watervulnerability

Shasta Trinity National Forest Watershed Vulnerability Assessment, Pacific Southwest Region (R5)
Two primary ecological/climatological provinces cover the majority of the Forest; the Southern Cascade
and the Klamath Mountain Range (Miles and Goudey 1997). There are also two river systems that drain
the Forest (Figure 1): the Sacramento River Basin and the Trinity River, which drains into the Klamath
Basin.
The Southern Cascade lies on the east side of the Forest and contains the headwaters of the Sacramento
River Basin. Elevations in the Southern Cascade range from 2,000 to 14,000 feet elevation, the range in
precipitation is from 8 to 80 inches, with a growing season of 25 to 175 days. The Southern Cascade
includes a number of active volcanoes, including Mount Lassen on the southern end and Mount Shasta to
the north.
The Klamath Mountain Province lies on the west side of the Forest and contains most of the Trinity River
portion of the Klamath Basin as well as a the portion of the Sacramento River Basin that surrounds Shasta
Lake. Elevations in the Klamath Province are a little lower than the Southern Cascade, ranging from 200
to 9,000 feet elevation. Climate variability is great with precipitation ranging from 18 to 120 inches and a
growing season of 25 to 225 days. The spectacular Trinity Alps run east-west to east along the northern
edge of the Forest within this province. The southernmost portion of the province is the headwaters of
California’s agricultural heartland, the Central Valley.
Figure 2. Shasta Trinity National Forest Hydrologic units included in Watershed Vulnerability Assessment.
HUC-4 (left), HUC-5 (center) and HUC-6 (right) were the three scales used in the analysis.
SCALES OF ANALYSIS
This assessment included analysis at three scales: sub-basin (HUC-4), watersheds (HUC-5) and
subwatersheds (HUC-6) (Figure 2). The Shasta-Trinity Watershed Vulnerability Assessment (WVA) was
unique among the WVA pilot Forests in that multiple scales were utilized. A subbasin (HUC-4) was the
largest assessment unit and represents the largest tributaries of the large rivers on the forest (Table 1).
Subbasins range in size from roughly 300,000 acres to 1.6 million acres. Each subbasin is subdivided into
watersheds (HUC-5) which range in size from roughly 40,000 acres to 200,000 acres. Watersheds are
comprised of subwatersheds (HUC-6) which range from roughly 7000 acres to 57,000 acres.
187 Assessing the Vulnerability of Watersheds to Climate Change