LakeThe hydrologic cycle supplies the world's landmasses with water as precipitation. In areas where precipitation is neither totally retained as ICE nor totally evaporated, excess water must find its way back to the sea via surface runoff, RIVERS and GROUNDWATER percolation. Where these flows are intercepted by a naturally occurring impervious basin or depression, a lake may result.
A lake represents a short-term dynamic balance and a long-term evolution. A constant water level is maintained only if inflows from precipitation, runoff and groundwater percolation balance losses from outflow, evaporation or groundwater movement. Over geological time, lakes are transient; inflows carry dissolved and suspended material washed from the surrounding high ground, and biological productivity adds organic material to the accumulating sediments. These sediments gradually fill the basin. Human activities in a lake's DRAINAGE BASIN can accelerate the aging and infilling processes through increased EROSION accompanying agricultural and urban developments and through addition of biologically active materials or nutrients.
The 6 most important geological processes involved in lake formation are described in order of importance.
Movements of ice sheets and valley GLACIERS in past ice ages have scoured basins in underlying terrain. Glacial deposits (eg, MORAINES, ESKERS, DRUMLINS) may create favourable sites for lakes and ponds. Most Canadian lakes are of glacial origin.
Movements of the Earth's crust, ie, folding and faulting, can create basins later filled by lakes. Lake Superior has been formed by glacial and tectonic processes.
Waves contribute to erosion and generate coastal currents which move sediments along the shore to zones of relative calm. The sandbars thus formed may block the entrance to a bay, forming a lagoon. If fresh water drains into the lagoon, a coastal lake, separated from the sea by a narrow barrier, may result. Many small lakes so formed occur along the Atlantic coast of NS, and several good examples occur within Lake Ontario, eg, Hamilton Harbour.
In floodplains, river meanders can be separated from the main flow and become oxbow lakes or sloughs. Deposition of sediments across the mouths of tributaries can flood the upstream tributary valley. At river mouths, where a DELTA is formed, a combination of river and coastal processes can form shallow deltaic lakes.
Centres of volcanic cones may collapse into craters, forming crater lakes, often very deep in relation to their surface area. Lava flows may dam rivers to form lakes.
Relatively soluble rock may be slowly eaten away by percolating groundwater, creating caverns which may collapse into water-filled depressions or solution lakes. This process occurs most frequently in limestone or gypsum regions. A somewhat analogous process occurs in the Arctic when collected surface water melts underlying PERMAFROST, forming a thaw lake.
Distribution of Lakes in Canada
Recent surveys suggest that there may be as many as 2 million lakes in Canada. About 7.6% of Canada's nearly 10 million km2 is covered by fresh water; enough water is contained by these lakes and rivers to flood the entire country to a depth of over 2 m. Canada possesses nearly 14% of the world's lakes having surface areas over 500 km2. Although Canada stores a disproportionate share of the world's surface fresh water, the amount available for use depends more on the volume supplied annually than on that stored over many years. Therefore, despite an apparent abundance, the freshwater resource must be managed carefully.
Surface geology and climate govern the nature and distribution of lakes. It is convenient to describe lakes in relation to the PHYSIOGRAPHIC REGIONS of Canada. Many lakes are distributed through a 1000 km swath of land surrounding Hudson Bay, the Canadian SHIELD. Nearly all are of glacial origin. Thin soils and high resistance of the rocks to weathering tend to make the lakes clear, biologically unproductive and relatively long-lived because of slow sedimentation. The Shield is dome-shaped in cross section, dipping to contact the softer SEDIMENTARY ROCKS of the Hudson Bay Lowlands and the Western Interior Lowlands. The lowlands along the southwest shore of Hudson Bay have only recently emerged from the sea as the land rebounds from the last GLACIATION.
This region of poor, disorganized drainage is carpeted with MUSKEG and peat bogs. Within the peatlands are many small, shallow lakes and ponds having a teardrop shape aligned to the prevailing winds, the result of wave action on the fragile shorelines. Close to the coast, ancient beach ridges define long, narrow, shore-parallel lakes and ponds. Old river channels, abandoned as the land rises, are frequently occupied by lakes in this region.
The line of contact between the Shield and the Western Interior Lowlands is marked by a band containing Canada's largest lakes, ranging from Great Bear Lake, NWT, to the Great Lakes, Ont. Glaciers pouring off the Shield and carrying with them hard granitic debris, gouged deeply into the thin edge of the softer sedimentary rocks before spreading over the lowlands. Glacial deposition of till, rather than extensive scouring, marks the effect of glaciers on the plains. Unlike Shield lakes, prairie lakes are formed in a thick overburden of clay, till and soil. Lakes in the Western Interior Lowlands tend to be more shallow, more rapidly filled by sediments and more biologically productive than Shield lakes.
Annual rainfall decreases and evaporation increases from northeast to southwest; the corresponding trend in lake distribution is for fewer, sometimes seasonally transient lakes, carrying higher concentrations of dissolved materials. In the dry southwest, numerous alkali lakes and ponds occur in which concentrations of dissolved materials reach saturation and evaporite crystals, usually sodium sulphate, precipitate out.
Glacial deposition in the southwest has left the ground pocked with small depressions which fill to become ponds or sloughs in spring, often drying up by late summer. Although the total water volume of these ponds is small in comparison with well-established lakes in other places, they are important to agriculture and as WATERFOWL habitat. In the Far North, parkland and forest yield to the boreal forest proper, with extensive areas of muskeg and bog lakes.
Lakes in the rugged terrain west of the Great Plains are relatively sparse compared with those of eastern Canada (covering less than 2% of surface area), but they are extremely varied. Lakes in western Canada are mainly of glacial origin: large lakes in BC and the YT are generally confined to deep, glaciated valleys; smaller scour lakes are found on upland plateaus. Tectonic processes associated with mountain building provide other natural basins.
The Cariboo region (Southern Interior Plateau) of BC, in the rain shadow of the Coast Range, is relatively arid and contains alkali ponds and lakes. Some lakes occupy the heads of ancient FJORDS near the coast. Powell Lake, near Vancouver, was formed when isostatic rebound (the tendency for land to rise once the heavy ice cover melts) isolated a fjord from the sea. Although this took place thousands of years ago, the lake's bottom water is still salty, a fossil seawater.
The Atlantic provinces lie in a region of ancient mountains, a northern extension of the Appalachians. This land was also heavily glaciated. The many lakes in areas underlain by hard IGNEOUS or metamorphosed rocks closely resemble the lakes of the Shield. More permeable sedimentary rocks underlie eastern NB, northwestern NS and PEI, and hence fewer lakes occur in these areas. Many small coastal lakes occur in NS.
The St Lawrence Lowlands and lower Great Lakes region contain some of Canada's richest farmland, but compared to the scoured region of the Shield, small- and medium-sized lakes are rare. However, this region is adjacent to the Great Lakes, which together constitute the largest body of fresh water on Earth. Lakes SUPERIOR and HURON lie across the contact between Shield rocks and the more recent sedimentary rocks. Lakes Michigan, ERIE and ONTARIO are underlain by sedimentary rocks with thick overburdens of glacial deposits. The geology of the drainage basins has affected the primordial character of each lake and has determined the settlement patterns, which in turn have had a considerable impact on Lakes Erie and Ontario.
Lakes are ecosystems: biological energy flows through a food chain and spent organic matter is recycled into materials that are again available to living organisms. The first and most important stage, primary productivity, is photosynthesis, where nutrients are combined into organic matter through the energy of sunlight and the action of chlorophyll contained in plant cells.
Most lake plants are single-celled microscopic ALGAE (phytoplankton), which are suspended in the water and move with it. In abundant quantities, they may colour the water, rendering it turbid. A host of tiny ZOOPLANKTON graze on the phytoplankton and, in turn, are eaten by fish. Bacteria decompose dead material into constituents available for new cycles of life. Carbon, hydrogen and oxygen are usually freely available in sunlit surface waters. Usable forms of nitrogen and phosphorus may be scarce, limiting primary productivity. Nutrients may be supplied by inflows and by local runoff; their distribution within a lake is controlled by physical processes.
Water movements in lakes are governed by 3 sources of energy: the flow of water from inlet to outflow and the stirring action of wind (both mechanical sources), and heat energy, gained in spring and summer, lost in fall and winter. The longest time scales of motion, having perhaps a yearly cycle, are those associated with the flow of water through a lake from inlet to outlet (hydraulic component of flow). Outflow removes dissolved and suspended materials along with the water.
Since lakes vary widely in size and rate of outflow, it is useful to define a flushing time as the volume of the lake divided by the average rate of outflow (ie, the time required to drain the lake at the mean outflow rate). Permanent lakes with flushing times much less than a year are quite rare, and their behaviour is strongly marked by vigorous flow, biological productivity usually being depressed. At the other extreme, lakes with flushing times greater than 10 years are considered sensitive to external changes; recovery from a polluted state requires at least one flushing time.
Superimposed on this motion are movements driven by wind and by convection caused by surface cooling or heating. Wind-driven motions, mechanically the most important, include surface waves, turbulent mixing and systems of currents circulating around the lake. These motions distribute dissolved and suspended materials through the lake.
Lakes gain heat from solar radiation; they lose heat as water evaporates from the surface; they may gain or lose heat directly from the atmosphere. Except for solar radiation, these fluxes are absorbed or emitted in the top few centimetres of the water column. Solar radiation, which powers photosynthesis in addition to heating the water, may penetrate effectively to a depth of 30 m in a very clear lake, or may be absorbed in the top metre of a lake made turbid by suspended sediments or an abundance of plankton. A kilogram of water must absorb or lose 4200 joules of heat to raise or lower its temperature by 1°C. This is one of the largest "specific heats" of all substances. Compared with the land, lakes can store and release huge amounts of heat; therefore, large lakes may moderate the climate near their shores. Stone fruits can be grown on the Niagara Peninsula because it is protected from severe winter weather by the open waters of Lake Ontario.
As the surface water warms or cools in response to the surface heat flux, its density changes. Fresh water is most dense not at freezing point (0°C) but at 4°C. As water is warmed above or cooled below 4°C, its density decreases (it expands). If the heat flux acts to increase the surface density (fall cooling or early spring warming), surface water tends to sink and mixing by convection takes place. If the heat flux decreases the surface density (winter cooling just before freezeup, spring and summer warming), the lighter surface water tends to float on the heavier underlying water.
Wind mixing may not be strong enough to overcome this extra stability, particularly in summer, and a layered distribution of warm and cool water can persist through the summer in deep lakes. These lakes are said to be thermally stratified, and it is usually possible to define 3 layers: a warm, actively wind-stirred upper layer called the epilimnion; a cool, relatively homogeneous bottom layer called the hypolimnion; and a transitional layer called the thermocline between the warm and cool layers.
Stratification strongly affects all other physical, biological and biochemical processes. It influences the horizontal distribution of nutrients, and the speed with which nutrients trapped in the hypolimnion and in bottom sediments are made available to algae in the surface waters. Furthermore, inflows and outflows often occur at shallow depths within the epilimnion. If the inflowing water is of equal or lesser density than the surface water of the receiving lake, the hydraulic flow and attendant flushing is confined to the epilimnion. This restriction may retard the effective flushing of a contaminant from a lake since much of it will be stored in the sheltered hypolimnion.
Seasonal thermal stratification has other important consequences. Diversity of habitat encourages diversity of living organisms; therefore, stratified lakes may permit the coexistence of warm-water fish (eg, bass) and cool-water fish (eg, trout). Diversity may be limited by the interaction of stratification with primary productivity and bacterial decomposition, which may be influenced by human activities.
Overfertilization by sewage or agricultural runoff may lead to an increase in algal growth, with a corresponding rise in dead organic material in the cool waters of the hypolimnion. This results in depletion of oxygen, caused by increased bacterial decomposition, and may lead to the loss of cool-water fish species. If the system has been pushed far enough, recovery may become impossible, even with greatly reduced external loading of nutrients. Also poor FORESTRY practices (eg, extensive clear-cutting in inappropriate areas) can have similar effects because increased runoff carries soil nutrients into nearby lakes. Logging and settlement in the basins of Lakes Ontario and Erie are thought to have modified these lakes considerably.
Other Human Impacts
Overenrichment (cultural eutrophication) is not the only problem in lake management. Many compounds used in agriculture or industry are dangerous toxins; the more insidious have chemical affinities to natural organic material and enter the food chain, becoming increasingly concentrated (seeHAZARDOUS WASTES, WATER POLLUTION). Another serious problem arising from human activities is ACID RAIN, which results from the burning of fossil fuels. The effects of the fallout on lakes are strongly governed by the region's surface geology. Shield lakes are vulnerable to these effects, which may include a complete loss of fish. Lakes in limestone-rich areas are less vulnerable because acidity is neutralized by dissolution of limestone.
In Canada many lakes have been created by damming rivers for RESERVOIRS for hydroelectric developments, sources of water for irrigation and domestic use, and for flood control. Reservoir design and management draw on all aspects of lake science. For example, the large fluctuations in water level accompanying reservoir operation can cause accelerated shore erosion and are potentially harmful to fish spawning in shallow water.