The creek then continues to flow southwards and Devil Canyon Creek draining the eastern slope of the Santa Margarita Mountains with its tributary, Cold Spring Canyon Creek, comes in from the left at RM 15 (RK 24.1). The Devil Canyon Creek confluence approximately marks the point where San Mateo Creek leaves the Cleveland National Forest. Soon, the narrow -deep canyon gives way to a broad alluvial valley. San Mateo Creek flows past Camp Pendleton, a United States Marine Corps amphibious training base. At RM 3.7 (RK 5.95) Cristianitos Creek, the largest tributary, joins from the right. Cristianitos Creek begins in extreme southeastern Orange County and drains about , that includes its tributaries of Talega Canyon Creek and Gabino Canyon Creek and its tributary La Paz Canyon Creek that enter it from the left.