Lake glossary

Water level

Percent full

How much usable water a reservoir is holding, as a share of what it holds when full. It compares conservation storage with conservation capacity, so a lake at 60% full has 60% of the water it is designed to supply, not 60% of its depth.

A reservoir can be well below 100% and still be normal for the time of year. Our lake pages say which.

Conservation pool also called Conservation pool elevation

The level a reservoir is normally kept at. Water up to this line is the usable supply for towns, farms, power, and recreation. When people say a lake is “full”, this is the level they mean.

Above it sits the flood pool, kept empty on purpose to catch heavy rain.

Flood pool

Space deliberately left empty above the conservation pool to hold storm water. A lake sitting in its flood pool is above its normal full line, which is why a reading can exceed 100% full after heavy rain.

Dead pool

The level below which water can no longer flow out through the dam's normal outlets. Water below this line exists but cannot be released or, at a hydroelectric dam, used to generate power.

For big Colorado River reservoirs this is a much-discussed threshold. See our guide on Lake Mead.

Conservation storage

The amount of water a reservoir is holding between dead pool and the conservation pool. This is the number that matters for supply, and it is what percent full is calculated from.

Acre-foot

The volume of water that would cover one acre to a depth of one foot: about 325,851 US gallons, or 1,233 cubic metres. Reservoir storage is usually counted in acre-feet or thousands of them.

Roughly what two or three average US households use in a year.

Lake elevation also called Water surface elevation

The height of the lake's surface above sea level, in feet or metres. This is the number most official gauges report for lakes and reservoirs, and the one we show.

It is not the depth of the lake, and not the height of water above the gauge.

Gauge height also called Stage

The height of the water above a fixed zero point chosen for that particular gauge. Useful for seeing whether water is rising or falling, but it is not a height above sea level and cannot be compared between sites.

Confusing the two is a common error. A lake at 1,065 ft above sea level might read 27 ft of gauge height. We always label which one a number is.

NGVD 1929 and NAVD 1988

Two reference systems for “zero” sea level in North America. The older National Geodetic Vertical Datum of 1929 and the newer North American Vertical Datum of 1988 can differ by a foot or more at the same place, so the same lake can have two slightly different official elevations.

We show whichever the gauge publishes, and say which one it is.

IGLD 1985

The International Great Lakes Datum of 1985, the shared US and Canadian reference used for Great Lakes levels. Because the Great Lakes are managed jointly, both countries quote heights against this system.

Drawdown

A deliberate lowering of a reservoir, usually to make room before a wet season, to allow repair work, or to send water downstream. Not the same as drought, though the effect on boat ramps is identical.

Provisional data

A reading that has been published automatically but not yet reviewed by the agency that collected it. Nearly all live water data is provisional, and figures can be corrected later.

Good enough for planning a day at the lake, not for legal or engineering use.

Water temperature

Surface water temperature

The temperature of the top layer of a lake, typically measured within the first metre. It is what a swimmer feels on entry, and it is the number we show.

It can differ by several degrees across one lake on the same afternoon.

Thermocline

The narrow depth band where a lake's temperature drops sharply, separating the warm upper layer from the cold water below. Swimmers notice it as a sudden cold layer; anglers use it to work out where fish are holding.

Stratification

The tendency of a lake to settle into layers through summer, with warm light water sitting on top of cold dense water. Autumn cooling and wind mix the layers again, which is called turnover.

Turnover

The point in spring or autumn when a lake's layers mix and the whole water column reaches a similar temperature. Surface temperature can change quickly around it, and fishing often changes with it.

Lake model

A scientific simulation that calculates a lake's temperature from the weather, the lake's shape and depth, and past measurements. It is an estimate, usually within a degree or two of reality, and it fills gaps where there is no sensor in the water.

Our Alpine lake temperatures come from such a model, run by a research institute, and we label them as models rather than measurements.

Cold water shock

The body's involuntary reaction to sudden immersion in cold water: a sharp gasp, then rapid uncontrolled breathing. It peaks between about 50 and 60 °F (10–15 °C), not at freezing, and it is what makes cold water dangerous long before hypothermia.

Explained in full in our guide on how cold is too cold to swim.

Wind and weather

Knot

One nautical mile per hour, used for wind and boat speed. One knot is about 1.15 mph or 1.85 km/h, so a 20-knot wind is roughly 23 mph.

Gust

A brief peak in wind speed, higher than the sustained wind. Gusts decide whether conditions feel manageable: 12 mph with 30 mph gusts is a very different day from a steady 12 mph.

Sustained wind

The average wind speed over a short period, measured 10 metres above the surface, which is the standard height for weather reporting. It is the figure we show as “wind now”.

Beaufort scale

A long-standing way of describing wind by its effect rather than a number, from calm through light breeze and fresh breeze up to gale and storm. We give the plain description alongside the speed.

Offshore wind

Wind blowing from the shore out across the water. It looks deceptively calm from the beach because there are no waves at the edge, and it is the condition that carries inflatables, paddleboards, and swimmers away from land.

Fetch

The open distance wind travels across water without obstruction. A long fetch builds bigger waves, which is why the same wind produces choppy water at one end of a lake and calm water at the other.

On the water

Boat ramp also called Launch

The concrete slope used to put a boat into the water. When a reservoir drops far enough, the water's edge retreats past the end of the concrete and the ramp closes, which is why low lake levels matter to boaters more than to swimmers.

No-wake zone

An area where boats must travel slowly enough to leave almost no wake, usually near shorelines, marinas, and swimmers. Some lakes impose a lake-wide no-wake rule when the water is unusually high, to stop wake damaging flooded shorelines.

Spawning temperature

The water temperature range at which a fish species breeds. It governs when spawning happens far more reliably than the calendar, which is why the same lake can vary by weeks from year to year.

Species by species in our guide on bass and crappie spawning temperatures.

Stocking

The release of hatchery-raised fish into a lake by a state agency, to support fish populations and angling. Records of what was released, and when, are usually published by the state.

Still unclear?

Read how our data works for the sources behind every figure, the questions we get asked most, or our guides. If a term is missing, tell us and we will add it.