A customer on Lime Rock Road, near Sonora California, reported bites from buffalo gnats. She collected specimens from a light fixture for us. In addition to numerous gnats and beetles, there were, indeed, a number of black flies. Specimens collected 13 April 2017. Thank to Dr. Lynn Kimsey for ID confirmation.
The source could well be Sullivan Creek, as it is within flight distance of black flies, although any stream can serve as a breeding site. The flight range can be five miles, although most, in one study, traveled 1.5 miles. (source: “Flight range, longevity, and habitat preference of female Simulium euryadminiculum Davies (Diptera: Simuliidae“)