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.