the arroyo hondo mainstem is formed by the confluence of isabel creek and smith creek at the northern tip of joseph d. grant county park at an elevation of above sea level. it flows northerly to its confluence with calaveras reservoir at an elevation of 765 feet. isabel creek begins at about about one mile south of mt. helen, then flows north through the isabel valley, then east and north of mt. isabel and mt. hamilton until it is joined by smith creek. arroyo hondo joins calaveras creek in calaveras reservoir. calaveras creek is, in turn, tributary to alameda creek in alameda county, california and ultimately flows into san francisco bay.