watervulnerability

Chugach National Forest Watershed Vulnerability Assessment, Alaska Region (R10)
• Tailings piles deposited along the creek have confined the channel and caused downcutting.
Greater flow velocities, scouring, and erosion may occur with predicted increases in precipitation
and extreme events. Salmon redds may be subject to scouring.
• The creek channel has been moved and straightened, increasing the gradient and water velocity.
Again, scouring and erosion are likely to increase with precipitation and extreme events.
• Mining has removed the trees in the riparian areas. This has resulted in the loss of future LWD
that would add roughness to the channel and moderate water velocities. The loss or pool-forming
LWD reduces fish habitat.
• Mining activity has reduced the fine-grained sediment and organic material from the floodplain,
so re-establishment of the riparian vegetation has been minimal. Without healthy vegetation, the
streambanks are more sensitive to erosion from high flows during extreme events.
The topography and current climate conditions, however, may reduce the sensitivity of the watershed to
climate change. Even with the predicted increases in temperature and precipitation, the watershed will
still remain relatively cold and dry. In addition, some current and proposed restoration work could lessen
the sensitivity. In brief:
• Cold winter temperatures (even at sea level), high mean elevation, low precipitation, could all
reduce sensitivity to snowline increase and rain-on-snow events.
• Continued cold winter temperatures at high elevations should result in fewer freeze/thaw cycles
and instances of wet heavy snow falling on dry snow layers. Avalanche danger and its sediment
transport may not be sensitive to changes.
• Short duration, high intensity storms are relatively rare, and the flow response from such events is
limited by high initial infiltration. This reduces sensitivity to flooding and high flows.
• A restoration project along one mile of stream reconnected floodplain, created meanders to
reduce gradient, added LWD and secondary channels. Several miles of unrestored channel
remain.
• Current low stream temperatures, minimal lake area, make water temperatures less sensitive to
warming.
• Low acreage of glaciers/permanent ice field reduces sensitivity to the effects of increased glacial
melting – higher flows, moraine transport.
• Low road density and minor current mining operations do not contribute significant amounts of
sediment to the streams.
One non-aquatic stressor that may have already been worsened by climate change is increased timber
mortality due to the spruce bark beetle. Warmer winters have been cited as one reason for increases in the
beetle population and infestation of the stands. Continued warming trends could lead to further increases
in the beetle population and greater tree mortality.
With high numbers of dead trees, the watershed is expected to be more vulnerable to fire, although the
extent of risk is in question. Fire and the resulting loss of vegetation could lead to greater erosion of the
hillslopes. More in-depth analysis of the fire potential is needed, but the general outlook is that the risk of
fire will increase as described here:
• The spruce bark beetle infests about 11% of the watershed, resulting in high levels of dead trees
and fuel loading. Predicted warmer summer temperatures with only small increases in
precipitation may increase fuel drying and fire hazard.
• Increased temperatures, growing season, and precipitation could increase grass and shrub growth,
increasing fuel load (Haufler et al. 2010).
290 Assessing the Vulnerability of Watersheds to Climate Change