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.
2025 Monthly Precipitation Totals within Boulder, St. Vrain, and Coal Creek Basins
Total precipitation in 2025 was somewhat below average, with 18.8 inches at the Boulder Creek station compared to the 1991-2020 average of 21.2 inches. A late March storm brought heavy precipitation to all three basins, and May was the wettest month of the year, with 4.4 inches in Boulder compared to a typical 3.2 inches. In contrast, April and July received only about half of their typical precipitation, and fall was very dry, with October and November together receiving only about a quarter of the usual amount in Boulder.
Some monthly totals are not shown for the St. Vrain and Coal Creek stations due to gaps in the NOAA record for those stations.
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
2025 Precipitation Graphs
The largest precipitation event of the year occurred on March 29 and 30, when a storm delivered between 1.7 and 2.4 inches across all three basins. Spring storms continued in May and early June, including multiple events of more than an inch in Boulder and Coal Creek. Summer rainfall was localized, with notable storms in late June on Coal Creek and smaller events in August on St. Vrain and Boulder Creeks, while July was drier than normal across the watershed. A storm on September 23 and 24 brought more than an inch of rain to all three basins, after which only a few smaller events occurred through the end of the year.
Boulder Creek Station
2025 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
2025 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
2025 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.