Ice

• human history coincides with a geological epoch with an abundance of snow and ice but comprising about only 1% of earth history
• the Pleistocene Ice Age, the last two million years of earth history, ended only 10,000 years ago (arbitrary limit), compared to the duration of a typical interglacial measured in 10s thousands of years; thus the earth is likely just between major glaciations
• the most recent advance of mountain glaciers, the Little Ice Age, began about 900 years ago and peaked around 1850
  
  
  • fresh Little Ice age moraines are clearly evident in the Rocky Mountains, 100s of metres beyond the present glacier margins
  • for example, the early travelers between Banff and Jasper used Wilcox pass because the terminus of the Athabasca glacier was in bottom of the valley now traversed by the Icefields Parkway

Glacier Ice

• unlike all other forms of ice, because it forms from the metamorphosis of snow
• storage of most of the world's fresh water
• there are an estimated 100,000 glaciers in Canada but most of the glacier ice is locked in polar ice caps with low mass balances
  
  
  • the glaciers of Alberta and British Columbia are the headwaters of the large rivers (e.g., Columbia, Fraser, North and South Saskatchewan) that supply most of the water used by Western Canadians
  
  
• by storing water seasonally (winter), and for up to thousands of years, glaciers like other storages regulate stream flow by augmenting inputs when precipitation is low , masking the variability of precipitation inputs, and prolonging the melt season into the summer
• maximum contributions of glacier meltwater occur when glaciers are large relative to the size of a catchment and they have large inputs and outputs, i.e., glaciers in wet temperate mountain ranges (e.g., the Coast Mountains)

glacier mass balance

• the fundamental concept in glaciology and glacial hydrology
  
  
• accumulation (input)
  
  
  • precipitation
  • blown snow (drift or ural glaciers)
  • avalanches and ice falls
  
  
• wastage (output)
  
  
  • melt resulting from influxes of solar radiation, sensible heat of air and rain, latent heat of condensation (dew and frost), and heat conducted from supraglacial debris (enhanced melt up to a threshold depth of debris; thick deposits insulate the ice)
  • calving: more than 50% of the wastage of the Antarctic ice sheet
  
  
• changes in storage
  
  
  • positive mass balance: more water stored as snow and ice
  • negative mass balance: less water stored as snow and ice, declining reserve of fresh water
  
  

Canadian glacier inventory

• initiated in 1945 to monitor glacier contributions to river systems
• data obtained from maps, air photos and field surveys (surface elevations, flow velocities, ice marginal positions)
• the data are used for:
  
  
  • forecasting water supply and flooding
  • monitoring climatic change as reflected by changes in glacier mass balance
  • determining potential for irrigation and generating hydroelectricity

Ground ice

• seasonal in temperate climates
• permanent in cold environments
• ground ice is present in a variety of forms (Mackay, 1972, World of underground ice, Annals of the Association of American Geographers, vol. 62), but there are two main types:
  
  
  • pore (interstitial) ice: freezing of the water in the pore spaces in soil and rock
  • segregated ice: bodies (lenses, wedges, seams, etc.) of pure ice that forms and grow as soil water is diffused towards the lower vapour pressure at the freezing plane
  
  

hydrologic significance of ground ice

1. permafrost
   
   
   • permanently frozen ground usually contains ice
   • thus the distribution of permanent ground ice is linked to the distribution of permafrost
   • this water can be stored as ice for tens of thousands of years, i.e., much permafrost and ground ice is relict from the colder climate of the late Pleistocene
   
   
2. seasonally thawed ground
   
   
   • in the active layer, water trapped above the permafrost table participates in the hydrological cycle during a short spring and summer
   
   
3. segregated ice
   
   
   • develops by attracting water, and thereby depleting the water content of surrounding sediments
   • needle ice is a form of segregated ice that forms on wet ground on cold nights, thereby desiccating the soil surface
   
   
4. lack of baseflow
   
   
   • interflow and overland flow from a saturated active layer tend to be the dominant source of runoff, because groundwater is either frozen or trapped below permafrost
   
   

Lake and River Ice

• there are large variations in the structure and strength of lake and river ice as a function of weather (snow, rate of freezing) and the dynamics of lake and rivers
• because wide rivers are similar to lakes and there can be turbulent flow (currents) in lakes, lake and river ice are not mutually exclusive, rather the ice cover on surface water is best classified as

  
  

  static ice

  plate ice that forms in still water or with laminar flow, where vertical heat exchange is limited by a lack of turbulence (convection); most of the ice on lakes, but also on rivers near the banks and on quite water (e.g., pools)
  
  

  dynamic ice

  forms by the coalescence of ice pans on flowing (river) water and lake currents

Freezing (nucleation) of water

• water freezes at a lower temperature than it melts, because supercooling and freezing nuclei are necessary for the formation of ice
• typical nucleation thresholds (freezing points, in 0 ^oC):

modes of nucleation

• primary: source of ice is internal
  
  
  • homogenous: spontaneous nucleation; occurs only in pure supercooled water, i.e., not in natural water
  • heterogeneous: ice grows from crystals that form in the water on freezing nuclei (suspended particulate matter)
    
    
    • surface supercooling: in still water or with laminar flow when temperatures below 0 ^oC develop in the surface water; strong temperature gradient between the water and cold air overlying air
    • frazil ice: crystals suspended in turbulent water
    
    
  
  
• secondary: ice cover initiated by external ice crystals, i.e., snow or frost

Static ice

• forms primarily by surface supercooling, with negligible heat transfer by conduction in still water or laminar flow; heat loss is concentrated at the surface where a strong temperature gradient is established
• at the nucleating threshold for natural water (~ 1 ^oC), ice crystals from at the surface and become nuclei for the formation of ice needles which grow downward into the lake or still river water (e.g., border ice along the shore)
• the rate of freezing depends on the rate of heat loss:
  
  
  • with a small temperature gradient (cool air), the needles form slowly and rotate (long axis parallel to the water surface) forming ice with a layered structure
  • with a larger temperature gradient (cold air), the ice needles can form so quickly that they interlock as they rotate, forming plate ice with a complex structure

secondary nucleation

• snow on a superccoled water surface will initiate ice formation
• sublimation of ice crystals (e.g., fog overlying cold water) is another mechanism of secondary nucleation

snow ice (white ice)

• snow ice forms by
  
  
  • the freezing of snow slush after a heavy snowfall occurs onto cold water
  • heavy snowfall forcing plate (black) ice into the lake; water moves through cracks and saturated the snow cover
  
  
• thus where heavy snowfalls are common and not removed by wind (e.g., humid microthermal climates), the strength and structure of lake ice depend on the amount and timing of snowfalls
• typically the static ice cover consists of granular snow ice overlying snow slush (the drowned lower part of the winter snow pack) and resting on the black ice cover

Ice cover on the Great Lakes

• Superior
  
  
  • 60% ice covered in a normal year
  • freeze up begins in shallow water on the north shore and east of Duluth, Minnesosta
  
  
• Erie
  
  
  • shallowest of the Great lakes and thus usually freezes over
  
  
• Ontario
  
  
  • generally only about 15% ice covered, because it receives more runoff than any other Great Lake
  • completely frozen over only twice in the past 100 years

Dynamic Ice

• turbulence (stream flow and lake currents) circulates heat throughout the depth of streams or the lake epilimnion and therefore dynamic ice forms from frazil ice, which is ice that has formed in quiet water and become suspended in the flow
  
  The frazil ice crystals agglomerate and float to the surface to form frazil flocks or slush floes, which oscillate in the turbulent water, becoming exposed to cold air and freezing into ice pans (pancakes). Ice pans collide with one another , developing a dish shape (circular and concave). Frazil ice and ice pans are generated from the same stretches of calm water, on successive nights and / or cold days until the concentration of ice pans results in a continuous ice cover. The ice cover extends upstream from ice bridges that form over clam water or constrictions in the channel. On the St. Lawrence River, ice pans travel as much as 40 km per day.
  
  
• in the more turbulent flow, the ice pans can be destroyed and thus an ice cover may be thin or absent from rapids, except possibly in cold climates or cold years
• ice also can form in the still water behind objects on the stream bed (e.g., boulders); this anchor ice can restrict the flow possibly causing winter flooding or it can break off and float to the surface