For more detailed information and advice regarding:
Erosion or other geomorphic concerns - Contact a geotechnical engineering firm or soil-bioengineering firm (try searching for “biotechnical stream stabilization california” in any search engine).
Appropriate streamside vegetation - Contact a native plant nursery.
To report a problem on public property:
Trash or pollution - Call the city or county clean water or pollution prevention program.
For erosion, flooding or large debris (e.g. downed trees) - Contact the city or county Department of Public Works.
Violations of creek or wildlife protection laws or ordinances (several Bay Area cities and counties do have ordinances limiting construction activities in or near streams) - Contact the city or county Department of Public Works and/or the California Department of Fish and Game.