Tools You Can Use
- St. Vrain Creek and Left Hand Creek cameras
- Longmont weather forecast
- includes Watches and Warnings from the National Weather Service
- National Digital Forecast Database
- Graphical forecasts across the country
- Colorado Flood Threat Bulletin
- includes an interactive state flood threat map that is updated daily
- Colorado Surface Water Conditions
- provides gage height for various points along St. Vrain Creek and Left Hand Creek
Learn more about Flood Preparedness & Safety >
Understanding the Variables
Whether or not these factors cause flooding often depends on daily weather conditions over the region such as temperature and precipitation.
Deep snow can melt into a lot of water. Deep snow very rarely causes flooding by itself. Often, heavy rain and rapid warm ups combine with rapidly melting snow to cause major flooding problems.
Frozen soil can not absorb as much water as unfrozen soil. Rain or rapid snow melt atop frozen soil can cause flooding that wouldn't have occurred if the soil were not frozen.
Saturated soil can't absorb rain and water from melting snow. The excess water becomes runoff that rapidly flows into rivers and streams. Unsaturated soil acts like a sponge, absorbing some of the water from rain or snow melt. Saturated soil by itself does not cause flooding. Usually, heavy rain or rapid snow melt combined with saturated soil causes the flooding.
Reservoirs are large, mostly manmade basins that hold water for irrigation and drinking. Reservoirs can alleviate river flooding by absorbing and spreading out flood crests flowing down the river. This would reduce the height in which the water rises downstream of the reservoir. If the reservoir is already full, then it can not absorb any water from swollen rivers.
High River and Stream Levels
Streams or rivers that are already at bankfull can be a precursor to major flooding. Heavy rain or rapid snow melt that flows into an already full river will cause the river to overflow its banks and flood nearby locations. A prolonged dry spell, however, can alleviate flooding concerns.
Widespread, Heavy Rain
This is perhaps the most important and influential factor of them all. Long periods of heavy rain can cause flooding even if all other factors are unfavorable for flooding. Often, heavy rain is a cause of some of the factors listed above such as wet soils, high stream levels and full reservoirs.
In addition to regular monitoring via technical tools and forecasts, City staff will also perform on-site, visual creek monitoring if predetermined flood triggers are met. In such cases, City staff will monitor conditions in waterways that have the potential to create flooding in Longmont. Localized flood locations and the major irrigation/storm water ditches are also monitored. An additional part of monitoring is the inspection of utility crossings of the river, bridges, problematic drainage grates, and trash racks throughout the city.
Monitoring locations include vehicular and pedestrian bridges along:
- St. Vrain Creek
- Left Hand Creek
- Spring Gulch #1
- Dry Creek
- Spring Gulch #2
- Lykins Gulch
- Oligarchy Ditch
You Can Help, Too
Residents can also be on the lookout for potential incidents along the creek corridors. If you see any of the following three conditions occurring, please report it to the appropriate number listed below.
- Debris (trees, granular, etc.) in or near a stream bed or ditch that is impeding flow or causing some other hazard. Call the PWNR Customer Service Center at (303) 651-8416.
- Very large debris (a shipping crate into a bridge) causing damage or a life safety issue of some kind. Call 911.
- Overflow (water overflowing stream bed or water overtopping the stream bed and flooding adjacent property). Call 911.