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Assessment of Watershed Vulnerability to Climate Change Helena National Forest April, 2012 Prepared By: Laura Jungst Hydrologist Helena National Forest, Helena, Montana 46 Assessing the Vulnerability of Watersheds to Climate Change
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
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White River National Forest Watersh
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Coconino National Forest Watershed
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Sawtooth National Forest Watershed
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Shasta Trinity National Forest Wate
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Umatilla National Forest Watershed
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Ouachita National Forest Watershed
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Chugach National Forest Watershed V
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