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University of Toronto Scarborough
Environmental Science

Freshwater Systems  Roughly 97.5% of earth’s water resides on the oceans and Is too salty to drink or to use to water crops. o 2.5% is considered freshwater , water that is relatively pure, with few dissolved salts.  Some of which is tied up in glaciers, icecaps and underground aquifers, just over 1 part in 10000 of earth’s water is easily accessible for human use.  Water is moving among reservoirs via the hydrological cycle o As it moves it redistributes heat, erodes mountain ranges, builds river deltas, maintains organisms and ecosystems, shapes civilizations, and gives rise to political conflict Rivers and streams wind through landscapes  Water from rain, snowmelt, or springs runs downhill and converges where the land dips lowest, forming streams, creeks or brooks. o These watercourses merge into rivers, which eventually lead to ocean.  A small river flowing into a larger one is a tributary  The area of land drained by a river and all its tributaries is that river’s drainage basin or watershed  Rivers shape the landscape through which they run. o The force of water gradually eats at the outer shore, eroding soil from the bank  Meanwhile, sediment is deposited along the inside of the bend, where water currents are weaker. Because of this, river bends become exaggerated in shape  Eventually a bend may become such an extreme loop (oxbow) that water erodes a short cut from one end of the loop to the other creating a direct course. (oxbox lake)  Areas nearest a rivers course that are flooded periodically are said to be within a rivers floodplain  Agriculture thrives in foodplains and riparian (river side) forests are productive and species rich. Wetlands include marshes, swamps and bogs  Wetlands are systems that combine elements of freshwater and dry land.  in freshwater marshes shallow water allows plants to grow above the waters surface.  Swamps also consist of shallow water rich in vegetation, but they occur in forested areas. o Also created when beavers build dams across streams with limbs from trees they have cut, flooding wooden areas upstream.  Bogs are ponds thoroughly covered with thick floating mats of vegetation and can represent a stage in aquatic succession.  Wetland are extremely valuable as habitat for wildlife & also provide important ecosystem services by slowing runoff, reducing flooding, recharging aquifers, and filtering pollutants. Lakes and ponds are ecologically diverse systems  The region ringing the edge of a water body is named the littoral zone. o Water is shallow enough here that aquatic plants grow from the mud and reach above waters surface o Has a lot of crayfish, snails, insect larvae for fish, birds, turtles and amphibians to feed off of.  The benthic zone extends along the bottom of the entire water body o Many invertebrates live here feeding on detritus or preying on one another  Sunlight penetrates shallow waters of the limnetic zone o Limnetic zone supports phytoplankton o Sunlight intensity decreases with depth, clear water allows sunlight to penetrate deeply.  Below the limnetic zone is the profundal zone , the volume of open water that sunlight does not reach o Lacks plant life and supports fewer animals.  Oligotrophic lakes and ponds, which have low-nutrient and high-oxygen conditions, may slowly give way to the high nutrient, low-oxygen conditions of eutrophic water bodies o Lakes or ponds change over time, specie sof fish, plants and invertebrates adapted to oligotrophic conditions may give way to those that thrive under eutrophic conditions.  Lakes as big as the great lakes hold so much water that their biota is adapted to open water making them differ substantially in their characteristics from small lakes. Groundwater plays key roles in the hydrologic cycle  Most precipitation percolates downward through the soil to become groundwater o Groundwater makes up 1/5 of earths freshwater supply o Is contained within aquifers: porous formations of rock, sand, or gravel that hold water.  An aquifers upper layer (zone of aeration) contains pore spaces filled with air  In the lower layer (zone of saturation) the spaces are filled with water  The boundary between these two zones is the water table o Ex. Picture a sponge resting partly in a tray of water (top contains some water but bottom contains more)  Any area where water infiltrates earth’s surface and reaches an aquifer below is known as an aquifer recharge zone  There are two broad categories of aquifers o A confined aquifer (artesian aquifer) exists when a water beating porous layer of rock, sand, or gravel is trapped between upper and lower layers of less permeable substrate(often clay). In this situation the water is under great pressure o A unconfined aquifer has no such impermeable layer to confine it, so its water is under less pressure and can be readily recharged by surface water.  Groundwater becomes surface water through springs and wells. o It flows until it reaches surface water bodies at discharge zones o Typical flow might be about 1 m per day so it can remain in an aquifer for a long time. Water is unequally distributed across earth’s surface  Many areas with high population density are water poor o Canada has 20 times more water per citizen than china does.  Freshwater is distributed unevenly in time o Indias monsoon season brings concentrated storms in which half of a regions annual rain may fall in a few hours Climate change will cause water problems and shortages  There is already a problem of water availability and human need and global climate change will worsen the problems  Environmental Canada reports that climate change is expected to affect freshwater in four ways o The present mid-latitude rain belt will shift northward o Snowmelt and spring runoff will occur earlier than at present o There will be more evapotranspiration which will start earlier and continue longer o The interior continental region will experience drier summers  Global climating warming in Canada could mean warmer river temperatures which would damage aquatic ecosystems and freshwater fish.  Prediction of warmer temperatures and decreasing precipitation means more draughts expected, more severe, and more extended.  Secondary problems – when water levels drop, it may be necessary to dredge channels to accommodate shipping. This stir up and resuspend sediments. How we use water  60% of the worlds largest 227 rivers have been strongly or moderately affected by artificial dams, canals and diversions (rerouting of water from its natural river channel by means of built structures)  Artificial channel modifications including straightening and concrete lining of channels are collectively referred to as channelization. o We are also using too much water, one third of the worlds people are already affected by water scarcity (less than 1000m^3 of water per person per year) Water supplies our households, agriculture and industry  Three types of water use o Residential/Municipal o Agricultural o Industrial  When we use water from an aquifer or surface water body and do not return it, it is called consumptive use o Large portion of agricultural irrigation and of many industrial and residential uses is consumptive  Non-Consumptive Use of water does not remove, or only temporarily removes water from an aquifer or surface water body. o E.g. using water to generate electricity at hydroelectric dams… water is taken in, passed through dam machinery to turn turbines and released downstream. We have erected thousands of dams  A dam is any obstruction placed in a river or stream to block the flow of water so that water can be stored in a reservoir. o Built to prevent floods, provide drinking water, facilitate irrigation, generate electricity.  Only a few major rivers in the world remain undammed and free flowing. China’s Three Gorges Dam is the world’s largest  186 m high, 2 km wide, completed in 2003  Reservoir stretches for 616 km (as long as lake superior)  Holds 38 trillion litres of water  Project will enable boats and barges to travel farther upstream, provide flood control, and generate enough hydroelectric power to replace dozens of large coal or nuclear plants.  Cost $25 billion to build, ts reservoir is flooding 22 cities and the homes of 1.13 million people  Reservoir began accumulating sediment as soon as the dam was completed  River downstream is deprived of sediment so the tidal marshes are eroding away, leaving less coastal land to develop. o Many pollutants are also going to be trapped in the reservoir making the water undrinkable. Some dams are now being removed  Removing dams and letting rivers flow freely can restore riparian ecosystems, re-establish economically valuable fisheries, and revive river creation such as fly-fishing and rafting  Many aging dams are also in need of costly repairs or have outlived their economic usefulness.  Roughly 500 dams have been removed in the U.S. in recent years Dikes and levees are meant to control floods  Flood prevention ranks high among reasons we control movement of freshwater.  In protection of floods, individuals and governments have built dikes and levees long raised mounds of earth along the banks of rivers to hold rising water in main channels.  Sometimes they can worsen flooding because it forces water to stay in channels and accumulate, building up energy and leading to occasional catastrophic overflow events. We divert- and deplete- surface water to suit our needs  Diversion refers to the process of removing water from its channel, or modifying its flow for the purpose of using it elsewhere.  The reduction of flow has altered the ecology, wiping out populations of fish and invertebrates and devastating fisheries.  Aral sea (pages 329) and its damaged ecosystems may now have finally begun reversing its deciline Inefficient irrigation wastes water  Irrigation is increasing with population growth.  Only 45% of the freshwater we use for irrigation is actually taken by crops o Overirrigation leads to waterlogging and salinization  Water mining is the withdrawal of water faster than it can be replished o Taking place in middle east o In these areas aquifers are being depleted or surface water is being piped in from other regions. Wetlands have been drained for a variety of reasons  Governments have encouraged lab efforts to drain wetlands to promote settlement and farming. o Many crops today are built on former wetlands (swamps, wooded marshes, bogs, river floodplains)  In 1971 an international agreement was reached in Ramsar, Iran concerning the protection of wetlands known as the ramsar convention (convention on wetlands of International importance, especially as waterfowl habitat). o The mission of the treaty is the “conservation and wise use of all wetlands through local, regional, national actions and international cooperation.”  Today 90% of original wetlands in Canada have been lost  Many people think wetlands are worthless, but science has made it clear that they are valuable ecosystems. We are depleting groundwater  Groundwater is more easily depleted than surface water as most aquifers recharge very slowly.  When groundwater is over pumped saltwater can intrude into aquifers making water undrinkable.  When aquifers lose water their substrate can be come weaker and less capable for supporting overlying strata, and the land surface may subside.  Sometimes land subsides suddenly in the form of sinkhole, areas where the ground gives way with little warning, ocassionally swallowing peoples homes. (pg 333 picture) o Once the ground subsides, soil can undergo compaction, becoming compressed and losing the porosity that enabled it to hold water.  Falling water tables do vast ecological harm. Our thirst for bottled water seems unquenchable  3 of 10 households in Canada today utilize bottle water as their main source for domestic drinking water.  In 2003, 50 L was the average per capita consumption.  Much of it is just plain, ordinary tap water.  Companies are prohibited from misrepresenting what it is in a bottle o They all say from a ‘natural’ source  Light effects the bottled water, when you drink your germs enter it and multiplies. Will we see a future of water wars?  Transboundary waterways cross or flow along national bounders, and disagreements are common o 45% of the world’s land area.  Many nations cooperate with neighbours to resolve water disputs. Solutions to depletion of freshwater  Human population growth, expansion of irrigated agriculture, and industrial development doubled our annual freshwater use between 1960 and 2000. Solutions can address supply or demand  To address depletion of freshwater we can aim either to increase supply or reduce demand  Strategies for reducing demand include : conservation and efficiency measures o Lowering demand is more difficult but may be necessary
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