U.S. Army Corps of Engineers Reservoir Reallocations
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Click on the tabs below to learn how reallocations work and to explore reallocations by the U.S. Army Corps of Engineers.
The U.S. Army Corps of Engineers manages 537 reservoirs in the continental United States. Congress authorizes the construction of reservoirs to meet specific purposes such as flood control, hydropower, navigation, and water supply. The design and operation of reservoirs includes allocating volumes of reservoir storage (typically referred to as “pools”) for each purpose. There are three broad categories of pools (Figure 1): flood control, conservation (also referred to as multi-purpose), and sediment (also referred to as inactive or dead storage).
Figure 1: The flood control pool is intended to be empty until needed to hold flood waters and prevent downstream flooding. The conservation pool is intended to be full of water that can be used to meet a specified purpose. Conservation pools may be further subdivided into pools dedicated for specific purposes such as water supply, hydropower, water quality, recreation, navigation, and so on. The sediment pool is intended to fill with sediment over time (usually 50 to 100 years).
When a significant change to reservoir operations is needed, the Corps may seek to reallocate, or move, the amount of storage volume allocated to each pool. The total reservoir volume is fixed unless major structural changes, such as raising the dam, occurs. Therefore, the volume of water dedicated to different pools may be adjusted in a zero-sum game. The storage volume would need to decrease in one pool (origin of reallocated storage space; origin pool) and be increased in another pool (destination of reallocated storage space; destination pool). For instance, if water supply is authorized as a new purpose, then either the volume dedicated to other purposes in the conservation pool would need to decrease (such as reducing the volume of water available for hydropower) or the volume of the conservation pool would need to be increased by reducing the volume allocated to the flood control pool (Figure 2). There are several types of reallocations the Corps has utilized over time: raising a reservoir, reallocating from the flood control pool, reallocating within the conservation pool, altering seasonal storage to only conservation or only flood control pool, temporarily using the sediment pool, and operating reservoirs along a river as a system.
Figure 2: (Top) The authorized pools of a reservoir. (Bottom Left) The Flood Control pool is the origin pool (storage space taken away) and the water supply pool is the destination pool (gaining storage space). (Bottom right) The conservation pool, specifically they hydropower pool, is the origin pool (losing storage) while the water supply pool is the destination pool (gaining storage).
Click on the tabs above to explore reallocations by U.S. Army Corps of Engineers. The data used in this tool can be downloaded below.
The map shows the location of U.S. Army Corps of Engineers reservoirs and their reallocation status.
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Number of reallocations shows the number of reservoirs each year that have implemented a reallocation, use surplus water, or have proposed a reallocation.
Volume of reallocations shows the volume of water reallocated (implemented, surplus, or proposed) each year.
Change in origin pool shows the percentage of the flood control pool reduced and conservation pool altered by reallocations at the Division or District. Note that flood control and conservation pool volumes were missing for numerous reservoirs in the Great Lakes & Ohio River and Mississippi Valley Divisions (particularly the Vicksburg District). Blue indicates flood control pool is the origin pool. Red indicates the conservation pool is the origin pool. Darker shades indicates implemented reallocations. Lighter shades indicates surplus + proposed reallocations.
Reallocation by reservoir shows the cumulative effect of reallocation(s) on the origin pool of a reservoir. The denominator for Percent Origin Pool is grouped by flood control, seasonal storage, and conservation pool (e.g. hydropower, water quality, navigation). See Introduction for more details.
Water supply over time shows the volume of storage space dedicated to water supply over time by Division. The majority of reallocations were for water supply purposes. Original refers to water supply storage that was part of the original authorized purposes for the reservoir and activated refers to storage space that was reserved for future use that has since been activated. Future water supply includes both surplus water and proposed reallocations.
Summary shows the percent of the conservation pool dedicated to water supply by original purpose, reallocation (implemented), or proposed in the future (includes surplus water).