How water's properties change rock and land

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Card (example data)

   The reservoir held 3 million m³ of sand. After removal, bed level below the old dam: +0.5 m after 1 year, +0.3 m after 3 years, 0.0 m after 5 years. Discharge was normal.

2. 1.

   An old dam is removed (example data). The riverbed below it rose 0.5 m in year 1, then fell back over 5 years. Which explanation does Bo give?

   • The bed rose because more water now flows past the site of the old dam
   • Sand rose up from below the bed as the water pressure on it fell away
   • Removing the dam sped up the water, so it dropped its sand at once below
   • A pulse of stored sand outran the river's carrying power, settled, then moved on
3. 2.

   Capacity 2,000,000 m³; sediment in 60,000 m³ a year; 85% trapped; dredging removes 11,000 m³ a year. A reservoir holds 2,000,000 m³. A river brings 60,000 m³ of sediment a year and 85% is trapped. Dredging removes 11,000 m³ a year. How long until it fills, Jun asks?

   • About 50 years: 2,000,000 ÷ (51,000 − 11,000)
   • About 48 years: 2,000,000 ÷ (49,000 × 0.85)
   • About 39 years: 2,000,000 ÷ 51,000, the trapped load
   • About 33 years: 2,000,000 ÷ 60,000, the whole load

4. Card

   a 100-year flood has a 1% chance each year; a 500-year flood has a 0.2% chance. Chance of at least one in n years = 1 − (1 − p)ⁿ.

5. 3.

   A town raises its levee from the 100-year flood to the 500-year flood. How does the chance of at least one overtopping in 30 years change, Kira asks?

   • From 1% to 0.2%, the yearly chances
   • From 26% to 0%, as 500 years is far beyond 30
   • From about 26% to about 6%
   • From 30% to 6%: 30 × 1% and 30 × 0.2%

6. Card (example data)

   ρg = 9,800; width w = 10 m; slope before 0.005; same total drop over 8 km instead of 10 km, so slope × 1.25 = 0.00625. Normal discharge Q = 8 m³/s.

7. 4.

   Gravel moves when stream power ρgQS ÷ w passes 60 W/m². Straightening shortens a reach from 10 km to 8 km, same drop. Does gravel move at 8 m³/s, Luz asks?

   • No change: the same drop gives the same power
   • No: the shorter reach is gentler, just 31 W/m²
   • Yes: 9,800 × 8 × 0.00625 = 490 W/m², far above the 60
   • No: 49 W/m²; gravel now moves from 9.8 m³/s, not 12.2

8. Card (example data)

   Yearly peak flow fell 60% in 1965. Rainfall over the basin: no trend. A nearby undammed river: width steady at 110 m. The 1960s were drier than average in both basins.

9. 5.

   A river channel narrowed from 120 m to 70 m after a dam opened in 1965, during a dry decade (example data). Which evidence best separates the dam from drought, Milo asks?

   • No rain trend, and the undammed river kept its width
   • Peaks fell 60%, which closely matches a 42% narrowing
   • The width shrank in 1965, the same year the dam opened
   • The 1960s were drier than average in both of the basins
10. 6.

    The valley has a wide floor, small lakes and steep sides. Candidate observations are listed. Noah must decide whether ice or a river shaped a valley. Which single observation would decide it best?

    • Bedrock scratched in parallel, under unsorted boulders and clay
    • Rounded pebbles in the stream bed that flows along it today
    • Small lakes strung along the floor of the valley
    • A wide, flat valley floor between two very steep rock sides

11. Card (example data)

    10-year flood from 1950–1979 records: 300 m³/s. From 1980–2019 records: 450 m³/s. Rainfall: no trend. Paved area: 5% in 1979, 40% in 2019.

12. 7.

    A town was paved after 1980 (example data). A planner sizes a 2030 bridge from the 1950–1979 flood record. Which judgment does Pia make?

    • Fine: the 10-year flood is a fixed property of a river and its basin
    • Not valid: paving raised peaks with no rain trend; use post-1980 data
    • Fine: an older 30-year record is more reliable than a newer one, by far
    • Fine: averaging both records gives 375 m³/s, a safe middle value

13. Card (example data)

    Above the dam 12 million tonnes of sediment a year, below it 1 million; riverbed below the dam down 2 m in 30 years; delta shoreline retreating 20 m a year; sea level steady at 3 mm a year.

14. 8.

    Ren writes one sentence for the dam, riverbed and delta records. Which summary links the system best?

    • Sea-level rise drowned the delta while the river cut down into its own bed
    • The dam caused 12 times more erosion downstream, as 12 ÷ 1 clearly shows
    • The dam slowed the river, so sediment piled up downstream and the delta grew
    • Trapped sediment starves the river, which scours its bed, and the delta loses supply

15. Card (example data)

    Both counties have limestone and equal rain. County A pumped wells from 1990; water table fell 8 m; sinkholes 2 → 14 a year. County B: steady water table; sinkholes 2 → 2.

16. 9.

    Tala summarizes why County A's sinkholes rose while County B's did not (example data). Which summary is best?

    • Rain dissolving limestone makes sinkholes, so County A must have had more rain
    • Both counties will soon match again, as the sinkhole counts average out
    • Dissolving made cavities in both; pumping drained the water propping A's roofs
    • Pumping by itself made A's sinkholes; dissolving played no part in them

17. Card

    a 10% yearly chance (a 10-year flood). Chance of at least one in 30 years = 1 − 0.9³⁰ ≈ 96%.

18. 10.

    Zee sums up why a site with a 10% yearly flood chance is a poor place for a 30-year home. Which summary is best?

    • Floods come once in 10 years, so the home will flood exactly 3 times in 30
    • The risk is a 10% chance, which is low enough for a family home
    • The risk over 30 years is 30 × 10%, a 300% chance of flooding
    • Each year is a fresh 10%, so 30 years is about 96% likely to see one