Head (hydrology)

Above the head of the reservoir 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 flooding below.

Upstream of the actual reservoir is likely to be a pre-dam, which typically have a constant water level so the head is reinforced.

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.

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 are.

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).