Three Mile Reservoir (L.&C.) dam
Three Mile Reservoir (L.&C.)
Three Mile Reservoir, also known as Dam Nancy, is a private-owned structure located in Lewis and Clark County, Montana. Built in 1926, this earth dam stands at a height of 70 feet and has a maximum storage capacity of 300 acre-feet. Its primary purpose is for irrigation, serving the surrounding area by storing water from Three Mile Creek.
Despite its age, Three Mile Reservoir poses a high hazard potential due to its poor condition assessment. The dam has a drainage area of 5.6 square miles and a maximum discharge of 2600 cubic feet per second. The reservoir covers a surface area of 3 acres and has a normal storage capacity of 180 acre-feet. Its location in the Helena Valley makes it a critical water resource for the region.
With the Montana Department of Natural Resources and Conservation regulating its operations, Three Mile Reservoir remains under state jurisdiction for inspection, enforcement, and permitting. The dam is in need of maintenance and potential risk management measures to ensure its continued functionality and safety for the community that relies on it for irrigation water. For water resource and climate enthusiasts, the Three Mile Reservoir serves as an important example of the ongoing challenges faced in managing aging infrastructure in the face of changing environmental conditions.
Dam data reference
Condition Assessment
- Satisfactory
- No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
- Fair
- No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action.
- Poor
- A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency.
- Unsatisfactory
- A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
- Not Rated
- The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Hazard Potential Classification
- High
- Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
- Significant
- Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environmental damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
- Low
- Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
- Undetermined
- Dams for which a downstream hazard potential has not been designated or is not provided.
Plan around the weather
Same NOAA / yr.no feed Snoflo's iOS app uses. Watch the precipitation column on the meteogram -- rain on the basin upstream typically lifts inflow 24-72 hours later.
Next 5 days, hour by hour
Temperature line with weather symbols on top, snow + rain accumulation as columns, humidity as a dotted line.
5-day forecast table
Every 3 hours, broken out across temperature, snow, rain, humidity, and wind. Each cell is colour-coded relative to the column min/max.
| Time | Condition | Temp (°F) | Snow (in) | Rain (in) | Humidity (%) | Wind (mps) | Wind dir |
|---|---|---|---|---|---|---|---|
| Loading detailed forecast… | |||||||
15-day temperature & precipitation
Daily temperatures, snow, and rain projected over the next two weeks.
Nearby streamflow gauges
USGS streamgauges around Three Mile Reservoir (L.&C.) -- inflows here typically show up in storage 24-72 hours later.
| Streamgauge | Discharge | View |
|---|---|---|
| Tenmile Creek Near Helena Mt | 26 cfs | → |
| Tenmile Creek Near Rimini Mt | 17 cfs | → |
| Missouri River Bl Hauser Lake Nr Helena Mt | 3,230 cfs | → |
| Prickly Pear Creek Near Clancy Mt | 36 cfs | → |
| Little Prickly Pear Cr At Wolf Cr Mt | 88 cfs | → |
| Missouri River Bl Holter Dam Nr Wolf Cr Mt | 3,280 cfs | → |
Make a day of it
Boat launches, lakeside camping, fishing access, and other reservoirs near Three Mile Reservoir (L.&C.).
Boat launches
- Smith Road Helena Valley Northeast
- Cross Drive Helena Valley Northeast
- Hauser Dam Road Helena Valley Northeast
- White Sandy Boat Ramp
- Helena Regulating Reservoir Road Lewis And Clark County
- York Road Lewis And Clark County
Campgrounds
- Fort Harrison Rv Military - National Guard
- Cromwell Dixon
- Cromwell Dixon Campground
- Moose Creek Cabin
- White Sandy Campground
- Black Sandy State Park
Fishing spots
- Silver Creek
- Rattlesnake Creek
- Spring Meadow Lake
- Little Prickly Pear Creek
- Prickly Pear Creek
- Missouri River Fishing Access Site
Track Three Mile Reservoir (L.&C.) in the Snoflo app
Save this dam as a favorite and get the local NOAA / yr.no forecast plus regional flow context wherever you are.
About Three Mile Reservoir (L.&C.)
Where does the data for Three Mile Reservoir (L.&C.) come from?
Structural and regulatory data come from the U.S. Army Corps of Engineers' National Inventory of Dams (NID). Weather forecast comes from NOAA / yr.no -- the same feed Snoflo's iOS app uses.
How often is the report updated?
NID structural data refreshes annually as the Corps publishes updated assessments. The weather forecast refreshes throughout the day.
What does the High hazard rating mean?
The Corps of Engineers' hazard potential classification grades probable consequences if the dam fails: High = probable loss of human life; Significant = no probable loss of human life but possible economic loss / environmental damage; Low = no probable loss of human life, only minor economic / environmental losses. See the Dam Data Reference card above for the full definitions.
What's "% of normal"?
The current storage value compared to the historical average storage on this calendar day. 100% = right on average; values above 100% mean above-normal storage (wet year); values below mean below-normal (dry year or drought).
Can I get alerts when storage crosses a threshold?
Yes -- alerts are managed in the Snoflo iOS app. Favorite this dam, set a threshold, and you'll get a push the moment conditions cross.
Other water bodies near here
Snoflo-tracked reservoirs and dams within driving distance of Three Mile Reservoir (L.&C.).