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
This section describes 2023 streamflow conditions within the Boulder, Coal and St. Vrain Creek Basins. Both Boulder and St. Vrain Creeks have monitoring locations regularly spaced from upstream-to-down, and on key tributaries. In Coal Creek however, for most of 2023 only one monitoring point was available, located far upstream, well above the water quality monitoring reach. This upstream gage has been included in this review, however its proximity to the water quality monitoring locations should be taken into account.
The Coal Creek streamflow gage near the mouth (above Boulder Creek Confluence) was destroyed in 2022 during the Marshall fires and was restored to working order late September 2023. It has been included in the list below however a graph is not included because it was not complete for the study year.
Streamflow monitoring locations most relevant to assessing streamflow impacts on 2023 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 (SVCLOPCO)
Explore additional streamflow monitoring stations on Boulder and St. Vrain Creeks on the Map page.
All streams within the study area exhibit seasonal fluctuations in streamflow. Streamflow typically increases in late spring and early summer due to snowmelt in the upper watershed. The major drivers in streamflow during the rest of the year include agricultural diversions and storm events. Streamflow was average or below average through the fall and winter months and in the late summer. The exceptional rainfall events of May and June however can clearly be seen in the Hydrographs below with flows peaking well above historical averages.
In Boulder Creek, rains were consistently high starting mid-May and continuing through early July. While not as intensely concentrated as in the other basins, Boulder Creek did see peaks in July more than 3 times the average of the last 5 years (2018 - 2022). An early storm in the eastern portion of the basin on May 12th, which did not reach the upper gage, caused a peak flow the mouth of Boulder Creek more than 5 times the previous 5-year average. Interestingly, the September storm which created a very high precipitation peak on September 5th had minimal effect on streamflow.
Coal Creek saw two major flow peaking events, in mid-May and again in mid-June. Recorded flows during these events peaked at flow more than 10 times the average of the past 5 years.
The St. Vrain had a slow start to the high flow season, showing flows well below average for the first week of May. Once the rains set in however, peak flows were also seen on the St. Vrain that dwarfed those of the past 5 years. The most intensely high flowing period on the St. Vrain was from June 5th - June 30th with flows peaking on June 17th with flows 3 to 7 times higher than the previous 5-year averages.
Water quality this watershed appears to have been affected by the exceptionally high amounts of rain and extended rain events that characterized the spring of 2023. As is called out in the water quality discussion sections of this report, several water quality constituents that are typically deposited into streams from rainfall runoff can be seen as elevated in the months affected by the spring rains. An important caveat to this possible correlation however is that intentional storm flow monitoring was not conducted during 2023. Apparent relationships between higher concentrations of some water quality constituents and the high precipitation events is based on observation of the data alone and would need further study beyond this analysis to be confirmed.