watervulnerability

Chugach National Forest Watershed Vulnerability Assessment, Alaska Region (R10)
line maps and bar graphs are available for Alaska communities. Raw data are available for use
with GIS.
• Total salmon spawning area not known.
Sensitivity
The sensitivity of the Eyak Lake watershed is due mainly to natural conditions –steep slopes, shallow
lake, and high precipitation. Human activities, such as road building and other development, have been
relatively minimal but there are stressors that might affect watershed’s ability to respond to the predicted
increases in temperature and precipitation.
• The mean high elevation of the watershed makes the watershed less sensitive to the effects of
higher temperatures on the glaciers and snowpack. However, the current storm patterns from the
relatively warm ocean already cause frequent rain-on-snow events. These are likely to increase
and occur at higher elevations.
• The relatively high percentage of area covered by glaciers and icefields makes the watershed
more sensitive to the effects of melting glaciers: increased flows and erosion of glacial moraine.
• Stream temperatures could rise with predicted changes in air temperatures but should be well
within the suitable range for salmonids. Power Creek temperatures were no more than 10 °C at a
downstream location (Sea-Run Fisheries 2006) and should rise no more than the predicted 2 to 3
°C air increase. All streams are relatively steep and short, so there is little opportunity for streams
to warm.
• Water temperatures for most of Eyak Lake are dominated by stream and groundwater flows.
Summer surface temperatures are generally less than 13.5 °C and data at two sites suggest there is
a thermocline at about 1 m (Crawford 2010).
• Parts of Eyak Lake could be sensitive to higher water temperatures. The west arm of the lake has
less circulation, is less than 3 m deep, and currently has recorded surface temperatures of 15 °C
(the top end of the optimal range for salmonids).
• Watershed is naturally flashy due to 19.7% being unvegetated, 23.1% having steep slopes
(>70%), along with thin soils, high precipitation, and long duration of storm events.
• Flooding already occurs in residential areas along Eyak River and Eyak Lake. Floods have
occurred in 1983, 1985, 1986, 1995, 2004, and 2006.
• Hydroelectric power generation is sensitive to flows in Power Creek, which are at a minimum in
winter when precipitation is bound as ice and snow. Higher precipitation, warmer temperatures,
and rising snowline could increase winter power generation.
• Salmon spawning in the lake and smaller tributaries not sensitive to redd displacement by floods.
Those fish spawning in the main channel of Power Creek may be susceptible to substrate
mobilization.
• The risk of avalanches could increase as warmer temperatures create more frequent wet, heavy
snowpacks. There are a high percentage of avalanche-prone slopes of 58 -173%. There have been
three fatal incidents in past 15 years.
• Landslides. High precipitation, long steep slopes characteristic of glacial U-shaped valleys, thin
soils, underlying bedrock, and glacial till increase propensity for landslides.
Stressors
Stressors from residential development include hydrocarbon input to the lake from the streets and snow
dumping, nutrient input from fertilizer and leach fields, minor sediment input from unpaved roads, and
runoff from two subdivisions. Low levels of hydrocarbons have been detected in water samples, but the
overall effects of oil and the other stressors have not been quantified.
279 Assessing the Vulnerability of Watersheds to Climate Change