Streamflow
How does streamflow impact water quality?
Streamflow plays a crucial role in determining water quality in streams by influencing the transport of pollutants, sediments, and nutrients. Higher flows often bring additional suspended sediments into the streams but may dilute other pollutants found in the watershed.
Variations in flow can impact the habitat and health of aquatic organisms, as many species depend on specific flow conditions for survival and reproduction.
Analysis
Streamflow monitoring locations most relevant to assessing streamflow impacts on 2025 water quality are listed here, and daily average flow graphs are displayed below. Clicking on any of the station names in this list will open the Colorado Division of Water Resources data page for that site.
Boulder Creek Basin Monitoring Locations
- Boulder Creek at North 75th St. Near Boulder, CO (BOCNORCO)
- Boulder Creek at Mouth Near Longmont, CO (BOCLONCO)
Coal Creek Basin Monitoring Locations
St. Vrain Creek Basin Monitoring Locations
- Left Hand Creek at Hover Road Near Longmont, CO (LEFTHOCO)
- St. Vrain Creek Below Boulder Creek at Hwy 119 Near Longmont, CO (SVCBBCCO)
- St. Vrain Creek Below Ken Pratt Blvd at Longmont, CO (SVCLOPCO)
- St. Vrain Creek At Hygiene, CO (SVCHGICO)
Explore additional streamflow monitoring stations on Boulder and St. Vrain Creeks on the Map page.
All streams within the study area exhibit expected seasonal fluctuations in streamflow linked to rainfall, snowmelt, and human water use (diversions). Streamflow typically increases in late spring and early summer due to snowmelt in the upper watershed. Major drivers in streamflow during the rest of the year include agricultural diversions and storm events. In 2025, spring runoff was lower in volume than the previous 5-year average at most locations, particularly on Left Hand and St. Vrain Creeks, where May and June flows were roughly 35 to 65 percent of average. Snowmelt runoff peaked in mid-June, typical for the watershed, but the high-flow period was shorter than usual. Flows on upper Boulder Creek were closer to average and were above average in August. Short-lived spikes in flow at most stations correspond to the late March and late September storms (see Precipitation section).