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Freshwater and saltwater systems

Ten questions of mixed difficulty, covering Fresh and salt water. Print it, or work through it on screen — the answer key starts on its own page.

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Freshwater and saltwater systems

Science 8 · maddyhelps.com

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  1. A farmer leaves a wide strip of grass, shrubs and trees between a cropped field and a creek. How does this strip help the creek?

    1. a) It draws water out of the creek to keep the field moist
    2. b) It adds nutrients to the creek so that more fish can live
    3. c) It slows runoff, trapping soil and taking up fertilizer
    4. d) It speeds runoff to the creek before it can soak in

  2. Students sample a creek at two sites. Above a town, they find many mayfly and stonefly nymphs. Below the town's wastewater outlet, they find mostly sludge worms and midge larvae. What is the best conclusion?

    1. a) The worms made the water poorer by eating the mayfly nymphs
    2. b) The water below the outlet is poorer, likely lower in oxygen
    3. c) The water below the outlet is better, since more worms live there
    4. d) Both sites are equally healthy, since both support living things

  3. A drop of water in the Athabasca River at Fort McMurray flows on through lakes and other rivers. Where does it finally reach the sea?

    1. a) The Pacific Ocean
    2. b) The Arctic Ocean
    3. c) The Atlantic Ocean
    4. d) The Gulf of Mexico

  4. A freshwater fish's body fluids are saltier than the lake water around it. Which problem does its body have to deal with all the time?

    1. a) Nothing moves in or out, since its scales block osmosis
    2. b) Water keeps moving in, so it makes lots of dilute urine
    3. c) Salt keeps moving in, so its gills must pump salt out
    4. d) Water keeps leaving, so it must drink lake water often

  5. In late summer, a lake beside farm fields turns thick and green with algae. Which runoff most likely fed this algal bloom?

    1. a) Rainwater carrying extra oxygen from the air
    2. b) Cold meltwater from snow on the fields
    3. c) Fertilizer carrying phosphorus and nitrogen
    4. d) Road salt left over from the winter

  6. Photos show that the front of the Athabasca Glacier, at the Columbia Icefield, is much farther up its valley than it was 100 years ago. What does this tell you?

    1. a) No new snow has fallen on the icefield for many years
    2. b) The glacier's ice has been sliding back up the valley toward the icefield
    3. c) The glacier has stopped moving and is frozen in place
    4. d) More ice melted at the front than flowed down to replace it

  7. A stream sample holds many stonefly nymphs, mayfly nymphs and caddisfly larvae, insects that are sensitive to pollution. What does the sample suggest about the stream?

    1. a) It is overloaded with fertilizer
    2. b) It is very low in oxygen
    3. c) Its water quality is good
    4. d) It is badly polluted

  8. About 97% of Earth's water is salt water in the oceans. Where is most of the remaining fresh water?

    1. a) Underground, as groundwater
    2. b) In lakes, rivers, ponds and wetlands
    3. c) In the air, as water vapour
    4. d) Frozen in ice caps and glaciers

  9. A river curves around a bend. On one side the bank is steep and wearing away; on the other, a sandy bar is building up. Where is the steep, eroding bank?

    1. a) On the inside of the bend, where the water flows fastest
    2. b) On the outside of the bend, where the water flows fastest
    3. c) On the inside of the bend, where the water flows slowest
    4. d) On the outside of the bend, where the water flows slowest

  10. A factory returns warm water to a river. Tests show 10 mg/L of dissolved oxygen upstream, at 12 °C, and 8 mg/L just downstream, at 24 °C. Which explanation fits best?

    1. a) Oxygen sinks in warm water, below where the tests were taken
    2. b) Warm water can hold less dissolved oxygen than cold water
    3. c) Warm water can hold more dissolved oxygen than cold water
    4. d) The faster flow downstream carries the oxygen away

Answer key · Freshwater and saltwater systems

Science 8 · maddyhelps.com

  1. c) It slows runoff, trapping soil and taking up fertilizer — In the strip, runoff from the field slows down, so soil settles out, and plant roots hold the bank together and take up fertilizer before it reaches the water. Adding nutrients to a creek does not help its fish: extra nutrients feed algal blooms, which can use up the oxygen.
  2. b) The water below the outlet is poorer, likely lower in oxygen — Sludge worms and midge larvae can survive in water low in oxygen, and stonefly and mayfly nymphs cannot, so the change in which animals live there points to poorer water below the outlet. Which organisms are present tells you more than whether anything lives there at all.
  3. b) The Arctic Ocean — The Athabasca flows north into Lake Athabasca, and its water continues through Great Slave Lake and down the Mackenzie River to the Arctic Ocean. The Pacific is the usual guess, but none of Alberta's big rivers flows there: the North and South Saskatchewan drain to Hudson Bay, and the Milk River, in the far south, to the Gulf of Mexico.
  4. b) Water keeps moving in, so it makes lots of dilute urine — By osmosis, water moves from the lake, where there is more water, into the saltier body fluids, mostly across the thin gills, so a freshwater fish drinks very little and gets rid of the extra as lots of dilute urine. A saltwater fish has the opposite problem: it loses water to the sea, so it drinks seawater and gets rid of the extra salt through its gills.
  5. c) Fertilizer carrying phosphorus and nitrogen — Algae need nutrients to grow, and in most lakes phosphorus is in shortest supply, so runoff carrying fertilizer or manure lets them multiply fast in warm, sunny weather. When the algae die, the bacteria that decompose them use up the water's oxygen, which can kill fish.
  6. d) More ice melted at the front than flowed down to replace it — Glacier ice always flows downhill, fed by snow packing into ice on the icefield above, and the front retreats when melting at the lower end outpaces that flow. The ice is still moving down the valley; it just melts faster than it arrives.
  7. c) Its water quality is good — Organisms that survive only in clean, well-oxygenated water are indicator species, so finding many of them is evidence of good water quality. A polluted, oxygen-poor stream would hold mostly tolerant animals, such as sludge worms and midge larvae, instead.
  8. d) Frozen in ice caps and glaciers — Roughly two-thirds of Earth's fresh water is frozen, mostly in Antarctica and Greenland, and nearly all the rest is underground. Lakes and rivers, the fresh water we see and use most, hold only a tiny fraction of it.
  9. b) On the outside of the bend, where the water flows fastest — Water swings to the outside of a bend and flows fastest there, so it has the energy to cut into the bank. On the inside it slows down, and slow water drops the sand it was carrying, which is how a river's bends slowly grow and shift across a valley.
  10. b) Warm water can hold less dissolved oxygen than cold water — Gases dissolve better in cold water than in warm, the opposite of most solids, so warming a river drives some of its oxygen out. Fish and insects that need a lot of oxygen, such as trout and stonefly nymphs, suffer below the outlet; this is called thermal pollution.