Reservoir or Man-made lake - An artificial lake, basin or tank in which a large quantity of water can be stored.

Storage reservoir or Conservation reservoir - A reservoir which is operated with changing water level for the purpose of storing and releasing water.

Pumped storage reservoir - A reservoir filled entirely or mainly by water pumped from outside the natural catchment.

Direct supply reservoir-A reservoir from which water is conveyed by aqueduct direct to supply.

Regulating reservoir - A reservoir from which water is released so as to regulate the flow in a river.

Active storage (Fig. 1) - The volume of the reservoir that is available for use either for power generation, irrigation, flood control or other purposes. Active storage excludes flood surcharge. It is the reservoir capacity less inactive and dead storage. The terms Useful storage or Usable storage or Working storage are sometimes used instead of Active storage but are not recommended.

Inactive storage (Fig. 1) - The storage volume of a reservoir measured between the invert level of the lowest outlet and minimum operating level.

Live storage (Fig. 1) - The sum of active and inactive storage volumes. Where there is no inactive storage, e.g. in some irrigation reservoirs, live storage and active storage describe the same storage which is generally termed “live storage”.

Dead storage (Fig. 1) - The storage volume of a reservoir measured below the invert level of the lowest outlet.

Flood storage - The part of the active storage used specifically for flood control. “Flood storage” should not be confused with “flood surcharge”.

Reservoir capacity or Gross capacity of reservoir or Gross storage or Storage capacity - The gross capacity of a reservoir from the river bed up to retention water level. It includes active, inactive and dead storages.

Bank storage or Ground storage - Water that has infiltrated from a reservoir into the surrounding land where it remains in storage until water level in the reservoir is lowered.

Retention water level or Top water level or Normal top water level or Full supply level or Normal water level (Am.) - For a reservoir with a fixed overflow sill it is the lowest crest level of that sill. For a reservoir, the outflow from which is controlled wholly or partly by movable gates, siphons or by other means, it is the maximum level at the dam to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge.

Flood surcharge or Surcharge (Fig. 1) - The volume or space in a reservoir between the retention water level and the maximum water level. Flood surcharge cannot be retained in the reservoir but will flow over the spillway until retention water level is reached. The term “Wet freeboard” for describing the depth of flood surcharge is not recommended (see freeboard).

Maximum water level (Fig. 1) - The maximum water level, including flood surcharge, which the dam has been designed to withstand.

Minimum operating level (Fig. 1) - The lowest level to which the reservoir is drawn down under normal operating conditions. The lower limit of active storage.

Freeboard - The vertical distance between a stated water level and the top of a dam. Thus Net freeboard or Dry freeboard or Flood freeboard is the vertical distance between the maximum water level and the top of the dam (Fig. 1)
Gross freeboard or Total freeboard is the vertical distance between the retention water level and the top of the dam.
That part of the Gross freeboard attributable to the depth of flood surcharge is sometimes referred to as the Wet freeboard, but this term is not recommended as it is preferable that freeboard be stated with reference to the top of dam.

Backwater curve - The longitudinal profile of the water surface in an open channel where the depth of flow has been increased by an obstruction such as a weir or dam across the channel, or by increase in channel roughness, or by decrease in channel width, or by a flattening of the bed slope. The water surface for some distance upstream of the point where the regime changes has a longitudinal curve which is concave upwards.

Drawdown curve - The longitudinal profile of the water surface in an open channel where the depth of flow has been decreased by decrease in channel roughness, or by increase in channel width, or by a steepening of the bed slope. The water surface for some distance upstream of the point where the regime changes has a longitudinal curve which is convex upwards.

Axis of reservoir - Line joining the mid points of the surface of the reservoir, when full to retention water level, at successive cross-sections through the reservoir area.

Length of reservoir - The distance along the axis of the reservoir from the dam to the farthest point where the principal river or a tributary enters the reservoir.

Reservoir surface (Fig. 2) - The surface of a reservoir at any level.

Reservoir area - The surface area of a reservoir when filled to retention water level.

Reservoir shore or Reservoir banks (Fig. 2) - The strip of land skirting the reservoir at or near retention water level (see also chapter Geomorphology).

Road or Railway diversion or Road or Railway relocation (Am.) - The realignment of a road or railway made necessary by the construction of a reservoir.

Reservoir operation - The filling and drawing down of a reservoir within the active storage limits according to a predetermined plan.

Drawdown - The difference between retention water level and a lower water level in a reservoir at a particular time.

Fluctuation - With reference to a reservoir, the variation in water level, up or down, as a consequence of reservoir operation.

Drawndown range - The difference between the upper and lower limits of active storage in a seasonal reservoir, or of fluctuation in a weekly or daily storage reservoir.

Drawndown zone - That part of the banks of a reservoir corresponding to the drawndown range.

Fetch - The straight line distance between a dam and the farthest reservoir shore. The “fetch” is one of the factors used in calculating wave heights in a reservoir.

[Hollow intake weir with several small bottom sluices for continuous or discontinuous flushing used on mountain streams with appreciable bed load transport]

[Device that detects the build up of gravel in a gravel trap and which, for a determined gravel depth, controls subsequent flushing]

Flushing channel or Flushing canal - “Flushing” meaning to cleanse by flow of water is commonly used in irrigation and water supply practice and also in connection with sand traps and gravel traps at river intakes. Hence the expressions: flushing channel, flushing canal, intake with automatic flushing. The terms “Scour” and “Scouring” are more commonly used in connection with reservoirs.

Aqueduct - An artificial way of conveying water, i.e. by canal pipe or tunnel. Hence, the expressions: “connecting aqueduct”, “diversion aqueduct”.