Reservoirs in Idaho
12 of 301 graded Idaho lakes (4%) are man-made reservoirs in the USACE National Inventory of Dams. 12 carry a High or Significant hazard rating.
Reading the Idaho reservoir landscape
Idaho has 12 reservoirs in the USACE National Inventory of Dams — a limited count by national standards, reflecting the state's mix of natural lake topography and limited need (or political space) for major federal impoundments. Where reservoirs do exist, they tend to be either flood-control structures on smaller tributaries or municipal water-supply lakes — both with distinct access and recreation profiles.
Idaho's reservoirs serve a mix of purposes — flood control, water supply, hydroelectric power generation, navigation, and recreation often co-exist on the same impoundment, with one designated as the primary and others as secondary. The operating rules (pool levels, release schedules, drawdown timing) follow the primary purpose, which shapes everything from shoreline access through fish-population dynamics through year-round recreational use patterns.
The USACE National Inventory of Dams (NID) catalogs every dam in the United States above a size threshold — typically 25 feet in height or 50 acre-feet in maximum storage, plus all dams classified as high-hazard regardless of size. The reservoir records below pull NID fields (year built, hazard class, maximum storage, surface area at normal pool, dam length, primary purpose) and pair them with water-quality data where the impoundment also appears in EPA's lake-monitoring inventory.
Why these Idaho reservoirs were built
All 12 Idaho reservoirs (by surface area)
| Reservoir | County | Grade | Surface area | Year built | Purpose | Hazard |
|---|---|---|---|---|---|---|
| Palisades Lake | Bonneville | C | 16,230 ac | 1957 | Flood Risk Reduction | High |
| Lake Walcott | Cassia | D | 12,400 ac | 1906 | Fish and Wildlife Pond | High |
| Island Park Lake | Fremont | D | 8,680 ac | 1938 | Irrigation | High |
| Deadwood Reservoir | Valley | A | 3,800 ac | 1931 | Hydroelectric | High |
| Arrowrock Reservoir | Elmore | NR | 3,256 ac | 1915 | Flood Risk Reduction | High |
| Ririe Reservoir | Bonneville | NR | 1,560 ac | 1977 | Flood Risk Reduction | High |
| Black Cnyn Reservoir Ns of | Gem | A | 1,100 ac | 1924 | Hydroelectric | High |
| Mountain View Lake | Owyhee | D | 680 ac | 1969 | Irrigation | Significant |
| Littlewood Lake | Blaine | D | 590 ac | 1939 | Irrigation | High |
| Little Wood Reservoir | Blaine | NR | 590 ac | 1939 | Irrigation | High |
| Mann Creek Reservoir | Washington | NR | 283 ac | 1967 | Irrigation | High |
| Soldiers Meadow Reservoir West Arm | Nez Perce | NR | 150 ac | 1922 | Irrigation | High |
Frequently Asked Questions
How many reservoirs are in Idaho?
12 of 301 graded Idaho lakes (4%) are man-made reservoirs in the USACE National Inventory of Dams. 12 are rated High or Significant hazard, meaning failure would put downstream lives or property at substantial risk. The oldest matched dam dates to 1906.
What's the largest reservoir in Idaho?
The largest matched reservoir in Idaho is Palisades Lake in Bonneville County at 16,230 acres of normal surface area, impounded by the Palisades Dam (completed 1957).
What's the difference between a reservoir and a natural lake?
A reservoir is a body of water impounded by a man-made dam — typically built for flood control, hydroelectric power, water supply, irrigation, or recreation. A natural lake formed without human intervention, usually from glacial scouring, volcanic activity, or river meandering. Reservoirs typically have shoreline that fluctuates with seasonal water level management, while natural lakes have more stable shorelines. The USACE National Inventory of Dams classifies every regulated dam in the United States, which is how we identify which LakeQuality lakes are actually reservoirs.
What does the hazard class mean?
USACE assigns each dam a hazard potential classification based on what would happen downstream if the dam failed — NOT how likely the dam is to fail. "Low" hazard means dam failure would cause no probable loss of life and only low economic loss. "Significant" means probable loss of life is unlikely but appreciable economic damage would occur. "High" means probable loss of life and significant economic damage. The classification is about consequence, not condition. Many High-hazard dams are perfectly safe; they're rated High because populated areas have grown downstream over time.
Where does this reservoir data come from?
Every reservoir record on this page is matched to the USACE National Inventory of Dams (NID), the federal database of all dams ≥25 ft tall or impounding ≥50 acre-feet, plus any dam that poses a Significant or High hazard regardless of size. The NID is published by the U.S. Army Corps of Engineers and updated annually. Surface area, storage capacity, dam height, and hazard classification come directly from NID records.
Where to stay near Idaho reservoirs
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More on reservoirs and water quality
Dam-built reservoirs and natural lakes graded side by side on the same nutrient and algae measurements — the gap, the reasons behind it, and the exceptions.
Surveyed maximum depth against A–F nutrient-and-algae grades: the pattern, the shallow lakes that still grade an A, and why land use beats bathymetry.
How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.
How phosphorus and nutrient runoff drive algae growth and turn lakes green, where the nutrients come from, and what it means for a lake's grade.
Data source
Reservoir data from the USACE National Inventory of Dams (NID), the federal database of all regulated dams. Dam attributes (height, year completed, storage capacity, hazard class) are matched to LakeQuality lakes by proximity and dam-name similarity. Some lakes may be reservoirs that aren't matched if the dam record uses a name substantially different from our lake name.