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 drainage basin sustainability. A practitioner of hydrology is called a hydro ...

, 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 (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...

natural conditions prevail; below it the water level above the riverbed 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 flood A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice and snow. Flash f ...

ing 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, aqua ...

, 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 chamber in a permanently fixed position i ...

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, 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 The Moselle ( , ; ; ) is a river that rises in the Vosges mountains and flows through north-eastern France and Luxembourg to western Germany. It is a bank (geography), left bank tributary of the Rhine, which it joins at Koblenz. A sm ...

) 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 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). 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 ...