This is defiantly the Two-Striped Garter Snake Thamnophis hammondii which is the only Garter known in the San Gabriel Mountains proper. I was born and raised in Covina Calif. and grew up with these snakes. I spent a lot of time fishing and herping the San Gabriel River and other areas of the Eastern San Gabriels. Unfortunately this snake is protected by law in Calif. (only place where it occurs except for Baja Calif) and has been for several years. It is illegal to either collect or possess T. hammondii within Calif. I collect garters and they are my favorite so I have been following their status closely with Calif F&G. The Valley Garter Snake T. s. fitchi is not found below central California (Santa Barbara County.)
Randy