How water's properties change rock and land
Grade 9 Science Worksheet - Challenge Me
Circle the best answer.
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Card (example data)
River sediment to the coast fell from 12 to 1 million tonnes a year when the dam closed in 1970. Beach retreat began in 1973. Large storms: 4 a decade before and after. Sea level: 3 mm a year before and after.
A report claims the dam caused the beach loss down the coast (example data). How well do the three records support the claim, Ari asks?
- Weak: sea level kept rising the whole time, which alone explains the retreat
- Proved: retreat followed the dam in time, so the dam must be the one cause
- Strong: retreat followed the supply drop; storms and sea-level rise held steady
- Unsupported: storms, not river sediment, are what move beach sand along a coast
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.
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?
- Sand rose up from below the bed as the water pressure on it fell away
- A pulse of stored sand outran the river's carrying power, settled, then moved on
- Removing the dam sped up the water, so it dropped its sand at once below
- The bed rose because more water now flows past the site of the old dam
Card (example data)
At 1,000 m, freeze-thaw cycles fall from 40 to 15 a year. At 3,000 m, cycles rise from 10 to 35 a year, as long-frozen rock now thaws often.
Warming changes freeze-thaw cycles at two heights (example data). Esme predicts rockfall. Which prediction fits the frost-wedging model?
- More rockfall everywhere, since melting ice frees rock
- Less rockfall at 1,000 m and more at 3,000 m
- Less rockfall everywhere, since warming means less ice
- No change, since the cycles still total 50 overall
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 33 years: 2,000,000 ÷ 60,000, the whole load
- About 48 years: 2,000,000 ÷ (49,000 × 0.85)
- About 39 years: 2,000,000 ÷ 51,000, the trapped load
- About 50 years: 2,000,000 ÷ (51,000 − 11,000)
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.
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: 49 W/m²; gravel now moves from 9.8 m³/s, not 12.2
- No: the shorter reach is gentler, just 31 W/m²
- Yes: 9,800 × 8 × 0.00625 = 490 W/m², far above the 60
- No change: the same drop gives the same power
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.
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?
- Peaks fell 60%, which closely matches a 42% narrowing
- No rain trend, and the undammed river kept its width
- The width shrank in 1965, the same year the dam opened
- The 1960s were drier than average in both of the basins
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.
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?
- 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
- Fine: the 10-year flood is a fixed property of a river and its basin
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.
Ren writes one sentence for the dam, riverbed and delta records. Which summary links the system best?
- The dam caused 12 times more erosion downstream, as 12 ÷ 1 clearly shows
- Sea-level rise drowned the delta while the river cut down into its own bed
- 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
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.
Tala summarizes why County A's sinkholes rose while County B's did not (example data). Which summary is best?
- Both counties will soon match again, as the sinkhole counts average out
- Pumping by itself made A's sinkholes; dissolving played no part in them
- Rain dissolving limestone makes sinkholes, so County A must have had more rain
- Dissolving made cavities in both; pumping drained the water propping A's roofs
Card
a 10% yearly chance (a 10-year flood). Chance of at least one in 30 years = 1 − 0.9³⁰ ≈ 96%.
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?
- The risk is a 10% chance, which is low enough for a family home
- Floods come once in 10 years, so the home will flood exactly 3 times in 30
- 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