LAKES

• lakes seem like permanent features, but they are transitory in terms of geologic time
• lakes are "born of catastrophes to mature and die quietly and imperceptibly" (Hutchinson, 1957: Treatise on Limnology)
• most lakes "die" as they become increasingly eutrophic and the lake basin fills with mineral and organic sediments
• some lakes disappear catastrophically: shallow lakes during episodes of drought or ice- or earth-dammed lakes when the dam is removed
• "born of catastrophe" refers to the origin of lake basins

genetic classification of lake basins

1. tectonic
   
   
   • epeirogenesis (continent building): isostacy and broad folding (warping); rift valleys (e.g., Lake Baikal, and East African lakes)
   • orogenesis (mountain building): long narrow lakes in folded and faulted mountain belts
   • volcanism: crater lakes (e.g., Crater Lake, Oregon), lava dams
   
   
2. glacial
   
   
   • ice-dammed lakes (e.g., Glacial Lake Regina)
   • moraine-dammed lakes
   • kettles (prairie sloughs)
   • ice-scoured basins (e.g., the Great Lakes)
   
   
3. landslide basins
   
   
   • drainage dammed by mass wasting events
   • e.g., Hebden Lake, Montana, formed when the Madison River was impounded by a rockslide during the 1960 earthquake
   
   
4. solution basins in calcareous rocks
   
   
   • tend to fill with surface runoff and then drain into underground drainage networks (pipes, caves, etc.)
   
   
5. fluviatile
   
   
   • oxbow lakes
   • wide rivers
   • plunge pools below waterfalls
   
   
6. deflation basins
   
   
   • form by wind erosion in dry environments, and thus any water tends to be temporary
   
   
7. coastal and shoreline basins
   
   
   • lake or seawater impounded by coastal or shoreline deposits, baymouth bars, barrier reefs, atolls
   
   
8. meteor impact
   
   
   • e.g., Ungava Lake in northern Quebec
   
   
9. artificial
   
   
   • mostly for generating hydroelectricity, flood control or water supply in dry environments (e.g., Lake Diefenbaker)
   
   

Lake districts

• tectonism and glaciation, which account for most lake basins, are concentrated in time (i.e., catastrophic on a geologic time scale)
• they also have a definite spatial distribution and thus most of the world's lakes occur in a few large areas of former tectonic or glacial activity

1. Canadian shield
   
   
   • 7.6% of Canada is lake, a much higher proportion than any other country
   • nearly all these lakes, including the Great Lakes, occupy ice-scoured basins
   • for example, in an area of 13,710 km² southwest of Reindeer Lake, Saskatchewan, there are approximately 7500 lakes
   • Great Slave Lake, Alberta, is the deepest lake of glacial origin; it also produced a 109 pound lake trout, the Alberta record
   • beyond the shield, most of the large lakes in Canada are in ice-scoured mountain valleys, e.g., Kootenay, Okanogan and Arrow Lakes
   
   
2. Fenno-Scandian (Finland and Scandinavia) shield
   
   
   • scoured by the Fenno-Scandian ice sheet that occupied northern Europe
   
   
3. East African rift zone
   
   
   • location of most of the world's deepest lakes, besides lake Baikal
   
   

Role of lakes in the hydrological cycle

• lake storage of runoff regulates stream outflow by sustaining low flows and suppressing peak discharges
• more than 98% of accessible surface water is lake water
• lakes are aquatic habitat; the chemical and ecological cycles are strongly influenced by physical limnological processes (e.g., upwelling and downwelling, spring and fall turnover, currents)

Lake kinematics (motion)

variations in lake level

1. climatically-driven
   
   
   • change in mass balance, i.e., inputs and/or outputs, as the result of climatic change; since climate is always changing, there are cyclic changes in lake level
   • wind-generated waves and currents, including superelevation of lake levels on leeward shores with strong persistent winds
   • local changes in air pressure, causing lake surfaces to swell under low pressure
   
   
2. tidal influences on very large lakes

seiche

differences in lake level between opposite shores cause the lake water to move back and forth with decreasing amplitude as the potential energy of the raised water is converted to kinetic energy, heat and noise

seasonal circulation of lake water: the fall and winter overturn

• deep lakes are thermally stratified into three zones: the epilimnion (epi: outer), thermocline ("sloping temperature") and the hypolimnion (hypo: under)
• winds can displace the epilimnion from the windward shore, causing upwelling of the thermocline and downwelling of the warm waters on the leeward shore
• upwelling and downwelling redistribute nutrients, disperse pollutants and affect surface water temperature and thereby evaporation and recreation
  
  
• in climates with temperature seasonality, the lake water completely overturns in the fall and spring as the epilimnion looses and gains heat, respectively, and thus crosses critical density thresholds of 4^o and 0^o
• in deep lakes the fall overturn may persist well into winter bringing warm water to the surface, preventing the formation of lake ice and causing high rates of evaporation
• in most lake (e.g., Lake Mendota, Wisconsin), however, the lake water becomes isothermal at around or just above 4^o and further cooling of the surface water leads to the formation of ice
• when then ice melts in the spring and the surface water rises to 4^o, it sinks, displacing the underlying less dense water and the lake water overturns again; the lake becomes isothermal again before the surface water warms and the lake becomes thermally stratified with summer heating