Lake Conditions & Hydrology Glossary

The designated target water-surface elevation (in feet above mean sea level) that an operating agency (such as the U.S. Army Corps of Engineers or a regional water authority) manages a reservoir to hold for normal conservation, recreation, and municipal storage. Elevations above full pool typically indicate flood control operations; elevations below reflect seasonal drawdown or drought conditions.

The national survey reference frameworks used by USGS and dam operators to measure absolute water elevation. NGVD29 (National Geodetic Vertical Datum of 1929) is an older vertical datum based on mean sea level, while NAVD88 (North American Vertical Datum of 1988) is the modern, gravitationally corrected reference. Depending on geographic location, the datum offset can range from a few inches to more than four feet. LakeRadar always normalizes or clearly labels the reported datum.

Gauge height (or stage) is the height of the water surface above an arbitrary local zero point set by the gauge installer (for example, 14.2 ft). In contrast, water elevation adds the gauge zero datum to produce a true altitude above sea level (e.g., 1,042.8 ft MSL). Both indicate whether water is rising or falling, but absolute elevation is required to compare against full pool.

The intentional lowering of a reservoir's water level below full pool by dam operators. Drawdowns are routinely scheduled in late autumn and winter to create storage capacity for spring snowmelt and rain runoff, perform shoreline repairs, or expose invasive aquatic vegetation to freezing temperatures. Significant drawdowns can render shallow boat ramps unusable.

An algorithmic estimate of active reservoir water volume expressed as a percentage of total conservation capacity. Because most dam operators do not publish continuous live acre-foot telemetry, LakeRadar applies standard area-capacity power-law approximations based on measured elevation, maximum depth, and surface acreage. It provides an indicative trend, not a bathymetric survey.

An official advisory issued by the National Weather Service when sustained winds or frequent gusts reach 18 to 33 knots (21 to 38 mph), or hazardous seas/waves are observed or forecast. While not a ban on boating, recreational vessels, small sailboats, and personal watercraft should exercise extreme caution or remain in sheltered waters.

A high-priority warning issued by the NWS for sustained winds or frequent gusts of 34 to 47 knots (39 to 54 mph). Conditions at this level produce large, breaking waves and pose severe hazards to virtually all recreational watercraft.

A meteorological measure (expressed in Joules per kilogram, J/kg) of atmospheric instability and updraft energy available to create convective storms. Values exceeding 1,000 J/kg indicate potential for lightning; values above 2,500 J/kg signal volatile environments capable of generating severe thunderstorms, hail, and dangerous microbursts even on initially sunny afternoons.

Gust Factor is the ratio between peak wind gusts and mean sustained wind speed (e.g., 24 kt gust / 12 kt sustained = 2.0). A gust factor above 1.6 indicates erratic, squally winds. Fetch is the unobstructed distance of open water over which wind blows in a single direction; longer fetch allows waves to build in both height and steepness.

A theory developed by John Alden Knight demonstrating that fish and wildlife activity increases during specific lunar alignments. Major periods occur when the moon is directly overhead or directly underfoot (lasting ~2 hours). Minor periods occur at moonrise and moonset (lasting ~1 hour). Feeding activity is highest when a solunar window coincides with dawn, dusk, or a falling barometer.

Changes in atmospheric pressure directly compress or expand the air bladders of freshwater fish (such as bass, walleye, and crappie). A falling barometer (approaching front) often triggers aggressive pre-frontal feeding. A rapidly rising barometer behind a cold front creates high pressure and clear blue skies, typically causing fish to hunker down tight to heavy structure.

During summer, deep lakes stratify into three distinct thermal layers: the warm upper layer (epilimnion), the cold oxygen-poor bottom layer (hypolimnion), and a steep transition layer in between known as the thermocline. Fish frequently concentrate at the thermocline where water temperature is comfortable while dissolved oxygen remains sufficient.

A standardized measurement of water transparency taken by lowering a black-and-white disk (Secchi disk) until it disappears from view. Turbid or muddy water (low Secchi depth) limits light penetration and favors vibrating or noisy lures, while clear water requires natural presentations and lighter fluorocarbon lines.

A composite 0–100 rating reflecting how conducive current and forecast conditions are for a specific recreation type (Fishing, Boating, Sailing, Paddling, Swimming). It is an algorithmic decision aid, not a safety guarantee. Read our Methodology for exact parameter weightings.

A statistical confidence indicator that decreases if sensors are remote, if weather model grids are broad, or if telemetry is delayed. High confidence means recent in-situ sensors and high-resolution forecasts are available.

LakeRadar's editorial standard: we only publish lake conditions when verified attribution can be established to an official federal, state, or regional authority. We would rather display an honest “no data” indicator than invent synthetic readings.