hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...

, the head is the point on a watercourse up to which it has been artificially broadened and/or raised by an impoundment. Above the head of the

reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...

natural conditions prevail; below it the water level above the

riverbed A stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow ( channel). The lateral confines or channel margins are known as the stream banks or river banks, during all but flood ...

has been raised by the impoundment and its flow rate reduced, unless and until banks, barrages, weir sluices or dams are overcome (overtopped), whereby a less frictional than natural course will exist (mid-level and surface rather than bed and bank currents) resulting in flash flooding below. In principle, a distinction must be drawn between the head of a reservoir impounded by a

dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...

, and the head of a works resulting from a barrage or

canal locks A lock is a device used for raising and lowering boats, ships and other watercraft between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a fixed chamber in which the water lev ...

Head of a reservoir

A head's location varies with the height of the water level against the dam. Since there is only an extremely low flow within the

so no water level gradient, the head can be clearly seen: where the farthest watercourse discharges into the reservoir. Upstream of the actual reservoir is likely to be a

pre-dam A forebay is an artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made.< ...

, which typically have a constant water level so the head is reinforced. The term does not apply to embankment (storage/settling) reservoirs, to which water is pumped from below.

Head of a works

{{See also, River engineering#Canalization of rivers On large rivers in all but arid climates, the head of a works is rarely fixed rigidly, as, within the impounded reach a significant flow rate and water gradient is sometimes seen. The head can only be found by calculation or defined by observations with and without impoundment. Depending on the flow rate and control of the barrage, locks or weir, position will greatly vary and will not necessarily be where the so-called

headworks Headworks is a civil engineering term for any structure at the head or diversion point of a waterway. It is smaller than a barrage and is used to divert water from a river into a canal or from a large canal into a smaller canal. at Web archive An ...

are. Many rivers (such as the Moselle) are barraged many times to make them navigable and/or to avoid uncontrolled flooding. In such a case only the higher stretches of river are uninfluenced by impoundment. As to the other stretches the river has long "level" pounds but no or few natural heads, instead having artificial structures until the top head. Ideal management of the higher heads will allow headroom to keep back some

flood meadow A flood-meadow (or floodmeadow) is an area of grassland or pasture beside a river, subject to seasonal flooding.Huhta, Ari‐Pekka, Rautio, Pasi (2014). Flood meadows in Finland - their development during the past century. ''Nordic Journal of B ...

water so as not to compound heavy precipitation and resultant run-off downstream; corollary channels with spare capacity are a further mitigation where land is at a premium (such as the

Jubilee River The Jubilee River is a hydraulic channel in southern England. It is long and is on average wide. It was constructed in the late 1990s and early 2000s to take overflow from the River Thames and so alleviate flooding to areas in and around the ...

). Ideal management of the lowest head will allow daily timed openings, at least in flood events, to coincide with an outgoing ( ebb), rather than

flood tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...