The California Streets and Highways Code defines Route 605 as "(a) Route 1 near Seal Beach to Route 405. (b) Route 405 to Route 210 near Duarte." However, the portion in subsection A has yet to be constructed.
I-605 follows most of the length of the San Gabriel River from the San Diego Freeway in Seal Beach to the Santa Fe Dam. Typically dry riverbed and flood basins are visible from many portions of the route, especially near the northern terminus. Also, the freeway features an HOV lane for carpools of two or more people throughout most of the route. The left shoulder is close to non-existent. The road also suffers from traffic jams regularly, especially near I-5 (the Santa Ana Freeway).
Signs along I-605's entrances and freeway junctions do not include destinations (control cities) such as "Seal Beach" or "Irwindale", but simply read "NORTH" or "SOUTH". In place of control cities, a simple "THRU TRAFFIC" designation is used on signs along I-605 itself.
Route 605 from Route 405 to Route 10 is known as the San Gabriel River Freeway, as named by Senate Bill 99, Chapter 1101 in 1967.
The I-605/I-105 interchange is known as the Joe A. Gonsalves Memorial Interchange, as named by Assembly Bill 96, Chapter 129 in 2001. Gonsalves became the first person of Portuguese ancestry to become a member of the California State Legislature in 1962
Interstate 605 began construction in 1963 and the first section was opened in 1964 from Interstate 405 to Pomona Freeway CA 60. The newest section (extension to Interstate 210) was opened in 1971 was originally signed as SR 243. Originally it was planned as I-13 running from Interstate 5 to I-15. There are plans to extend it to SR 1 further south in Orange County, but strong community opposition means that it is unlikely that the alignment will ever be built.