The community of Rainbow is located in the rough, foothill portion of northern San Diego County, bordered on the north by Riverside County. Rainbow Valley, a central valley approximately four square miles in size, is the focal point of the 14.1 square mile community plan area. It is situated among the foothills between the Aqua Tibia Mountains and Santa Margarita Mountains. The foothill peaks adjacent to the valley are both rocky and steep, reaching a maximum elevation of 1,800. Large live oaks and sycamores can be found on the valley floor while scrub oak and green chaparral cover the rocky hillsides.
Geographically, Rainbow can be separated into two distinct areas, the foothills and the valley. The foothills portion consists mainly of detached and exposed worn masses of granite covered with natural chaparral where soil conditions preclude other vegetative types. The runoff from winter rains is occasionally quite rapid and creates a sheet flow of water across the valley floor due to the steep slope and sparse soil cover. The valley portion is characterized by sandy loam which is underlain at a shallow depth in many places with hardpan. This geologic condition on the valley floor has caused extreme drainage problems for crops and residences alike.
- Rainbow Community Plan, San Diego County General Plan