Cal. Gov. Code § 23133
The boundaries of Riverside County are as follows:
Beginning at the corner common to Orange, San Bernardino, and Riverside Counties, being located at the point of intersection of the easterly boundary of the El Canon de Santa Ana Rancho with course number seven of the boundary line, established by joint survey in December 1876, and January 1877, as the line between Los Angeles and San Bernardino Counties; thence southeasterly along said line of survey to the most northerly point of that survey on file in Book 87 of Records of Survey, at Pages 60 through 61, in the office of the County Recorder of Riverside County; thence south 68°49'57" west a distance of 35.37 feet; thence south 48°16'56" west a distance of 150.70 feet; thence south 65°40'06" west a distance of 75.15 feet; thence north 84°32'29" west a distance of 155.61 feet to the beginning of a nontangent curve, concave to the northwest, with a radial bearing north 29°06'47" west and a radius of 1,550.00 feet; thence along said curve through an angle of 15°01'17" a distance of 406.35 feet to the easterly line of the Rancho Lomas de Santiago; thence south 02°53'27" east, along said easterly line, a distance of 3,273.18 feet to the south line of Sec. 36 T. 3 S., 8 W., S. B. B. & M.; thence south 89°07'27" east, along said south line, a distance of 818.46 feet; thence south 89°05'38" east, continuing along said south line, a distance of 2,484.00 feet to the southerly most point on said survey on file in Book 87 of Records of Survey, at Pages 60 through 61, in the office of the County Recorder of Riverside County, said point also being on said line of survey between Los Angeles and San Bernardino Counties; thence continuing southeasterly on said line of survey to the point of beginning of said joint survey, it being upon the northern boundary of San Diego County, as it was then established; thence southwesterly to a point on the eastern line of Rancho Mission Viejo or La Paz two miles north of the south boundary of T. 7 S., S. B. B. & M.; thence south along said boundary to the point of intersection of said line and the northwest corner of Lot 1 of Sec. 33, T. 7 S., R. 6 W., S. B. B. & M.; thence south 89°16'14" east along the north line of said Lot 1 and the north section line of said Sec. 33 a distance of 2,086.39 feet to the northeast section corner thereof; thence south 00°27'14" west, along the east section line of said Sec. 33, a distance of 2,647.46 feet to the east quarter corner of said Sec. 33; then south 01°00'50" west, continuing along said east section line, a distance of 1,320.33 feet to the southeast corner of the northeast quarter of the southeast quarter of said section; thence north 89°25'11" west, along the south line of the northeast quarter of the southeast quarter and the south line of Lot 3 of said Sec. 33, a distance of 2,042.68 feet to the eastern boundary line of Rancho Mission Viejo or La Paz; thence 00°00'35" east, along said boundary, a distance of 1,324.46 feet to the point of intersection with said line with the township line between T. 7 S. and T. 8 S., S. B. B. & M.; thence easterly along said township line to its intersection with the western boundary of Santa Rosa Rancho; thence southerly along the boundary of said rancho to where said boundary of said rancho intersects the range line between T. 8 S., T. 3 W., and T. 8 S., T. 4 W.; thence south on said range line to the point of intersection of the said line with the second standard parallel south; thence east along said parallel to its intersection with the interstate boundary between the States of Arizona and California on the centerline of the Colorado River, said interstate boundary and centerline being described in "Interstate Compact Defining the Boundary Between the States of Arizona and California", a certified copy of which is recorded and filed in the office of the County Recorder of Riverside County; thence northerly along said interstate boundary to its point of intersection with the east and west centerline of T. 1 S., R. 24 E., S. B. B. & M., or the prolongation thereof; thence westerly along the section lines to the southeast corner of Sec. 17, T. 1 S., R. 16 E., S. B. B. & M.; thence south to the southeast corner of Sec. 32, same township and range, said point being on the township line between T. 1 and 2 S., S. B. B. & M.; thence west on said township line to the northeast corner of T. 2 S., R. 1 W., S. B. B. & M.; thence south to the southeast corner of Sec. 12, T. 2 S., R. 1 W., S. B. B. & M.; thence west to the southwest corner of Sec. 8, T. 2 S., R. 3 W., S. B. B. & M.; thence north to the northwest corner of said Sec. 8; thence west to the quarter corner of the south line of Sec. 2, T. 2 S., R. 5 W., S. B. B. & M.; thence north to the quarter corner on the north line of said Sec. 2; thence west to the southwest corner of Sec. 31, T. 1 S., R. 6 W.; thence south along the section lines to the northern boundary of the Jurupa Rancho; thence southwesterly along said north boundary to the northwest corner of said Rancho; thence south along the west boundary of said Jurupa Rancho to the quarter corner on the east line of Sec. 9, T. 3 S., R. 7 W., thence west in a direct line to the center of Sec. 7, same township and range; thence south in a direct line, to the quarter corner on the south line of Sec. 19, T. 3 S., R. 7 W., thence west to the east boundary of the El Canon de Santa Ana Rancho; thence southerly along the easterly boundary of said Rancho to the place of beginning.
That portion of the Riverside County boundary line which coincides with the Arizona-California Interstate Boundary is shown on a series of Planimetric Maps appearing in Exhibit "A" of the above-mentioned Interstate Compact.
Ca. Gov. Code § 23133