Water Flow in the Snake River at Flagg Ranch, Wyoming

The U.S. Geological Survey operates a gaging station (USGS 13010065) on the Snake River near Flagg Ranch, WY. The gage has daily flow data dating back to October 1983. A record of surface water temperatures begins in November 1983 but is intermittent; those data are excluded from this summary.

The hydrograph for the Snake River at Flagg Ranch, WY, exhibits a general pattern of high early summer flows and lower baseflows in late summer and fall. Average annual peak flow between 1983 and 2020 was 6,735 cfs and on average occurred on May 26 (day 146 of the year). Minimum annual daily flows over this same period of record averaged 239 cfs. Flows in 2021 were below the long-term (1983 to 2020) average flows at Flagg Ranch and generally tracked the 25th percentile of flows for this record. Peak flow in 2021, at 3,390 cfs, was just half of the long-term average and came on May 20 (day 140 of the year), almost one week earlier than the long-term average. Base flows, the lower dry weather flow in a river, came earlier in 2021 compared to the historical record. The minimum flow recorded in 2021 at the Flagg Ranch monitoring location was 171 cfs—this was the 5th lowest annual minimum flow in 39 years of flow records.

The downstream view from the sampling location on the Snake River at Flagg Ranch, WY, in June 2021 (left) and August 2021 (right)

NPS

Summary of the average daily discharge (in cfs) in the Snake River at Flagg Ranch, WY (USGS 13010065), for the period of record and in 2021. The 25th and 75th percentiles of daily discharge are also presented. In general, a percentile between 25 and 75 is considered normal, while a percentile greater than 75 is considered above normal and a percentile less than 25 is considered below normal.

NPS