watervulnerability

Chugach National Forest Watershed Vulnerability Assessment, Alaska Region (R10)
harvest is still a small percentage of the commercial fishery (Lang 2010). Recreation and subsistence
harvest are likely to grow, but data are lacking.
Exposure/Risks
Hydrologic/Geomorphic
Assessing risk is difficult in the Eyak Lake watershed because the weather conditions are highly variable
already. At the nearby Cordova airport, the mean annual precipitation from 1949 to 2004 is 96 inches, but
the extremes have ranged from 54 to 139 inches (139 being 45% above normal). Thus, predictions that
the mean precipitation in the rainier Eyak Lake watershed will increase 3% from 177 to 184 inches do not
give a clear indication of how that will affect the hydrologic or geomorphic conditions. Such an increase
is well within what might be considered normal.
The significant changes are most likely to come from the extreme events, which are predicted to increase
and intensify, but aren’t readily quantified. Mass wasting from snow avalanches is likely to increase but
predicting such events is also not feasible. Thus, exposure and risk may be best discussed in general
terms.
The predicted increases in temperature and precipitation are likely to result in higher streamflows
throughout the year, more frequent rain-on-snow events in the fall and spring, and changes in the timing
of peak spring flows as the snowpack melts earlier. The predicted increase of extreme weather events,
including increased storm duration and intensity, will also lead to greater streamflows. Glacial melting is
expected to continue or accelerate, adding to flows in the summer, which could compensate for the
reduction in flows from an earlier snowmelt.
Geomorphically, these changes are likely to lead to increased snow avalanches, landslides, and other
erosive processes. Temperatures changing between freezing and thawing at the lower elevations will be
especially conducive to increasing snow avalanche danger. Many avalanche and landslide areas transport
material directly to Power Creek or Eyak Lake itself, adding to the bedload. Exposed glacial moraines
will be subject to erosion and transport by meltwaters.
The increased bedload material will be deposited in the Power Creek delta at the head of Eyak Lake, and
at Middle Arm and other smaller alluvial deposition areas around the lake. As with many deltas and
glacial outwashes, stream channels will fill and shift. The Power Creek delta will most likely extend
farther into the lake.
The main consequence of the hydrologic and geomorphic changes will be the increased risk of flooding,
especially in the subdivision just downstream from the outlet of Eyak Lake. Prolonged storm events in the
fall have caused flooding in this area a few times every decade and this is only likely to increase with
more precipitation and extreme events. Despite past flooding, development has continued on this
floodplain due to the general scarcity of level land on which to build and its location beyond the city
zoning areas.
The other exacerbating factor is that flows from the glacial Scott River in the adjacent watershed can spill
over into Eyak River, about 1/2 mile downstream from the development. As the Scott River deposits
sediment into Eyak River, the Eyak channel’s ability to drain its watershed is reduced, resulting in
increased flooding (Blanchet 1983, Hitch 1995). Similar increases in flows, bedload transport, and
channel shifting in the Scott River are thus likely to affect Eyak River as well.
281 Assessing the Vulnerability of Watersheds to Climate Change