Terrace formation in the upper headwater region of the Mattole River watershed across the Mendocino Triple Junction, northwest California

The Mattole River, in northwestern California, is located in a tectonically active and geologically complex area, the Mendocino Triple Junction (MTJ), where the North American, Pacific and Gorda plates meet. The Mattole River does not follow the classic river "concave-up" profile. Instead, the river headwaters have wide valleys of low gradient terraces with deeply incised active channels. As a result of differential uplift along the river, the longitudinal profile has two "convex-up" sections resulting in low gradients in the headwaters leading to higher gradients in the midcourse. Low gradients have accommodated terrace formation in the upper headwater region of the Mattole River, that record times of disequilibrium as the river responds to changes that are, in part, due to changes in climate and also the passage of the northwardly migrating MTJ and associated growth of a slab window. Age estimates of sediments in Baker Creek, determined using optically stimulated luminescence dating (OSL), suggest the timing of valley widening and filling occurred after the LGM to 17 ka and from 11 to 8 ka. Surveys of terrace surfaces were conducted along four headwater tributaries: Ancestor Creek, Baker Creek, Lost River and Thompson Creek. Similar flights of terraces in three of the four surveyed headwater tributaries, along with locations of knickpoints and convexities in the long profile, provide information about the fluvial system's response to changes in climate and the ongoing northward migration of the MTJ.