Precipitation
How does precipitation impact water quality?
Precipitation is a key factor in understanding a stream's chemical, physical, and biological characteristics. Increased flows from precipitation may temporarily lower the concentration of pollutants and move contaminants through the system. At the same time, water flows through our communities, picking up contaminants like pesticides, fertilizers, and sediments. This polluted runoff then enters storm drains which drain directly into local streams.
Heavy precipitation can also stir up the stream bed, releasing metals and nutrients from the soil, and in extreme cases lead to erosion of the stream banks, adding yet more sediment and nutrients to the stream.
Over time, sediment settling on the stream bed will cover gravel substrates that are crucial for organisms like fish, insects, and invertebrates. This can reduce habitat availability for spawning fish (such as trout) and macroinvertebrates, which rely on clean, coarse substrate for shelter and reproduction.
Analysis
The following section illustrates precipitation daily and monthly levels within the KICP monitoring areas, based on three NOAA weather stations (one in each key watershed).
- Boulder Creek Station - GHCND:USC00050848, Boulder, CO US
- St. Vrain Creek Station - GHCND:US1COBO0352, Longmont 1.8 ENE, CO US
- Coal Creek Station - GHCND:US1COBO0557, Lafayette 0.5 ENE, CO US
Precipitation Events are listed in tables below each of the daily graphs. These represent periods of measurable precipitation. For this report, a Precipitation Event has been defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.
2024 Monthly Precipitation Totals within Boulder, St. Vrain, and Coal Creek Basins
Heavy precipitation fell both earlier and later in the year than is typical, with the highest amounts in February, April and November across all basins, and in March also in St. Vrain and Boulder Creeks. Sporadic and localized precipitation in August affected Boulder Creek, but overall, late spring, summer, and early fall were drier than normal across the watershed with few notable precipitation events.
See more details by basin in the following sections.
1991 through 2020 Historical Monthly Average Precipitation for the Boulder Creek Station
See individual years from 1893 to current: NOAA Physical Sciences Laboratory
2024 Precipitation Graphs
Rainfall totals early in the year were higher than normal, due to heavy rain (not snow) events in February, March, and late April, with daily rainfall otherwise normal for the season. These rains occurred somewhat earlier than is typical for the season. Summer and fall precipitation were much lower than average, with some localized rain in August breaking that trend just for that month and just for Boulder Creek. Adding to the unusual rainfall patterns in 2024, November was unusually rainy, totaling 2 to 3 times above the 20-year average for the month.
Boulder Creek Station
2024 Precipitation Events
*A precipitation event is defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.
St. Vrain Creek Station
2024 Precipitation Events
*A precipitation event is defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.
Coal Creek Station
2024 Precipitation Events
*A precipitation event is defined as a period of consecutive days that experienced more than 0.1 inches of rain each day and totaled at least 1 inch over the event, or a single day with greater than 0.5 inches of rain.