watervulnerability

Sawtooth National Forest Watershed Vulnerability Assessment, Intermountain Region (R4)
Overall, the predictions for bull trout do not seem promising for long-term persistence for many
populations. The long-term climate patterns in tributary streams suggest both an expected decrease in the
total amount of cold water stream habitat and fragmentation of some colder areas into disconnected
“patches” of suitable habitat. Bull trout populations will likely increasingly retreat into these shrinking
summer cold water refuges to avoid warming conditions. These restricted tributary populations may
become more vulnerable to local extinction (Dunham et al. 1997; Dunham and Rieman 1999; Morita and
Yamamoto 2002; Rich et al. 2003; Isaak et al. 2007). Many remaining patches will be subjected to more
frequent winter peak flows, which will scour the streambed and destroy redds and/or kill newly emerged
fry. Populations may also be subjected to larger, more severe wildfires (McKenzie et al. 2004; Westerling
et al. 2006) that can remove riparian vegetation or catalyze severe channel disturbances such as debris
flows (Luce, et al 2005). Conceivably, the combined effects of shrinking patch size and increasing
frequency or magnitude of stream channel disturbance could chip away at what remaining resiliency these
populations have, leaving them in a poorer condition to withstand the next series of disturbances, and
accelerating the rate of local extinctions beyond that driven by temperature alone.
Forest Infrastructure
Developed recreation sites and trails within riparian conservation areas, water diversions, system roads,
bridges, and ownership were categorized according to Forest Plan and literature criteria, histograms, and
professional judgement, to determine the level of threat associated with each type of infrastructure (Table
6). Bayesian belief networks were then used to evaluate the overall amount of infrastructure and risk to
facilities within each subwatershed from winter peak flows caused by rain-on-snow events on the
Sawtooth NRA. Those subwatersheds that have moderate/high amounts of infrastructure and high risks
from increased winter peak flows were considered to have a high risk of damage to road and trail drainage
and facilities within riparian areas. Subwatersheds with less infrastructure were considered to have lower
risks from winter peak flow events.
Infrastructure
Low
Threat
Moderate High
Percent Federal Lands 85-100% 50-84% 8 sites/6 th Field
Water Diversions No Diversions 1-2 diversions/6 th Field >2 sites/6 th System Road Density - Miles of
Field
road/sq. miles (within admin
boundaries)
< 0.7 mi/mi 2 0.71-1.7 mi/mi 2 >1.7 mi/mi 2
Road Stream Crossings - Number of
road/stream crossings on perennial and
intermittent streams based current road
layer and NHD streams within total
subwatershed regardless of ownership
or administrative boundaries
0-11 crossings crossings >23 crossings
Bridges No bridges present 1-2 Bridges >2 Bridges
System Trails within RCAs < 0.7 mi/mi 2 0.71-1.7 mi/mi 2 >1.7 mi/mi 2
Table 6. Forest infrastructure and levels of risk
177 Assessing the Vulnerability of Watersheds to Climate Change