watervulnerability

Umatilla National Forest Watershed Vulnerability Assessment, Pacific Northwest Region (R6)
FINE SCALE ANALYSIS FOR BULL TROUT
Bull trout (Salvelinus confluentus) was used as our aquatic focal species in the WVA because bull trout
require cold (≤ 17 °C) and relatively low gradient, pristine waters to rear and spawn. They have a small
thermal niche and are very responsive to changes in stream temperature. Analysis of suitable habitat on
the UNF is necessary because bull trout are on the edge of their bioclimatic envelope (Beever et al. 2010,
Dunham et al. 2003); the UNF is a fairly low elevation, dry forest landscape. Bull trout populations in the
southern parts of the UNF can also be described as peripheral populations or species that are at the
geographic edge of their range; they often have increased conservation value because they maximize
within-species biodiversity, retain important evolutionary legacies, and may provide a “gene pool” for
future adaptation (Haak et al. 2010). Previous research suggests future stream temperature increases on
the forest, but influences on distribution and abundance of stream organisms is not well documented
(Rieman et al. 2007). To begin the analysis, current bull trout distributions were identified in the
Umatilla, Walla Walla, Tucannon, Lookingglass, and North Fork John Day (NFJD) drainages. Previous
stream surveys conducted by USFS and ODFW/WDFW were used to verify current bull trout
distribution.
Multiple Regression Stream Temperature Model
A multiple regression stream temperature model developed by the RMRS was used to model historic,
current and future (years 2033, 2058, 2080) suitable bull trout habitat. Stream temperature model
information and methods to the can be found at
www.fs.fed.us/rm/boise/AWAE/projects/stream_temperature.shtml
The regression model used observed mean weekly maximum temperature (MWMT) and physical
parameters or predictor variables and geomorphic variables that have direct effects on stream
temperatures. (The regression equation and coefficients can be found at
www.fs.fed.us/ccrc/wva/appendixes.)
Physical metrics:
• Water diversion
• Wildfire – Used data from the last 20 years; ~4km from the stream.
• Groundwater Dependent Ecosystems (resiliency): number of springs and wetlands per HU12,
rated
Geomorphic variables or metrics (National hydrologic data set):
• Cumulative drainage area (km 2 )
• Slope (%)
• Elevation (m)
Observed Stream Temperature and Climate Data
Observed summer MWMT were taken from 37 locations and provided a total of 333 stream observations.
A separate regression model was developed to predict historic and future stream temperatures using the
same physical and geomorphic predictor vales, however, air MWMT data (1979-2009) and flow (m 3 /s)
data (1957-2009) were considered. (Details about this regression model are available at
www.fs.fed.us/ccrc/wva/appendixes.)
219 Assessing the Vulnerability of Watersheds to Climate Change