Henrys Fork level at Island Park
Idaho · Fishing & wading
Measured at USGS 13042500 — HENRYS FORK NR ISLAND PARK ID · NWS forecast point ISLI1
Reading taken · as of 43 min ago
Low and clear — wadeable, and fish will be spooky
At 241 CFS, this gauge is in the lowest 10% of the 93 years USGS has recorded for this date (1933–2026), at 48% of the 505 CFS median.
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Flow history at Island Park
Daily values from USGS 13042500, updated every 15 to 60 minutes; NWS forecast for ISLI1 reissued through the day. Provisional.
Highest crests on record at Island Park
12 on record
| Stage | Flow | Date |
|---|---|---|
| 6.15 ft | 2,770 CFS | Apr 26, 1946 |
| 6.06 ft | 3,030 CFS | May 23, 1984 |
| 5.96 ft | 2,580 CFS | Jun 1, 1943 |
| 5.87 ft | 2,560 CFS | Jul 18, 1955 |
| 5.85 ft | 2,580 CFS | Aug 26, 1983 |
| 5.82 ft | 2,700 CFS | May 18, 1982 |
| 5.80 ft | 2,040 CFS | Jul 21, 2015 |
| 5.80 ft | 2,610 CFS | May 13, 1971 |
| 5.80 ft | 2,430 CFS | Jun 9, 1999 |
| 5.45 ft | 1,930 CFS | Jul 30, 2013 |
Crests published by the National Weather Service.
About Henrys Fork at Island Park
The lowest daily mean ever recorded at this station is 1 CFS, on 16 November 1938. The river effectively stopped. November is still the lowest month in the published statistics today, at a median of 214 CFS, and the contrast with the summer figures is what makes this gauge's shape unusual.
It peaks in July, not in June. The published medians are 1,160 CFS in July and 1,020 in August against 890 in May and 900 in June, on a 501 square mile drainage at 6,228 feet. A snowmelt river at this elevation does not normally carry more water in August than in May. The statistics cover 1933 to 2026, about 92 years, and the highest daily mean on file is 2,990 CFS from 22 May 1984.
Henrys Fork at Island Park — common questions
6 questions
Has this gauge ever gone dry?
It has come close. The record low daily mean is 1 CFS, measured on 16 November 1938, against a modern November median of 214 CFS. That reading is in the published USGS daily record as measured. The statistics window is 1933 to 2026, so it sits near the start of a long series and nothing since has approached it.
Why does this river carry more water in July than in May?
The published medians put July at 1,160 CFS, August at 1,020, May at 890 and June at 900. That is the reverse of the snowmelt shape you would expect at 6,228 feet, and it is the signature of storage being released through the summer rather than runoff arriving in the spring. November, at 214 CFS, is the low point of the year.
Does the Madison move with this gauge?
No, despite being 31.2 miles away in the same country. Across 32,128 shared days since 1938 the correlation between the two is 0.108, and this gauge carries a median of about 0.52 times the Madison below Hebgen. Both are held behind storage that is operated on its own schedule, so knowing what one is doing today tells you very little about the other.
What makes the flow change on Henrys Fork at Island Park?
Rain upstream, plus seasonal baseflow. The 12-month chart shows how it behaves through the year.
Is Henrys Fork at Island Park high or low for this time of year?
The USGS median flow for late September here is 505 CFS, from 93 years of record (1933–2026). Today sits in the lowest 10% of those years, at 48% of the median. The comparison is what makes the raw number mean anything: the same reading is a drought on one river and a flood on another.
How often does this reading update?
USGS transmits from gauge 13042500 every 15 to 60 minutes, and the National Weather Service reissues the flood forecast for ISLI1 a few times a day — more often when the river is rising. This page refreshes from both when you open it. The timestamp is in the gauge's local time.