The Arroyo Seco — "dry stream" in Spanish — is the canyon that runs from the San Gabriel Mountains south to the Los Angeles River, forming the western edge of Pasadena. The canyon park is a linear green space popular with joggers, cyclists, and dog walkers, with a paved trail running along the streambed. In winter after rain, the stream actually runs; in summer it's mostly dry but remains pleasant under the tree canopy.
The Hahamongna Watershed Park at the northern end of the Arroyo, near JPL, is the most natural-feeling section — chaparral hillsides, a small reservoir, and good birding. South of the Colorado Street Bridge the trail passes under the 1940 Arroyo Seco Parkway, America's first urban freeway. The parkway itself is a listed historic road and worth driving for the 1930s bridge engineering alone.