Longmont Reservoir dam
Longmont Reservoir
Longmont Reservoir, located in Lyons, Colorado, is a concrete gravity dam built in 1900 primarily for water supply purposes. With a height of 30 feet and a storage capacity of 75 acre-feet, this reservoir serves as a key water resource for the surrounding area. Situated along the North St. Vrain Creek, the reservoir covers an area of 5 acres and has a drainage area of 108 square miles, with a maximum discharge capacity of 3000 cubic feet per second.
Despite its age, Longmont Reservoir has been well-maintained and is in satisfactory condition according to its last inspection in 2015. Classified as a low hazard potential structure, the reservoir has a regular inspection frequency of every 6 years. While it lacks a spillway, the dam's risk assessment is considered high, indicating the importance of ongoing monitoring and risk management measures. With its strategic location and vital role in water supply, Longmont Reservoir is a significant reservoir for water resource and climate enthusiasts to study and appreciate in the Boulder County region.
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 Longmont Reservoir -- inflows here typically show up in storage 24-72 hours later.
| Streamgauge | Discharge | View |
|---|---|---|
| St. Vrain Creek At Lyons | 89 cfs | → |
| Left Hand Creek Near Boulder | 36 cfs | → |
| South St. Vrain Creek Near Ward | 37 cfs | → |
| Left Hand Creek At Hover Road Near Longmont | 1 cfs | → |
| Fourmile Creek At Orodell | 2 cfs | → |
| Boulder Cr At North 75th St Nr Boulder | 37 cfs | → |
Make a day of it
Boat launches, lakeside camping, fishing access, and other reservoirs near Longmont Reservoir.
Boat launches
- Lagerman Trail Boulder County
- Miramonte Road Boulder County
- Boat Ramp Larimer County
- Boat Launch Grand Lake
- Hilltop Boating Site
- Grand County
Campgrounds
- Meadow Park
- Glacier View Ranch
- Olive Ridge Campground
- Olive Ridge
- Carter Lake - South Side Campgrounds
- Hermit Park Open Space
Fishing spots
- Ralph Price Reservoir (Button Rock)
- South Saint Vrain Fishing Site #2
- South Saint Vrain Fishing Site #4
- South Saint Vrain Fishing Site #5
- Mccall Lake
- Pella Crossing
Track Longmont Reservoir 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 Longmont Reservoir
Where does the data for Longmont Reservoir 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 Low 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 Longmont Reservoir.