Glossary of Terms



Acre-foot – A unit of volume commonly used in reference to large-scale water resources. One acre-foot of water is equivalent to one foot in depth covering an area of one acre, or approximately 326,000 gallons of water. The average suburban household uses approximately one-half of an acre-foot per year.

Assistant Secretary of the Army – Civil Works (ASA) – The Assistant Secretary of the Army for Civil Works establishes policy direction and provides supervision of the Department of the Army functions relating to all aspects of the Civil Works program of the U.S. Army Corps of Engineers.

Aquifer – An aquifer is an underground layer of water-bearing sediment such as rock, gravel, sand, silt or clay, from which groundwater can be extracted using a well. Much of the water in deep aquifers has been trapped for thousands of years. While this water is relatively clean and often needs little treatment before drinking use, the resource is finite and can be depleted over time.


Call on water rights – A demand to the State Engineer by an entity owning water rights, in order for water in a stream system to reach their diversion works in order to yield water as expected from that water right. If a senior water right holder calls for its water rights, upstream junior water right holders are required to allow water to flow downstream to the senior right holder.

Clean Water Act – A federal law that outlines how the United States will restore and maintain the chemical, physical and biological integrity of the country’s waters, including oceans, lakes, streams, rivers, ground water and wetlands. The law provides protection to the country’s surface water from both point and non-point sources of pollution.

Colorado Water Conservation Board (CWCB) – A division of the State of Colorado’s Department of Natural Resources, the CWCB’s responsibilities range from protecting Colorado’s streams and lakes to water conservation, flood mitigation, watershed protection, stream restoration, drought planning, water supply planning and water project financing. The CWCB also works to protect the state’s water apportionments in collaboration with other western states and federal agencies. The Board consists of 15 members appointed by the Governor of Colorado. Eight voting members represent the state’s major water basins: the Arkansas, Colorado, Gunnison, Metro, North Platte, Rio Grande, South Platte, Southwest and Yampa/White. One voting member represents the City and County of Denver, and the tenth voting member is an Ex-officio, the Executive Director of the Department of Natural Resources.

Confluence – A flowing together of two or more streams.

Conservation-Multipurpose Pool – An existing designation by the U.S. Army Corps of Engineers for the storage capacity in Chatfield Reservoir that is allowed to hold multipurpose water, located between 5,385 and 5,432 feet above mean sea level.

Corps – U.S. Army Corps of Engineers. A division of the U.S. Department of Defense. The Corps is the federal agency responsible for investigating, developing and maintaining the nation’s water and related environmental resources. The Corps is the federal sponsor of the Chatfield Reservoir Reallocation Project.


Endangered Species Act – The federal law that governs how animal and plant species whose populations are dangerously in decline or close to extinction will be protected and recovered. The law protects not only threatened and endangered species, but also the ecosystems upon which they depend.


Feasibility Report/Environmental Impact Statement – A full disclosure document required by federal environmental law that details the process through which a federal project is developed, determines the viability of the option, considers a range of reasonable alternatives, analyzes the potential impacts resulting from each alternative, and demonstrates compliance with other applicable federal environmental laws and executive orders.

Flood Control Pool – An existing designation by the U.S. Army Corps of Engineers for the storage capacity in Chatfield Reservoir to hold flood waters, located between 5,432 and 5,500 feet above mean sea level.


Gravel Pit Storage – An excavated pit, generally with a clay stone floor and sand and gravel walls, used to store water. Some pits are lined with a bentonite slurry wall or clay liner to prevent water from seeping through the gravel pores.

Ground Water – Water found below the earth’s surface, such as in an aquifer.


Inactive/Sediment Pool – An existing designation by the U.S. Army Corps of Engineers for the storage capacity at the bottom of Chatfield Reservoir located between 5,377 and 5,385 feet above mean sea level. This pool elevation will not be affected by the reallocation project.


Joint Flood Control-Conservation Pool – A proposed designation for storage capacity in Chatfield Reservoir to hold multipurpose water and flood waters, as defined by the reallocation alternatives of the feasibility report/environmental impact statement. The pool will be confined to the elevations between 5,385 and 5,444 feet above mean sea level, creating 20,600 acre-feet of additional storage for multipurpose water.


Maximum Surcharge/Spillway Design Flood Pool – An existing designation by the U.S. Army Corps of Engineers for the storage capacity at the top of Chatfield Reservoir located between 5,500 and 5,521.6 feet above mean sea level. This pool elevation would not be affected by the reallocation project.

Mean Sea Level – The average height of the sea relative to a single point on land (a datum) after high frequency motions are averaged out; the basis for mapping the elevation of terrestrial features.

Mitigation – Reasonable measures taken to avoid, minimize, rectify, reduce or compensate for impacts to the physical environment resulting from federal actions. Federal law requires agencies to identify all relevant and reasonable mitigation measures, including avoiding the impact altogether by not taking a certain action or parts of an action; minimizing impacts by limiting the degree or magnitude of the action and its implementation; rectifying the impact by repairing, rehabilitating, or restoring the affected environment; reducing or eliminating the impact over time by preservation and maintenance operations during the life of the action; and/or compensating for the impact by replacing or providing substitute resources or environments.

Multipurpose Water – Water supply used for a variety of uses, such as municipal, industrial, agricultural and/or recreational uses, or for maintaining fisheries and wildlife habitat.


National Environmental Policy Act (NEPA) – Federal legislation requiring federal agencies to evaluate the environmental impacts of all proposed federally-funded or permitted actions, and to consider reasonable alternatives to those actions.

National Historic Preservation Act (NHPA) – Federal legislation requiring federal agencies to evaluate the impact of all proposed federally-funded or permitted projects on historical and archaeological sites listed on, or eligible for listing on, the National Register of Historic Places, and to consider reasonable alternatives to those actions.

Non-renewable Water – Water resources from a deep underground aquifer which have been trapped for thousands of years. This resource is not replenished by rain fall or melt-off, so the finite supply can be depleted and becomes more costly to extract water over time as the aquifer level drops.


On-channel – Refers to the location of Chatfield Reservoir directly on the South Platte River, where the reservoir captures flows from the river.


Pool Elevation – The maximum elevation, measured in feet, above mean sea level that a stored volume of water can rise in a flood control facility, as designated by the U.S. Corps of Engineers.


Reallocation – Refers to a change in the designation of storage space in Chatfield Reservoir. The reallocation merges the existing multipurpose pool with a portion of the existing flood control pool, creating a newly-designated joint flood-conservation pool. This newly allocated storage space would serve a dual purpose, primarily for storing flood waters and secondarily for storing multipurpose water.

Record of Decision (ROD) – A document prepared by the lead federal agency outlining the decision and documenting the required implementation and mitigation measures needed to complete a federally-funded or permitted project.

Renewable Water – Surface water resources that are replenished by seasonal rains and melting snowfall, such as rivers, streams and lakes.

Reservoir – An impoundment of collected water controlled by a dam.

Return Flows – The unused portion of water that returns to a stream or river after a beneficial use.

Reuse – To use again, to intercept for subsequent beneficial use, either directly or by exchange, water that would otherwise return to the stream system.


Semi-arid – Characterized by very little annual rainfall, usually from 10-20 inches (25-50 cm).

Spillway – A channel that carries surplus water over or around a dam or other obstruction from a reservoir, lake, or similar body of water.

Storage Capacity – A volume of space in Chatfield Reservoir available to hold water.

Surface Water – Water that collects in a river, stream, lake, wetland or ocean. It is naturally replenished by precipitation and lost to evaporation.


Watershed – An area from which water drains into a river, river system, or other body of water.

Water Rights – The right of a user to use water from a water source, such as a stream, tributary or aquifer. In Colorado, the seniority of water rights is determined by appropriation dates. The first person/entity to use a quantity of water from a water source for a beneficial use has a right to continue to use up to their full allocation annually for that purpose, provided the water source can supply it. The user with the next earliest appropriation date may use their full allocation next, and so on. In times of drought, users with junior appropriation dates might not receive their full allocation or even any water at all.

Water Yield – The amount of water supplied from raw water sources of a given water supply system.

Wetlands – Areas with standing water of a high water table either permanently or for some significant period each year. Generally includes swamps, marshes, bogs and areas with water-loving vegetation that grows in or around water. ­­