GEOMORPHIC CHANGE DETECTION ALONG THE PAJARO RIVER, CALIFORNIA USING REPEATED CROSS-SECTIONAL SURVEYS AND MULTI-TEMPORAL LiDAR

In March 2023, a flood breached a levee along the Lower Pajaro River, causing a 100-meter portion of the levee to fail, inundating the town of Pajaro. In the wake of this disaster, improved flood protection along the Pajaro has been a high priority, for local residents and for local, state, and federal leaders. As part of its plans for levee reconstruction, the US Army Corps of Engineers is planning a new setback levee designed to reconnect the river with approximately 200 acres of former floodplain, providing ecological benefits, enhanced infiltration, and further strengthening flood protection. Determining the geomorphic history of a river channel helps to guide engineering projects. In 2024, we surveyed repeated channel cross-sections along the Lower Pajaro River in 17 locations previously surveyed in 2016, as well as in 2004, 2008, and 1995 at select sites. We also compared these surveys with airborne light detection and ranging (LiDAR) from 2010 and 2018. For change detection between the 2010 and 2018 LiDAR surveys, an uncertainty model was applied to determine a limit of detection for vertical change based on point density and terrain complexity. This study aims to identify changes in channel geometry and cross-sectional area between survey dates, with a focus on intervals during which flood flows occurred. Since 1995, flow events that reached the base of the levee or higher have occurred in 1998, 2017, and 2023. The results of this study identified changes in the low-flow channel width and depth and sediment deposition along channel terraces during large flow events. Change detection from LiDAR shows reach-scale changes, including channel migration and point-bar deposition. In conjunction with this study, a 2-D sediment-transport model is being developed for the Lower Pajaro River and its tributaries.