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Glaciers are distributed globally, covering about 10% of the Earths surface and storing about 75% of the worlds freshwater. Hence, glaciers contribute significantly to river flow and water resources across the globe [Fleming and Clarke 2005], and provide important ecosystem services for society in terms of hydro-power, agriculture, and water supply [Barnett et al. 2005]. This is particularly the case in Alaska where approximately 35% of the runoff is from glaciers [Mayo 1986] and glacial runoff can have a marked effect on yearly, seasonal, and daily river discharge fluctuations where glacierization [glacier landcover] in a basin exceeds 5%. Much of this runoff originates from glaciers within Alaska national parks. Climatic conditions determine the total annual runoff from and the net storage of perennial snow and ice within the basin; hence inter-annual stream flow variations reflect glacier mass-balance fluctuations. Seasonally glacial rivers in the northern hemisphere typically have very low or no discharge in winter; flows begin increasing in early May as solar radiation increases to reach a summer peak at maximum glacier melt. Discharge then declines gradually until freeze-up in November and December. Glacierized environments are demonstrably one of the most vulnerable to climate change because of interconnections between atmospheric forcing, snowpacks/ glacier mass-balance, stream flow, water quality, and hydrogeomorphology [physico-chemical habitat], and river ecology [McGregor et al. 1995].
Not only are most glaciers shrinking [i.e. both thinning and retreating, see Figure 2], but that the rate at which they are changing has accelerated over the last 2-3 decades [Haeberli et al. 2007]. Over the last century, mid-latitude and arctic glaciers have generally been shrinking [including most in Alaska], while some in marginal environments have disappeared [Meier et al. 2003]. Measurements of glacier mass-balance outside polar regions are mainly negative, with a few exceptions e.g. Scandanavia.
Glaciers play a major role influencing river flow regimes with peak glacier-melt occurring during mid-late summer following retreat of the transient snowline [Hannah et al. 2005]. The hydrological behavior of basins with as low as 10 % ice cover are strongly influenced by the balance between accumulation [gain] and ablation [loss] of glacier mass [Fountain and Tangborn 1985]. Glaciers can maintain stream flow during the summer dry season when rivers in non-glacierized basins display low flow [Hannah et al. 2005]. Rivers with meltwater inputs provide habitat for fisheries [Richardson and Milner 2004] and a number of rare and endemic macroinvertebrate species [Brown et al. 2007].
Hydrological Response to Retreating Glaciers
For larger receding glaciers, an initial increase in glacial meltwater generation may occur due to increased energy inputs, earlier disappearance of reflective snow cover and exposure of lower albedo ice [Milner et al. 2009].
This initial flow increase will be followed by reduced glacial runoff in the long-term due to an ensuing negative glacier mass-balance [Stahl et al. 2008]. However for many smaller mountain glaciers reduced glacial meltwater runoff is occurring presently as glaciers recede.
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Glacier-fed Rivers and Their Biotic Communities
Many of the large glacier-fed rivers in Alaska [e.g. the Susitna River flowing from the south side of the Alaska Range in Denali National Park] possess a complexity of habitats adjacent to the main glacier-fed channel, including side-channels and side-sloughs. Juvenile king salmon use the side channels for rearing, whilst the turbidity and invertebrate drift into these channels can provide both cover and food resources. These side-channels and sloughs are sustained by melt waters of the main channel [Richardson and Milner 2004] and juvenile chinook salmon overwinter in these habitats. For many of these systems in northerly regions, winter is a critical period when overwintering mortality for salmonids can be high if they are unable to migrate to suitable refugia [i.e. channels off the mainstem]. A significant positive correlation exists between the survival of young salmonids during the winter and the amount of winter discharge or winter groundwater inputs [Fleming 2005]. Therefore, any changes in water source contributions that enhance winter flows would be beneficial to salmonid populations.
The effect of shrinking glaciers on fish populations will depend upon whether the reduction in mass-balance causes an initial increase in the glacial runoff contribution to the river system [as outlined earlier] or a decrease. Increased glacial runoff will cause an increase in summer flow peaks and potentially the duration of higher flows, which will enhance the migration of adult salmon to their spawning grounds, either in the river mainstem or in associated tributaries and off-channel habitats. Increased glacial runoff will also increase mainstem spawning habitat and cover in the side channels. This migratory and spawning habitat aspect of increased glacial runoff applies principally to larger systems fed by glaciers in Alaska and Canada.
Glacier-fed rivers support a unique flora and fauna driven by the overriding influence of water temperature and channel stability [Milner et al. 2001]. However there is a strong temporal element to these conditions and in the spring and autumn when the glacial component is reduced, improved water clarity and channel stability in the channel allow for periods of extensive algal growth and benthic invertebrate production. These can be looked on as windows of opportunity for these organisms before and after the harsher conditions of summer when glacier melt is at its maximum.
Glacier-fed rivers close to the source are unique in that they support a deterministic assembly of macroinvertebrate communities that are similar worldwide due to the overriding variables of low channel stability and water temperature [Milner et al. 2001]. A number of these taxa are cold stenotherms restricted to headwater reaches. With climate change and where glacial runoff becomes reduced, channel stability and water temperature will increase and suspended sediment/turbidity decrease. The development of stream macroinvertebrate communities in Wolf Point Creek in Glacier Bay with reduced percent glacierization of a remnant ice sheet provides some general insights into what will happen when glaciers retreat with reduced glacial runoff [Milner et al. 2008]. There was a strong negative relation between % catchment glacierization in the Wolf Point Creek watershed and the diversity of the benthic community [Figure 4] with only five taxa [all chironomids] found in the stream at 70% glacierization which increased to 24 taxa at 0% glacierization. Of the chironomids [non-biting midges], Diamesa were not found when % glacierization was