watervulnerability

Chugach National Forest Watershed Vulnerability Assessment, Alaska Region (R10)
Flooding will also affect Power Creek Road as it crosses the delta floodplain. Flooding already occurs
every few years, but the flows are not sufficient to severely damage the dirt road, nor does the minimal
amount of traffic seem to justify upgrading the road. Increased flows and a shift of the main channel,
however, could cut off access to the hydroelectric plant until waters subside.
One positive effect of the hydrologic changes may be the increased production of hydroelectric power at
Power Creek. The plant is a run-of-the-river facility with no reservoir, so when winter precipitation falls
as snow, and the river drops below 320 cfs, power generation is reduced. At the present time, maximum
generation is reduced from late October to mid-May and severely limited from late November to April.
The number of days with the mean temperature below freezing is predicted to decline dramatically, with
precipitation falling as rain later into the fall and earlier in the spring. Thus, the period of higher power
generation would be extended. Since the water use capacity of the turbines is well below the summer
flows, and summer precipitation is predicted to increase, the smaller snowpack and summer runoff should
still be sufficient to run the turbines at maximum capacity.
Because there is no reservoir, additional bedload from increased erosion should not be a problem. There is
a low dam with an inflatable bladder that can be deflated to allow accumulated sediment to be flushed
from behind the wall and pass downstream.
Biological Exposure/Risks
In the western lower 48 states, the main concerns for aquatic organisms are high water temperatures and
low flows that can have direct lethal effects. In coastal Alaska where precipitation will increase and water
temperatures will be higher but still relatively low (Bryant 2009), the effects of climate change could be
more subtle, but serious nonetheless.
Water Temperature
Water temperatures are expected to rise, but since existing stream temperatures in the Eyak watershed are
cool, increases would not be lethal or even beyond the optimum levels for salmonids, the organisms of
primary concern. Current lake temperatures are somewhat warmer, but Crawford (2010) shows that most
of the lake temperatures are influenced by the streams, except for the shallow west end. Even there,
surface temperatures in the summer have been moderate. If summer water temperatures increase about the
same as the predicted air temperatures (1.5 to 1.8 °C), the temperatures would still be within or close to
the optimal range. Thermal refugia would also be available near the mouths of some small creeks or in
deeper waters.
Increased water temperatures are more likely to have an effect on the egg and larval stages of fish and
aquatic invertebrates. As is clear from fish hatchery experience (Piper et al. 1982), higher water
temperatures accelerate the development of eggs and hatching. Based on a model by McCullough (1999),
a 1 °C increase in water temperature could cause coho salmon fry to emerge about 10-20 days earlier in
the Eyak area. If prey organisms do not follow the same pattern of earlier growth, the newly emerged fry
may lack food resources (Bryant 2009).
Such a scenario is described by Winder and Schindler (2004) where a species of zooplankton that
emerged according to photoperiod length was at a disadvantage compared to a species that hatched by
temperature. Unfortunately for sockeye salmon fry, their preferred prey species is the photoperiod
dependent species, which may have significant effects in the future. Hypothetically, similar disruptions
could occur with aquatic insect life cycles and the avian species dependent on them (McClure, et al,
2011). It is not known whether similar scenarios may occur in the Eyak Lake area because the specific
282 Assessing the Vulnerability of Watersheds to Climate Change