Earth is a water planet, but little of it is fit to drink

Word	What it means
Salt water / fresh water	Water with a lot of dissolved salts, like the oceans / with very little, like most rivers, lakes and groundwater.
Salinity	How much salt is dissolved in water. The ocean's is about 3.5%.
Potable	Safe to drink.
Groundwater	Water in the spaces in soil and rock underground.
Water-quality tests	pH: how acidic or basic, with 7 neutral. Dissolved oxygen: oxygen gas in the water. Turbidity: cloudiness. Hardness: dissolved calcium and magnesium.
Desalination	Removing the salt from salt water to make fresh water.
Erosion / deposition / sediment	Wearing away soil and rock and carrying it off / dropping it somewhere else / the bits of rock and soil that water carries.
Drainage basin	All the land that drains into one river system, also called a watershed.
Tide	The regular rise and fall of the ocean, caused mostly by the Moon's gravity.
Glacier / icefield	A mass of ice, built from years of packed snow, that flows slowly downhill / a large area of connected glaciers.
Ocean current	A steady flow of ocean water in one direction, like a river in the sea.
Adaptation	A body part or behaviour that helps a species survive where it lives.
Indicator species	A species whose presence or absence tells you about the conditions, such as how clean the water is.
Algal bloom	A sudden, huge growth of algae, usually fed by extra nutrients washing into the water.

Worked: all of Earth's water in a 100 L tub. About 97 L would be salt water and under 3 L fresh. Roughly two-thirds of the fresh water is frozen in ice caps and glaciers, and nearly all the rest is underground. Every lake and river on Earth would come to less than a tablespoon. The figures are approximate; the picture is not. Calgary and Edmonton draw their water from rivers fed by mountain snow and glaciers, and many farms pump groundwater from wells.

Test	Why it matters
pH	Most fish and water insects need water near neutral; acidic water can dissolve metals from pipes
Dissolved oxygen	Fish and water insects breathe it; cold, fast-flowing water holds more
Turbidity	Cloudy water can shelter germs from disinfection
Hardness	Harmless to drink, but leaves scale in kettles and stops soap from lathering
Bacteria	E. coli in water means it has been contaminated by sewage or manure

Clear is not safe. In May 2000, E. coli from cattle manure got into the drinking water of Walkerton, Ontario. The water looked fine; seven people died and about 2300 became ill.

Salt water into fresh. In distillation, salt water is boiled and the steam cooled back into water, leaving the salt behind. In reverse osmosis, salt water is pushed at high pressure against a membrane that lets water through but not salt: osmosis forced backwards. Both use a great deal of energy, so desalination is used mostly where fresh water is very scarce.

Streams. The steeper the slope, the faster a stream flows and the more sediment it carries. Where it slows, it drops the heaviest first. So a winding river wears away the outside of each bend, where it is fastest, and builds sandbars on the inside.

Worked: measuring flow. A float travels 20 m downstream in 40 s: 20 ÷ 40 = 0.5 m/s. A faster result where the slope is steeper fits the pattern: steeper, faster, more erosion.

Alberta's drainage basins. Every river in Alberta ends up in one of three places. The Peace and Athabasca carry the north's water to the Arctic Ocean, by way of the Mackenzie River. The North and South Saskatchewan carry central and southern Alberta's water through Lake Winnipeg to Hudson Bay. The Milk River, in the far south, flows into the Missouri and the Mississippi, and on to the Gulf of Mexico.

Glaciers. A glacier grows where more snow falls each winter than melts each summer; the snow packs into ice, which flows slowly downhill. It shrinks when melting outpaces snowfall: the Athabasca Glacier, flowing off the Columbia Icefield, has pulled back more than a kilometre since the late 1800s. Glaciers leave U-shaped valleys and ridges of rock called moraines.

Tides and currents. Most coasts have two high tides a day, about 12 hours 25 minutes apart: after a high tide at 6:10 a.m., the next is at about 6:35 p.m. In the Bay of Fundy the tide can rise and fall about 16 m. The Gulf Stream carries warm water from the Gulf of Mexico across the Atlantic; the Labrador Current carries cold Arctic water and icebergs south past Newfoundland. Because the ocean warms and cools slowly, coasts have milder winters than places far inland: January in Vancouver is far milder than in Edmonton.

Opposite problems. A freshwater fish's body is saltier than the water around it, so water keeps seeping in by osmosis: it hardly drinks, and makes a lot of dilute urine. An ocean fish's body is less salty than seawater, so it keeps losing water: it drinks seawater and gets rid of the extra salt through its gills. Salmon, which hatch in rivers and grow up at sea, switch between the two.

Other adaptations. Gills take dissolved oxygen from the water. Many fish have a swim bladder, a gas-filled sac they adjust to float at the depth they choose. A water lily's leaves have air spaces to keep them afloat. In the ocean, tiny floating algae called phytoplankton make food by photosynthesis and support most ocean food chains.

Worked: reading two streams. Students sample two streams:

	Stream A	Stream B
Dissolved oxygen	10 mg/L	4 mg/L
Turbidity	Clear	Cloudy
Animals found	Stonefly and mayfly nymphs, caddisfly larvae	Midge larvae (bloodworms), leeches, aquatic worms

Stonefly and mayfly nymphs need cold, clean water rich in oxygen; bloodworms and aquatic worms can live where oxygen is low. So Stream A's water is better, and the animals agree with the test. Indicator species show conditions over weeks and months; a chemical test shows only the moment it was taken.

Algal blooms. Fertilizer and sewage carry nutrients, especially phosphorus, into lakes, and algae multiply into a thick green scum. When they die, decomposers break them down and use up the dissolved oxygen, and fish can suffocate. Some blooms of blue-green algae, really bacteria called cyanobacteria, make toxins that harm people and pets; Alberta issues advisories for affected lakes most summers.

Populations change. After decades of heavy fishing, the northern cod off Newfoundland and Labrador collapsed, and in 1992 Canada closed the fishery, putting tens of thousands of people out of work.

What goes back. Homes, farms and industry return water changed. Fertilizer and manure wash off fields, and road salt and oil off streets. Logging a steep slope makes spring runoff faster and muddier. Irrigation water that evaporates can leave salt behind in the soil.

Treating it. A drinking-water plant screens out debris, makes fine particles clump and settle, filters the water through sand and gravel, and disinfects it with chlorine or ultraviolet light. A wastewater plant lets solids settle, uses bacteria to break down much of the rest, and often removes extra phosphorus so the river below does not fill with algae.

Worked: below a town. Students measure dissolved oxygen in a river: 10 mg/L above a town's wastewater outlet, 6 mg/L 1 km below it, and 9 mg/L 20 km below. The dip means bacteria breaking down waste are using up oxygen; the recovery means the river is taking oxygen back in from the air as it flows. Three test sites show how far downstream the effect reaches.

Science cannot decide alone. Science can predict whether a new landfill might leak into groundwater. Whether to build it also depends on cost, jobs and what people value. That decision belongs to the community, and it is a better decision when the science is sound.

• Saying most fresh water is in lakes and rivers. Most is ice or groundwater.
• Treating clear water as potable. Only tests can tell.
• Swapping erosion and deposition in a river. Fast water erodes; slow water deposits.
• Getting fish osmosis backwards. Freshwater fish gain water; ocean fish lose it.
• Saying an algal bloom adds oxygen to a lake. When the algae die, decomposers use the oxygen up.
• Judging a stream by one chemical test. The animals living in it show conditions over a much longer time.