Hydrology & Boating Glossary
Plain-English definitions for 20 essential water management, vertical datum, boat ramp, and reservoir engineering concepts.
Acre-Foot
A volumetric water unit representing the amount of water required to cover one acre of land to a depth of one foot.
Conservation Pool
The volume of water stored in a reservoir dedicated to beneficial uses such as drinking water, power, and recreation.
Datum
A reference surface or mathematical baseline from which vertical elevations or geographic coordinates are reckoned.
Dead Pool
The critical low-water elevation below which water cannot flow downstream through gravity outlets.
Drawdown
The deliberate or natural reduction of a lake or reservoir water surface elevation.
Evaporation Loss
The invisible volumetric loss of water from a lake surface into the atmosphere caused by heat, sun, and dry winds.
Flood Pool
The temporary storage capacity above full conservation pool designed to capture floodwaters and prevent downstream damage.
Full Pool
The designated maximum target elevation of a reservoir during standard conservation storage operations.
Gauge Height
The height of the water surface measured relative to an arbitrary, local gauge benchmark rather than mean sea level.
Inflow and Outflow
The volume rate of water entering a reservoir compared to the volume rate released downstream.
Lake Surface Elevation
The absolute vertical height of the lake surface measured against a standardized geodetic reference datum.
Lake Wind Advisory
An inland weather warning issued by the NWS when sustained winds over lakes make boating and navigation hazardous.
NGVD29 vs NAVD88
The difference between the historical 1929 vertical datum and the modern 1988 North American geodetic datum.
Normal Pool
The typical, authorized elevation of a lake or reservoir during standard non-flood operations.
Percent Full
The ratio of active water volume currently stored in a reservoir compared to its authorized full conservation capacity.
Provisional Data
Real-time telemetry readings published immediately by monitoring agencies prior to formal scientific review.
Rule Curve
A seasonal target elevation schedule established by dam engineers to guide reservoir storage decisions.
Small Craft Advisory
An official National Weather Service marine alert warning operators of small vessels of hazardous winds and rough chop.
Spillway
A engineered structural passage designed to release floodwaters safely over or around a dam.
USGS Gauge
An automated hydrologic monitoring station operated by the U.S. Geological Survey to measure water levels and discharge.