watervulnerability

Sawtooth National Forest Watershed Vulnerability Assessment, Intermountain Region (R4)
were considered low risk, 20 to 40% were considered moderate, and greater than 40% were considered
high risk. Changes in Winter 95 were determined by seeing how many days with the highest 5% flows
increased from current to 2040 and 2080. Subwatersheds with less than a 0.5 day increase were
considered low risk, 0.5 to 2 day increases were considered moderate risk, and increases greater than 2
days were considered high risk. Risk ratings for Winter 95 and MeanSummer were provided by Seth
Wenger, based on his recent work evaluating climate variables relative to geomorphic and land use in
determining the distributions of bull trout and other species in the Interior Columbia River Basin (Wenger
et al. (in press)).
EXPOSURE RESULTS
Winter Peak Flows (Winter 95) – The Upper Salmon
subbasin has many high-elevation subwatersheds and
is surrounded by 12,000-foot snow-capped peaks of
the White Cloud and Sawtooth Mountains. Cold, dense
air sinking from the mountains into the valley is the
main reason for the chilly early-morning temperatures
that are frequently the lowest in the lower 48 states. As
a result, mid-winter rain-on-snow events are currently
very rare. Rain-on-snow events that do occur typically
happen in late April to May. The high elevation terrain
and cold winter temperatures should help to buffer
snow packs from winter flooding. However, as air
temperatures increase, this natural buffering capacity
will diminish, especially in those subwatersheds where
temperatures hover around freezing. By 2100, air
temperatures in Idaho could increase by 5°F (with a
range of 2-9°F) in winter and summer (EPA 1998).
The VIC model projects that the risk from mid-winter
peak flows triggered by rain-on-snows events will
increase by 2080. Specifically, the highest 5% winterpeak
flows average 0.88 days under current conditions
(1977-1997), but increase to 2.6 days in 2040 and to
4.44 days in 2080 on the Sawtooth NRA under the
A1B emission scenario. Wenger et al. (in press) found
some areas in the interior Columbia River basin within
the 1977-1997 timeframe to have up to 8.4 days at the
highest 5% winter peak flow. Thus, the current risk of
mid-winter peak flows is relatively low in comparison
to other areas. However, these risks will be increasing.
By 2040, three (5.9%) of the 51 subwatersheds
analyzed have less than a 0.5 day (low risk) increase in
winter peak low from current; 34 (66.7%) have a 0.5 to
2 day (moderate risk) increase from current; and 14
(27.4%) have a greater-than-2-day (high risk) increase
from current (Figure 8). Meadow, Stanley Lake, and
Smiley Creek have the highest risk with each having
over a 4 day increase in winter peak flows by 2040.
168 Assessing the Vulnerability of Watersheds to Climate Change
Figure 8. Winter peak flow risk in 2040; highest
(red); moderate (yellow); and lowest (green)
Figure 9. Winter peak flow risk in 2080; highest
(red); moderate (yellow); and lowest (green)