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

The Pajaro River originates in the California Coastal Ranges and drains 3400 km² and discharges into Monterey Bay. The Lower Pajaro River has extensive urban development along its floodplain, including the City of Watsonville, CA and the unincorporated community of Pajaro, CA. In March 2023, a 100-meter portion of the levee failed along the Lower Pajaro River, flooding the nearby towns. The United States Army Corp of Engineers (USACE) is leading a levee setback and floodplain reconnection initiative for multi-benefit flood management. Understanding the geomorphic history and dynamics of a river channel is essential to inform flood management and restoration projects. We are collecting repeated channel cross-sections along the Lower Pajaro River and it's main tributary to assess changes in stream morphology and flood capacity since 2016. Portions of the river have decreased flood capacity due to aggradation while other reaches have incised. Repeated cross sections collected as part of this study will contribute to an ongoing assessment of channel capacity and will improve understanding of channel geometry along the Lower Pajaro River. Additionally, we have analyzed multi-temporal airborne light detection and ranging (LiDAR) surveys from 2010 and 2018, differencing these datasets to document change at a watershed scale. The LiDAR data were post-processed to extract the bare-earth terrain and to remove flight-path errors. Change detection from LiDAR has identified regions within the channel where the channel has migrated, where sediment has accumulated within sediment or degraded during this period. Data collected for this study is also being used to calibrate a 2-D sediment-transport model. This research will aid USACE in identifying challenges associated with channel morphological changes and sediment transport for a large-scale flood control project on the Pajaro River.