The climate along the Bull Run River is typical of the western Oregon Cascades foothills.
Annual precipitation ranges from 80 inches (2,000 mm) near the intake for the Portland water supply to as much as 170 inches (4,300 mm) near the headwaters.
Summers are dry, and winters, especially November through January, are wet.
At low elevations, most of the precipitation arrives in the form of rain, but at higher elevations 25 to 30 percent of the moisture arrives as snow.
Fog drip may add significantly to total precipitation in the vicinity of Bull Run Lake.
A study published in 1982 suggested that standard rain gauges placed in open areas might be underestimating the contribution of fog drip to heavily forested parts of the watershed by up to 30 percent.
Accumulated snow is rare at elevations up to 2,000 feet (610 m) above sea level but sometimes reaches 6 to 10 feet (1.8 to 3.0 m) above 4,000 feet (1,200 m).
Melting snow adds to streamflow in April and May, and dry soil inhibits streamflow in August.
Generally, temperatures are mild.
Lows in January range from just below freezing to about 25 °F (−4 °C), while July highs are usually about 80 °F (27 °C).

The Natural Resources Conservation Service (NRCS) of the United States Department of Agriculture operates snow telemetry (SNOTEL) stations at three places in the Bull Run watershed to help predict how much water will be available from melting snow.
Snow depths and density vary with time and location.
At the Blazed Alder Creek station, the highest of the three at 3,650 feet (1,110 m) above sea level, the mean snow-water equivalent (SWE) (the amount of water in the accumulated snow) ranged in 2009 from 0 in July–October to about 50 inches (1,300 mm) in April.
A station on the North Fork at an elevation of 3,060 feet (930 m) reported a minimum mean SWE of 0 in July–October 2009 and a maximum of about 37 inches (940 mm) in April.
 In the same year at the South Fork station, elevation 2,690 feet (820 m), the mean SWE varied from 0 in June–September to about 10 inches (250 mm) in March.
The climate along the Bull Run River is typical of the western Oregon Cascades foothills. Annual precipitation ranges from 80 inches (2,000 mm) near the intake for the Portland water supply to as much as 170 inches (4,300 mm) near the headwaters. Summers are dry, and winters, especially November through January, are wet. At low elevations, most of the precipitation arrives in the form of rain, but at higher elevations 25 to 30 percent of the moisture arrives as snow. Fog drip may add significantly to total precipitation in the vicinity of Bull Run Lake. A study published in 1982 suggested that standard rain gauges placed in open areas might be underestimating the contribution of fog drip to heavily forested parts of the watershed by up to 30 percent. Accumulated snow is rare at elevations up to 2,000 feet (610 m) above sea level but sometimes reaches 6 to 10 feet (1.8 to 3.0 m) above 4,000 feet (1,200 m). Melting snow adds to streamflow in April and May, and dry soil inhibits streamflow in August. Generally, temperatures are mild. Lows in January range from just below freezing to about 25 °F (−4 °C), while July highs are usually about 80 °F (27 °C).
The Natural Resources Conservation Service (NRCS) of the United States Department of Agriculture operates snow telemetry (SNOTEL) stations at three places in the Bull Run watershed to help predict how much water will be available from melting snow. Snow depths and density vary with time and location. At the Blazed Alder Creek station, the highest of the three at 3,650 feet (1,110 m) above sea level, the mean snow-water equivalent (SWE) (the amount of water in the accumulated snow) ranged in 2009 from 0 in July–October to about 50 inches (1,300 mm) in April. A station on the North Fork at an elevation of 3,060 feet (930 m) reported a minimum mean SWE of 0 in July–October 2009 and a maximum of about 37 inches (940 mm) in April.  In the same year at the South Fork station, elevation 2,690 feet (820 m), the mean SWE varied from 0 in June–September to about 10 inches (250 mm) in March.
Bull Run River (Oregon)