watervulnerability

Chugach National Forest Watershed Vulnerability Assessment, Alaska Region (R10)
species in the food chain and their life histories have not been studied. This lack of information makes it
difficult to assess the full effects of climate change.
Another concern is that increased metabolic rates for juvenile fish in warmer water may result in reduced
growth as a greater share of energy is expended for body processes when there is no increase in food
availability (Bryant 2009). Smaller size is linked to higher predation rates. If fish have lower fat reserves
going into the winter, winter survival rates will be a concern because food is less available then. Another
research need is to determine whether food is a limiting factor or whether greater primary production
from warmer temperatures and a longer growing season may lead to greater resources at higher trophic
levels.
Water Quantity
Water quantity is generally not a concern, as increased precipitation throughout the year (in addition to
the high current levels) should help to maintain flows in small streams. There is, however, some
uncertainty about the degree to which warmer winter temperatures will affect the snowpack. Winter
temperatures at sea level are expected to remain close to freezing until the middle of the century, but the
number of days below freezing will decrease, and more precipitation is expected to fall as rain at the
lower elevations. The question is whether the increased winter precipitation at the high elevations could
offset this loss of snow and maintain the snowpack and, in turn, summer flows.
The opposite concern is that flows may be too great. With increased precipitation, more frequent rain-onsnow
events, and more extreme storm events, high streamflows in the fall could mobilize gravels in
salmon spawning areas, displacing and killing the eggs in the redds. Material from landslides, triggered
by extreme precipitation, could scour spawning beds or be carried by high flows and deposited on redds
(Bryant 2009). Fine sediment deposition can not only smother salmon eggs; the deposition can cause
greater and deeper scouring (Montgomery et al. 1996), dislodging eggs that might have been buried at a
safe depth under other conditions.
These risks might also be increased because warmer temperatures could extend the flood-prone season
later into the year. Currently, by late October, most precipitation at higher elevations is falling as snow,
and streamflows drop. The somewhat late spawning run of coho salmon in the main channel of Power
Creek, which lasts into December, could be a local adaptation to avoid the risk of redd scour
(Montgomery et al. 1999). However, the benefits of late spawning are negated if heavy rain or rain-onsnow
events occur later in the year.
Overall, however, the risks to spawning are buffered by the variety of spawning habitats used by
salmonids. Sockeye salmon spawning in the lake is not subjected to scouring, although a large sediment
flux or landslide could bury some areas. Much of the spawning of coho salmon and sockeye salmon
occurs in the smaller, side channels of the Power Creek delta or in other tributaries that are not subject to
high flows. Cutthroat trout spawning areas are almost all in small tributary streams (Hodges et al. 1995).
Montgomery et al. (1999) and Tonina and McKean (2010) also stress that the channel type where
spawning occurs influences the risk of redd scour. Steeper-gradient confined channels are naturally more
prone to scouring, whereas less-confined channels allow flows and energy to be dispersed. In the case of
the Eyak Lake watershed, most of the salmon stream spawning occurs on poorly controlled alluvial fans
and in the Power Creek delta complex. As Tonina and McKean (2010) state:
Our analyses showed that such unconfined low-gradient streams have not a great danger of extensive bed
mobility, even at high flows. Consequently, in this landscape, alterations in flood timing due to climate
283 Assessing the Vulnerability of Watersheds to Climate Change