Reservoirs and dams

A reservoir converts a variable flow into a reliable supply. Rain and river flow arrive when they arrive; demand is continuous. Storage bridges the gap — and how much storage is needed depends entirely on how variable the inflow is.

Reservoir water balance

A reservoir is an accounting problem: inflows minus outflows equals the change in storage. Every drought is that sum turning negative for long enough.

Percentage full: Relative to each reservoir’s own capacity. A reservoir at 40% may hold far more water than another at 90%, so percentages compare a reservoir with its own past, not with other reservoirs.
Drought sequence: Low rainfall reduces runoff first, then storage, then — much later — groundwater. A dry summer alone rarely empties a reservoir; two dry winters can.
Compensation flow: A licensed release that continues during drought. It reduces available supply but protects the river below the dam.

Why percentage full misleads

The terms that get conflated

Gross capacity: Everything the reservoir can physically hold.
Usable or live capacity: What can actually be drawn out through the outlets. This is the operationally meaningful figure.
Dead storage: Below the lowest outlet. Cannot be used without emergency pumping — which is what São Paulo resorted to in 2015.
Compensation flow: A licensed minimum release to the river below the dam. Continues during drought and is not optional.
Yield: The reliable supply a reservoir can sustain through a design drought. Much lower than capacity, and the number that actually matters for planning.

Published capacity figures frequently do not say which of these they mean, and the difference between gross and usable capacity can be substantial.

Evaporation is a real loss

Evaporation is proportional to surface area, not to volume. A shallow reservoir with a wide surface in a hot climate loses far more proportionally than a deep narrow one. Lake Nasser loses a substantial fraction of the Nile flow it stores to evaporation each year — a cost of building storage in a hot arid climate that is rarely counted against the scheme.

Estimate evaporation loss →

What dams do and cost

What a large dam provides and what it costs

Row	Provides	Costs
Water	Reliable supply through dry seasons and multi-year droughts	Evaporation loss; downstream flow reduction
Energy	Low-carbon, dispatchable hydropower	Reservoir greenhouse gas emissions where vegetation is flooded
Flood control	Attenuates flood peaks downstream	Catastrophic consequences if the dam itself fails
Sediment	Traps sediment, extending downstream channel life in some cases	Starves the delta and the coast of sediment; the reservoir silts up and loses capacity
Ecology	Creates a new lake habitat	Blocks fish migration; alters the flow regime downstream species depend on
People	Supply, power and flood protection for millions	Displacement — the Three Gorges scheme resettled over 1.3 million people

The Aswan High Dam is the clearest single illustration. It gave Egypt multi-year storage and ended the annual Nile flood, which was transformative. It also ended the annual silt deposition that had fertilised Egyptian fields for millennia, requiring a permanent switch to chemical fertiliser, and it has contributed to erosion of the Nile delta.

Sources

Global Water System Project — Global Reservoir and Dam Database. Open — attribution required · CC BY 4.0
United States Bureau of Reclamation — Reservoir operations and Colorado River data. Open · US Government work — public domain
Ministerio para la Transición Ecológica y el Reto Demográfico — Spanish water resources and reservoir monitoring. Open — attribution required · Spanish public sector reuse (Ley 37/2007)