowledge they need to tackle the restoration and enhancement of today's Arroyo.

The Parkway also presents opportunities to advance a whole new model for transportation, not only in the Arroyo but all of Southern California. This summer the Gold Line will open, following along the route of the Parkway. Advocates for new bike trails have raised the idea that an extended commuter bikeway could serve as an important alternative to car traffic.

MTA's new rapid bus service, which has become popular in other parts of the region, is a natural fit for several different surface-street corridors that could stretch from Pasadena to downtown L.A.

Finally, planning efforts are under way to rehabilitate the Parkway itself and recapture elements of the parkway ideal through new landscaping, speed-limit reduction and reconfiguring the approaches to entrances and exits. These plans create simultaneous opportunities to maintain, develop and connect new green spaces and parklands along the route.

Ultimately, the Arroyo corridor offers Southern California the opportunity to create what transportation planners call a "multi-modal approach,' providing alternatives to the accident- prone, congested, air-polluting system in place today.

To walk or ride your bike on the freeway will be great fun, but underlying this and other events associated with ArroyoFest are also the seeds of a new connected vision of what the Arroyo Seco was and what it could become in the days and years to come.

-- Marcus Renner is the Education and Outreach Coordinator of the Urban and Environmental Policy Institute at Occidental College and on the ArroyoFest steering committee. Information on ArroyoFest and the UCLA-Occidental College study of the Pasadena Freeway can be found online at www.arroyofest.org