Appel scored five points in the first half when Stanford (20-4, 11-1) was held to 18.
But the junior center went 5-for-7 from the field in the second half, allowing Stanford to avoid being swept by Cal for the first time since 1985-86.
Ashley Walker and Lauren Greif scored 10 points apiece for the Bears (20-3, 11-1), who had not lost since falling to then-No. 5 Oklahoma 86-75 on Dec. 13. Cal's streak was the fifth-longest in the country.