Investigation of Strath Terrace Formation in the Upper Headwater Region of the Mattole River Watershed, Northwest California

Abstract:

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 low gradients with broad strath terraces and deeply incised channels. In fact, it could be stated that the river has a “convex-up” profile with a low gradient headwater leading to a higher gradient midcourse. Strath terrace formation in the upper headwater region of the Mattole River records times of disequilibrium of channel profile and bedrock incision. In order to investigate the distribution and relative ages of strath terraces, a detailed survey of the terrace surface and bedrock strath position was conducted along several headwater tributaries. Additionally, hand borings were excavated across the terraces to bedrock to provide a three dimensional image of terrace thickness. Terrace morphology and stratigraphy provide information on terrace forming mechanisms and timing. This study includes high-resolution geomorphic data regarding the relation of Mattole headwater terraces to the MTJ, as well as provides more temporal information about the relation between a fluvial system’s response to the ongoing northward migration of the MTJ.