watervulnerability

Helena National Forest Watershed Vulnerability Assessment, Northern Region (R1)
BACKGROUND AND FOREST CONTEXT
The Helena National Forest is located in west-central Montana within the Northern Region (R1) of the
USFS. The Forest consists of nearly 1 million acres of distinctive landscapes and lies on either side of the
Continental Divide, resulting in a very diverse climate and landscape (Figure 1). The Forest’s watersheds
make up the headwaters for both the Missouri and Columbia River basins. The western portion of the
Forest straddles the Continental Divide starting at the southern tip of the Bob Marshall Wilderness and
ending just east of Deer Lodge. The eastern side includes the lower, drier Big Belt Mountains. The Forest
is composed of a mixture of grass and sagebrush covered lowlands with pockets of lodgepole pine and
mountainous areas composed of Douglas fir, spruce and larch. Elevations do not exceed 10,000 feet
(3,000 m).
Figure 1. Helena National Forest (green) and nearby communities and rivers
The Helena National Forest has a continental climate modified by the invasion of Pacific Ocean air
masses. The Forest lies in the strong belt of westerly winds that move out of the Pacific Ocean and
deposit much of their precipitation on the mountain ranges in western Montana. Summers are warm in
most valleys and cooler in the mountains. Winter months are relatively cold. Most precipitation falls as
snow, and a deep snowpack accumulates in the mountains. East of the Continental Divide, occasional
down slope warming winds, Chinooks, can occur in the winter months, resulting in a rapid rise in air
temperature. The average annual precipitation ranges from 11.21 inches at Townsend in an intermountain
valley to 50.30 inches at Copper Creek on an alpine mountain ridge. Valleys generally receive two-thirds
to three-fourths of their annual precipitation during the growing season with seasonal peaks in May and
June and again in September. The mountainous areas receive a larger percentage of their precipitation as
snow during the winter. Average annual snowfall varies from 30 inches at Holter Dam to 108 inches at
Lincoln Ranger Station (Sirucek, 2001).
ANALYSIS OVERVIEW
The WVA was completed for all subwatersheds under the management of the Helena National Forest.
Three steps were completed to determine the vulnerability of each subwatershed (Hydrologic Unit Code
level 6 (HUC-6)) to predicted changes in climate. First, the sensitivity of each subwatershed was
determined, based on existing data representing the current condition of the subwatershed for each
individual resource value of concern. Next, an exposure analysis was conducted based on the selected
47 Assessing the Vulnerability of Watersheds to Climate Change