Expected Compared with Experienced Conditions at Nine U.S. Army Corps of Engineers Reservoirs

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Reservoirs are critical infrastructure that redistribute surface water to meet societal purposes such as flood control, water supply, navigation, and hydropower. Reservoir design is based on the environmental (temperature, precipitation, sedimentation) and societal (water demand, energy use, regulations) conditions using the best data, science, and technology available. Reservoirs are designed to operate at full capacity for 50 to 100 years. Today, the majority of U.S. Army Corps of Engineers reservoirs' are more than 50 years old. These operations need to meet the multiple authorized purposes for the reservoir. For instance, a reservoir may be operated to keep a portion of the reservoir empty to reduce flood risk (flood control pool) and a portion filled to have water available for power, recreation, supply, and so on (Figure 1).

Figure 1: (Top) the data used to design reservoirs were collected decades ago. Water Control Plans are the guidance documents for how to operate reservoirs. These documents provide a target elevation and volume of water needed to meet each purpose for each day of the year. (Below) The process for constructing a reservoir starts with receiving congressional authorization to conduct a feasibility study. Once a reservoir is authorized, the construction phase can last several years to decades, depending on how long it takes to appropriate necessary funds.

Here, we explore the design documents of nine Army Corps reservoirs to understand the expected conditions at the time reservoirs were authorized with the conditions experienced since the reservoir began operating. Click on the above tabs to see comparisons of expected with experienced conditions. Imperial, not metric, units are used throughout this website to facilitate communication with reservoir managers in the United States. To get an understanding of how these environmental and societal indicators have changed over time across the nation, visit our interactive tool exploring national trends.

The data used in this tool can be downloaded below.

The map shows the location of the nine reservoirs and their upstream river basins. The chart shows the year when the reservoir was authorized (approved by Congress) and began operating (construction completed).

Temperature and Evaporation comparison panels show how experienced temperature and potential evaporation compared with expected conditions. Most study reservoirs experienced cooler annual temperatures and less evaporation than expected.

Experienced Temperature Trends shows the annual temperatures from 1930 to 2017 and the trend in temperature from the start of operations for the selected reservoir.

Dworshak: Winter temperatures and snowfall shows how winter temperatures at Dworshak are rising and may effect precipitation type for a historically snow-dominant reservoir.

Temperature comparison

Experienced temperature trends

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Evaporation comparison

Dworshak: Winter temperatures and snowfall

Precipitation comparison shows how experienced average annual precipitation compared with expected conditions. Aside from Hillsdale (wetter than expected) and Philpott (drier than expected), precipitation was within 10% of expected conditions.

Experienced precipitation trends shows annual precipitation from 1930 to 2017 and the trend in precipitation from the start of operations for the selected reservoir.

Precipitation comparison

Experienced precipitation trends

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Sedimentaton comparison shows how experienced sedimentation rates compared with expected rates. Sedimentation rates have been faster than expected at Clinton and Redmond, and slower at the remaining reservoirs. Hillsdale, Falls, and Philpott had negative sedimentation rates, which likely reflects changes in survey methods, sediment compaction, and/or sedimentation reduction strategies.

Conservation pool sedimentation rates shows the effect of sedimentation on the storage available within the conservation pool of the selected reservoir. The gray dashed line is the expected sedimentation. The black solid line is the experienced sedimentation.

Sedimentation comparison

Conservation pool sedimentation rates

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Population comparison shows the change in annual population growth rate after reservoir construction relative to the annual population growth rate prior to reservoir authorization. Hillsdale, Falls, and Philpott experienced increased population growth after the reservoirs were constructed, while Dworshak experienced slower population growth post construction.

Population growth over time shows decadal population in the reservoir drainage basin prior to reservoir authorization, during construction, and since operation began for the selected reservoir.

Population comparison

Population growth over time

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Conservation pool authorized for water supply shows the percentage of the conservation pool originally and currently authorized for water supply (blue). While the water supply pool at Clinton and Hillsdale accounts for 70 to 80% of the conservation pool, less than 40% is currently under contract (i.e., can withdraw water). The demand is the percent of the contracted supply (not the conservation pool) currently being withdrawn. For examples, 35.7% of the conservation pool at Falls is allocated for water supply. The City of Raleigh has contracted with the Corps to use 100% of the water supply pool, but only withdraws 73% of the daily yield.

Other things to note. Hillsdale increased the amount of the water supply pool under contract in 2009 (right panel). At Redmond, the State of Kansas contracted for 55.8% of the conservation pool to be used for water supply in 1975. In 1996, Kansas contracted for an additional 20.3% of the conservation pool for a total of 76.1%. High sedimentation rates decreased the volume of the conservation pool such that the volume of water supply under contract accounted for 90% of the conservation pool. In 2013, the conservation pool was raised 2 feet, decreasing the percent of the conservation pool under contract for water supply to 66.7% (right panel).

Water supply over time shows how how water supply and demand have changed over time at the selected reservoir. Water supply (dark blue) refers to the percentage of the conservation pool currently under contract. Only a portion of the water supply pool is under contract in Hillsdale and Clinton, while the entire water supply pool is under contract at the other reservoirs. Demand (dark red) is the volume of water withdrawn shown as a percent of the conservation pool. A reallocation study is taking place at Falls to increase the water suppy pool by almost 10%.

Conservation pool authorized for water supply

Water supply over time

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Number of ESA fish species shows the number of threatened and endangered species listed under the Endangered Species Act (ESA; fish only) at each reservoir.

ESA fish species over time shows when species were listed for the selected reservoir. No new fish species have been listed for the four reservoirs in the western U.S. The majority of species listed for reservoirs in the South Atlantic occurred since 2010.

Number of ESA fish species

ESA fish species over time

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Annual operations target comparison shows a boxplot of the mean annual conservation storage at each reservoir. One hundred percent indicates the reservoir is always exactly operating as expected. Most reservoirs are around 100% of the operational target. Redmond is frequently above the operational target (the conservation pool is much smaller than the flood control pool; resulting in large percentages).

Monthly operations target comparison shows the average monthly conservation pool for each reservoir. Falls, Jordan, and Kerr are predominantly wetter in the winter and spring, and drier in the summer and fall months. Western reservoirs don't have quite as distinct of a seasonal pattern. Visit our interactive tool to learn more about reservoir operations and see how they have performed over time.

Annual operations target comparison

Monthly operations target comparison