PREDICTING SEDIMENT AGGRADATION FOLLOWING A SMALL DAM REMOVAL: MILL CREEK, CALIFORNIA, USA

Mill Creek, the largest tributary of the San Vicente River in the Santa Cruz Mountains, is part of a coastal watershed, which drains 18km² and connects the inland redwood forests with the Pacific Ocean. It was dammed twice in the past century, resulting in the obstruction of sediment flow, causing loss of fish habitat, trapping of sediment and nutrients upstream, and incision and coarsening downstream. In 2021 the lower Mill Creek dam was removed, in an attempt to reverse impacts of the dam, presenting a unique opportunity to study sediment transport, channel morphology, and salmonid recovery as the stream reverts back to pre-dam conditions following the drought and deluge weather conditions between 2021 and 2023. This dam is one of hundreds of small dams impacting coastal watersheds in California; as dam removal becomes a more feasible method of habitat rehabilitation for fish populations, the ability to understand how these watersheds react post-dam is extremely valuable. This study combines field data collection – through the Wolman pebble counting technique to collect grain size information and surveys which record channel characteristics – and an investigation of channel metrics, such as channel curvature and slope, using Geographic Information Systems (GIS) to investigate whether topography and particle size distributions have been influenced in a systematic way by the channel morphology. By studying channel metrics using remote imagery, and comparing these findings with field data, we aim to predict how future dam removals and changes in precipitation will impact coastal watersheds in California.