Visual. Look for flows of 300-500 cfs. While there is no gauge on this creek, it's likely running at the same time as the Cooper or Cle Elum in late spring. You can also check inflow to Kachess Reservoir (Box Canyon Creek is one of the creeks that feed into the reservoir) on the USBR Yakima Project site (keep in mind this is a calculated value and sometimes the numbers are a bit off). To see data from the last few days go to the data archive , select station KAC with parameter QU which is the calculated inflow.