Publication Date

Spring 2025

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

Advisor

Emmanuel Emmanuel; Daniel Malmon; William Russel

Abstract

Dams have many effects on stream morphology and ecology, including a loss of sediment supply downstream, reduced access to the entire stream for aquatic fauna, and a flooded area upstream of the dam. Dams are a hurdle for migrating organisms such as anadromous fish, which is a group of concern. An over 100-year-old dam on Mill Creek, a stream draining a coastal watershed in central California, was removed in the fall of 2021. The site is in the Santa Cruz Mountains, where endangered salmonids such as steelhead, Oncorhynchus mykiss, have historically lived. To study the change in the stream from the dam removal, pebble counts were conducted and cross sections were surveyed along Mill Creek and downstream of its confluence with its trunk stream before the removal and were repeated twice after the removal, once in the winter shortly after removal and once about one year post-removal. The winter survey included 20 cross sections upstream of the Mill Creek and San Vicente Creek confluence, and the summer survey included all 27 cross sections in the study site. The reach upstream of the dam eroded significantly and median grain size increased at the furthest upstream cross sections and fined slightly closer to the dam after the removal. Downstream of the dam site, cross section sites eroded and aggraded in different zones, and overall median grain size increased. In general, sediment eroded from the channel and banks, and the percentage of grain sizes beneficial for salmon spawning increased.

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