Published
Island Park Dam: the structure and the file behind the water
The structure, in the Bureau’s own rows
Island Park Dam is a federal earthfill structure at the north end of the reservoir country, built in the late 1930s and still run today as part of a reclamation project headquartered two counties south. The U.S. Bureau of Reclamation keeps a public project record for it, and the record is the kind of document a cabin buyer near this water should know how to read — because the numbers in it are the operating facts the shoreline lives with. The Bureau’s location line:
“Island Park Dam is located 38 miles north of Ashton, Idaho, on Henrys Fork. The dam is a zoned earthfill structure 91 feet high.”
— U.S. Bureau of Reclamation, Projects & Facilities record, Island Park Dam. Retrieved 16 October 2026
The project data sheet fills in the rest. Here is the structure, row by row:
| Project record row | Value |
|---|---|
| Original construction | 1937-1939 |
| Dam type | Zoned Earthfill |
| Structural height | 94.0 ft |
| Crest length | 9500.0 ft |
| Crest elevation | 6312.0 ft |
| Streambed at dam axis | 6228.0 ft |
| Project | Minidoka |
| National ID (dam registry) | ID00272 |
Two numbers worth slowing down on. The crest runs 9,500 feet — this is not a wall across a canyon mouth but a long earth embankment laid across a wide valley, nearly two miles of it. And the vertical span is modest: streambed at 6,228.0 feet, crest at 6,312.0, which is the whole reason the reservoir upstream is broad and shallow rather than deep. Storage, in the Bureau’s own sentence:
“Island Park Reservoir has a total storage capacity of 135,500 acre-feet (active 135,200 acre-feet).”
— U.S. Bureau of Reclamation, Projects & Facilities record, Island Park Dam. Retrieved 16 October 2026
A bathtub with a bladder
The spillway is the dam’s personality. Most people picture a gate or a chute; this one is neither:
“Spillway Type: Bathtub spillway with a 1 ft inflatable bladder on top”
— U.S. Bureau of Reclamation, project data sheet rows. Retrieved 16 October 2026
A bathtub spillway spills around the rim, all 360 degrees of a low ring — and on top of that rim sits an inflatable bladder one foot tall, which the operator can raise or lower to hold or release that extra foot of water. The normal operating line is the quiet part that shoreline owners should read twice:
“Normal Water Surface Elevation: 6303.0 ft with inflatable bladder, otherwise it is 6302.0 ft”
— U.S. Bureau of Reclamation, project data sheet rows. Retrieved 16 October 2026
One foot of bladder decides which side of 6,303 the water sits on. That is the managed-precision character of this reservoir: a band, not a level. The hydraulic rows, collected:
| Hydraulic fact | The record’s value |
|---|---|
| Spillway type | Bathtub ring with a one-foot inflatable bladder on top |
| Normal water surface | 6,303.0 ft with the bladder up; 6,302.0 ft without |
| Spillway crest | 6,308.2 ft |
| Spillway capacity | 6,600 cfs at 6,306.6 ft |
| Outlet works capacity | 3,260 cfs at 6,303.0 ft |
| Top of active conservation pool | 6,303.0 ft |
What the water does across the seasons — how the pool behaves from freeze-up through the irrigation draw — is a shoreline-buying question with its own page here.
Who owns it, who runs it, where the phones ring
The dam is federal; the daily operation is local, and the record prints both. The governance shape that matters to a buyer is in that pairing: a national agency owns the structure, and an irrigation district headquartered at the county seat decides the day-to-day water.
| Who | The record’s entry |
|---|---|
| Owner | Bureau of Reclamation, Upper Snake Field Office — 470 22nd Street, Heyburn, Idaho; 208-678-0461 |
| Operator | Fremont-Madison Irrigation District — PO Box 15, St. Anthony; 208-624-3381 |
| Project | Minidoka; Upper Snake River Division, alongside the Cross Cut Canal and Diversion Dam and Grassy Lake Dam |
| Registry | National dam ID ID00272 |
The Bureau places the dam inside a project family, too:
“The Upper Snake River Division of the projectincludes Island Park Dam, Cross Cut Canal and Diversion Dam, and Grassy Lake Dam.”
— U.S. Bureau of Reclamation, Projects & Facilities record, Island Park Dam. Retrieved 16 October 2026
Grassy Lake, up the Flagg Ranch way at the southeast edge of this basin, is the sibling structure; the two reservoirs work as one storage system, and the Bureau’s record says their stored water serves users in Fremont and Madison counties. For anyone buying a lot on this water, the practical translation is simple: the lake outside your window is a component in a two-reservoir irrigation system with a published paper trail — not a wild lake that happens to be pretty.
Twice rebuilt, twice added to
The dam you look at is not quite the dam they built. The record describes a federal safety modification program completed in the early 1980s:
“The Safety of Dams modifications were completed at Island Park Dam in the early 1980s. This consisted of replacing the deteriorated concrete in the spillway and excavation and replacement of liquefiable materials in the right abutment.”
— U.S. Bureau of Reclamation, Projects & Facilities record, Island Park Dam. Retrieved 16 October 2026
That second clause is the engineering story in six words: liquefiable materials — ground that behaves like a solid until an earthquake shakes the water out of it — were dug out of the right abutment and replaced. A dam sitting on volcanic valley fill got the treatment you would expect, decades after the 1937-1939 build. Then two additions followed, in the Bureau’s words:
“in the early 1990’s a privately owned powerplant was built at the dam. In the 2000’s a one foot inflatable rubber dam was installed on the spillway crest to allow better control of the 1 foot of surcharge.”
— U.S. Bureau of Reclamation, Projects & Facilities record, Island Park Dam. Retrieved 16 October 2026
So the structure has four ages:
- 1937-1939 — built: zoned earthfill across the valley, part of the Minidoka Project’s Upper Snake River Division.
- Early 1980s — rebuilt where it mattered: spillway concrete replaced, liquefiable ground excavated out of the right abutment.
- Early 1990s — a privately owned powerplant added at the dam.
- 2000s — the inflatable rubber dam installed on the spillway crest, putting the top foot of surcharge under positive control.
A buyer does not need to memorize the sequence — just to know the thing has a maintenance history, which is another way of saying it has an owner with a budget and a file that does not close.
The ground it rests on
The Bureau’s geology note ties the dam to the reason this basin exists at all. From the same record:
“Island Park Dam rests within the Middle Rocky Mountain- Physiographic Province near the northwest rim of the Island Park Caldera.”
— U.S. Bureau of Reclamation, Projects & Facilities record, Island Park Dam. Retrieved 16 October 2026
The foundation rows describe alternating volcanic layers — rhyolite common in the valley section and right abutment, basalt in the left — which is the caldera’s signature stack. This site’s page on building inside the caldera covers what that ground asks of a foundation; the dam’s own abutment rebuild is the same lesson at federal scale.
What the file means for a lot
- Know which water question you are asking. The structure, its elevations and its contacts live in the Bureau’s record; what the pool does to shoreline ownership and value across the year is the Shotgun Valley page’s story.
- Read the operating band, not a single level. Normal is 6,303.0 with the bladder, 6,302.0 without — a managed foot, decided by the operator, not the weather alone.
- Follow the water downstream. What leaves this dam in winter runs the canyon the box-canyon lots sit above — that page covers the release calendar.
- Check the setbacks before the view. How close to this water you may build is a county rule with distances of its own: the shoreline-setback page carries them.
- Use the public phones. The Bureau’s Heyburn office (208-678-0461) and the operator’s St. Anthony office (208-624-3381) answer water questions for a living — a rarity among the facts a cabin buyer needs.
The dam is the quiet landlord of the north reservoir country. It decides the top foot of the water, it has been rebuilt once and added to twice, and every number that governs it is public. A buyer who has read this far already knows more about the water outside the window than most people who have owned a lot on it for years.
Common questions
When was Island Park Dam built?
Original construction ran 1937-1939, per the Bureau of Reclamation's project record. The structure was then modified under the federal Safety of Dams program in the early 1980s — spillway concrete replaced and liquefiable materials excavated and replaced in the right abutment.
How big is Island Park Dam?
A zoned earthfill structure with a structural height of 94.0 feet and a crest length of 9,500 feet — nearly two miles of embankment across the valley floor, with the crest at elevation 6,312.0 feet above a streambed at 6,228.0 feet.
How much water does Island Park Reservoir hold?
Total storage capacity of 135,500 acre-feet, of which 135,200 acre-feet is active storage, according to the Bureau of Reclamation's record.
What is a bathtub spillway?
A spillway that overflows around its entire low ring rather than through a gated chute — the Bureau's row reads 'Bathtub spillway with a 1 ft inflatable bladder on top.' Spillway capacity is 6,600 cfs at elevation 6306.6 ft, and the outlet works carry 3,260 cfs at 6303.0 ft.
What does the inflatable bladder on the spillway do?
It holds or releases the top foot of the reservoir. The Bureau's operating row sets normal water surface at 6,303.0 feet with the bladder inflated and 6,302.0 feet without it; the device was installed in the 2000s for better control of that one foot of surcharge.
Who owns Island Park Dam?
The United States Bureau of Reclamation — administered from the Upper Snake Field Office at 470 22nd Street, Heyburn, Idaho, phone 208-678-0461. The dam carries national registry ID ID00272 and belongs to the Minidoka Project.
Who operates Island Park Dam day to day?
The Fremont-Madison Irrigation District, PO Box 15, St. Anthony, phone 208-624-3381, per the Bureau's record. The federal agency owns the structure; the irrigation district headquartered at the county seat runs the water — including the top foot governed by the spillway bladder.
Is there a powerplant at Island Park Dam?
Yes — a privately owned powerplant built at the dam in the early 1990s, per the Bureau's record, followed in the 2000s by the one-foot inflatable rubber dam on the spillway crest.
What is under Island Park Dam, geologically?
The Bureau places it 'near the northwest rim of the Island Park Caldera,' on alternating volcanic layers — rhyolite common in the valley section and right abutment, basalt in the left. The same caldera ground is what every foundation in the basin contends with.
Why should a cabin buyer care about the dam's paperwork?
Because the reservoir is a managed pool, not a natural lake: its top foot is an operating decision, its water serves irrigation users in Fremont and Madison counties as part of a system that also includes Grassy Lake, and the elevations, capacities and contact numbers that govern it are all published by the Bureau.