watervulnerability

Sawtooth National Forest Watershed Vulnerability Assessment, Intermountain Region (R4)
Water temperature increases are not surprising, as mean air temperatures have seen a 0.49 o C increase per
decade (1979-2008) at local weather stations and projections show air temperatures increases of another
3.9 o C by 2080. At the same time, annual stream flows have decreased 5%/decade (1957-2008) at local
USGS gauging stations and are projected to decrease an additional 54% by 2080. However, not every
future year is expected to see warmer air temperatures and lower stream. The most pronounced changes
will likely be associated with short-term cycles such as the Pacific Decadal Oscillation and the El Nino-
Southern Oscillation. As climate change progresses, long-term warming trends will result in more
frequent droughts and periods of unusually warm weather that were considered extreme in the twentieth
century. When these events occur, the most affected watersheds will be those that have a high percentage
of low-elevations terrain and channel conditions prone to heating (wide, shallow, lack of riparian
vegetation) (Crozier and Zabel 2006).
Projected decreases in thermally optimal bull trout habitat are similar to those by O’Neal (2002), who
concluded that 2%-7% of current trout habitat in the Pacific Northwest would be unsuitable by 2030, 5%-
20% by 2060, and 8%-33% by 2090. Williams et al. (2009) also concluded that cold-water fish habitat in
the Rocky Mountain region could lose up to 35% of its habitat by 2050 and 50% by 2100.
Ecological Departure
Bayesian belief networks were used to determine the overall influence of stream temperature, summer
baseflow, and winter peak flow changes due to climate change on current and historic bull trout habitat
(Figure 14). BBN’s were constructed through a series of meetings with Sawtooth National Forest and the
Rocky Mountain Research Station in 2010 to determine how much collective change would need to occur
before a certain level of ecological departure impacted aquatic habitat within each subwatershed that
supported current or historic bull trout populations.
Bayesian models predicted that habitat in 6 (16%) bull trout patches on the Sawtooth NRA would be at
high risk from ecologically-departed flow and temperature conditions. It also predicted that habitat would
be at moderate risk in 17 (46%) bull trout patches and at low risk in 14 (38%) bull trout patches. By 2080,
risks to habitat from changed flows and water temperatures increase greatly. Only one (3%) bull trout
patch (Big Casino Creek) would have low risk from ecologically-departed flow and temperature
conditions, while habitat in 22
(59%) patches would be at
moderate risk and 14 (38%)
patches would be at high risk.
Bull Trout Persistence
Bayesian belief networks were
used to determine bull trout
persistence in the future on the
Sawtooth NRA. Persistence of
bull trout was based on a
combination of factors. These
included (Figure 15): the influence
of increasing stream temperature,
decreasing summer baseflow, and more frequent winter peak flow events due to climate change; the
composite rating for risks and threats (i.e. landslide terrain, water diversions, route density, etc.); and
current biological (i.e., local population size, life history diversity, etc.) and physical (i.e., overall
watershed condition) baselines. The key assumption with this approach is that smaller, weaker, bull trout
populations will be more susceptible to climate change in patches with poor baseline conditions and with
173 Assessing the Vulnerability of Watersheds to Climate Change
Figure 14. Bayesian belief network for determining ecological departure from
changes in mean summer baseflows, winter 95 rain on snow risks, and changes in
optimal stream temperatures for bull trout