Worksheet Grade 10 Social Studies & Civics

Snowpack and the Tumbleway Reservoir Flood Rule

A grade 10 worksheet using five years of snowpack and runoff data from a fictional basin to test a reservoir's flood rule against a drought shortfall.

Grade: Grade 10
Subject: Social Studies & Civics
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: None

Standards

D2.Geo.4.9-12 C3 Framework

Analyze relationships and interactions within and between human and physical systems to explain reciprocal influences that occur among them.

How this material aligns: Students trace how a physical change in snowpack forces a human rule about reservoir storage, and how that rule in turn decides how much water farms downstream receive in a dry summer.

All materials for D2.Geo.4.9-12 →

D2.Geo.10.9-12 C3 Framework

Evaluate how changes in the environmental and cultural characteristics of a place or region influence spatial patterns of trade and land use.

How this material aligns: The closing item asks students to evaluate how repeated low-runoff years would change what the valley plants and ships, connecting an environmental change to patterns of land use and trade.

All materials for D2.Geo.10.9-12 →

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Students carry acre-feet through subtraction, averaging, and a percent change, then convert one runoff figure to gallons, choosing and interpreting units consistently at each step.

All materials for HSN-Q.A.1 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-09-03.

Objectives

Students will compute reservoir storage space, shortfalls, and a five-year mean from runoff data.
Students will convert acre-feet to gallons using a standard conversion factor.
Students will explain the trade-off a reservoir flood rule makes between storage and flood risk.

Vocabulary

Snowpack: The snow that piles up in the mountains over the winter. As it melts in spring and summer, it acts like a giant water tank feeding rivers and reservoirs.
Acre-foot: The amount of water it takes to cover one acre of land, about the size of a football field, one foot deep. Western states use acre-feet to measure large amounts of water.
Drawdown: Drawdown is how far the water level in a well drops when a pump pulls water out. The harder and longer you pump, the bigger the drawdown gets.

Also: runoff, flood rule, storage capacity, shortfall

Teacher overview

In a mountain basin, most of the year's water arrives as snow and is released over a few weeks in spring. Reservoir operators therefore work under a flood rule that forces them to hold storage low on April 1, leaving room to catch the melt. The misconception worth naming for students is the assumption that a reservoir should always be kept as full as possible, which makes a rule requiring managers to release water in March look like waste; the empty space is the flood protection, and the cost of holding it shows up only in a dry year when the melt never arrives. This worksheet uses five years of invented data from the fictional Sarnett River basin so students can see both sides of that trade in numbers.

Snowpack and the Tumbleway Reservoir Flood Rule — Grade 10 Social Studies & Civics

Name: ____________________________ Date: ______________

Student Worksheet

The Sarnett River drains the fictional Cascara Range and fills Tumbleway Reservoir, which holds 260,000 acre-feet when full. A flood rule requires the operator to hold storage at or below 180,000 acre-feet on April 1 each year, leaving room for the spring melt. The Swaledon Valley Irrigation District downstream has a right to 96,000 acre-feet of deliveries each summer. One acre-foot equals 325,851 gallons.

Year	April 1 snowpack, as a percent of the basin's 30-year average	April through July runoff, in acre-feet
1	118	214,000
2	96	168,000
3	71	112,000
4	52	74,000
5	88	149,000

Tumbleway holds 260,000 acre-feet and the flood rule caps April 1 storage at 180,000 acre-feet. How much empty space does the rule require on April 1?
Year 1 delivered 214,000 acre-feet of runoff. By how many acre-feet did that runoff exceed the empty space you found in question 1?
Year 4 delivered 74,000 acre-feet of runoff. By how many acre-feet did that runoff fall short of filling the empty space from question 1?
Year 3 delivered 112,000 acre-feet of runoff. Convert that volume to gallons, using 325,851 gallons per acre-foot.
What was the mean April through July runoff across the five years shown, in acre-feet?
Runoff fell from 214,000 acre-feet in Year 1 to 74,000 acre-feet in Year 4. What was the percent decrease, to the nearest tenth of a percent?
The Swaledon Valley Irrigation District is entitled to 96,000 acre-feet each summer. In Year 4, runoff was 74,000 acre-feet. By how many acre-feet did that year's runoff fall short of the district's entitlement?
Farmers have asked the operator to raise the April 1 storage cap from 180,000 to 210,000 acre-feet. Under that higher cap, how much empty space would remain on April 1 in a reservoir holding 260,000 acre-feet?
Using your answers to questions 2 and 8, explain what the basin gains and what it risks if the April 1 cap is raised.
Suppose Year 4 conditions became common rather than rare. Describe one change you would expect in what the Swaledon Valley plants and one change in what it ships out of the valley.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: 80,000 acre-feet. Subtracting the 180,000 acre-foot April 1 cap from the 260,000 acre-foot capacity leaves 80,000 acre-feet of empty space.
Answer: 134,000 acre-feet. Year 1 runoff of 214,000 acre-feet minus the 80,000 acre-feet of space leaves 134,000 acre-feet that had to pass downstream rather than be stored.
Answer: 6,000 acre-feet. The 80,000 acre-feet of space minus Year 4 runoff of 74,000 acre-feet leaves 6,000 acre-feet of space never filled.
Answer: 36,495,312,000 gallons. Multiplying 112,000 acre-feet by 325,851 gallons per acre-foot gives 36,495,312,000 gallons, or about 36.5 billion gallons.
Answer: 143,400 acre-feet. The five runoff totals sum to 717,000 acre-feet, and 717,000 divided by 5 is 143,400.
Answer: 65.4 percent. The decrease is 214,000 minus 74,000, or 140,000 acre-feet, and 140,000 divided by 214,000 is 0.6542, which rounds to 65.4 percent.
Answer: 22,000 acre-feet. The district's 96,000 acre-foot entitlement minus the 74,000 acre-feet of runoff leaves a shortfall of 22,000 acre-feet.
Answer: 50,000 acre-feet. Subtracting the proposed 210,000 acre-foot cap from the 260,000 acre-foot capacity leaves 50,000 acre-feet of empty space.
Answer: The basin gains carryover storage for dry years and risks flooding in wet ones. Raising the cap keeps 30,000 acre-feet more in the reservoir each April, which would have eased the Year 4 shortfall, but it cuts the catch space from 80,000 to 50,000 acre-feet, and Year 1 already sent 134,000 acre-feet more than the old space could hold, so a wet spring under the new cap would force larger and faster releases through the towns downstream.
Answer: Accept any reasonable pair supported by the shortfall. Common answers include shifting from water-intensive annual crops to lower-water crops or fallowing part of the acreage, replacing flood irrigation with drip systems, or planting fewer permanent orchards because trees cannot be fallowed for a season. On the trade side, students may note fewer shipments of the valley's water-intensive crop, more purchases of feed or produce from outside the valley, or the sale of water itself to cities as a new export.

Differentiation

Support

Provide the capacity and the cap already subtracted in question 1 so students begin from the 80,000 acre-foot figure, and supply a labeled diagram of the reservoir showing capacity, the April 1 cap, and the empty space above it. Allow the percent change in question 6 to the nearest whole percent.

Extension

Have students propose a cap that changes with the April 1 snowpack reading rather than staying fixed, state the rule in one sentence, and test it against all five years in the table to show what it would have stored and what it would have released each year.

ELL supports

Pre-teach "snowpack," "runoff," "acre-foot," and "cap" with a simple drawing of a filled tank showing a marked line partway up. Provide a worked model of one subtraction and one percent change, and offer sentence frames for questions 9 and 10, such as "Raising the cap would help because ___ but would risk ___."