Guides: understanding water levels

Plain-English explanations of the numbers on our lake and river pages, checked against the agencies that publish the data.

Using this site

How to Read a Water Level Atlas Lake or River PageA section-by-section walk-through of our lake and river pages: the numbers, badg…
What Does "Normal for the Date" Mean for Water Levels?How day-of-year percentiles define normal, below normal and above normal lake le…

Guide Curves and Rule Curves ExplainedWhat reservoir guide curves, rule curves and action zones are, how they set seas…
Acre-Feet and Percent Full: Reservoir StorageWhat an acre-foot is, how reservoir storage relates to lake elevation, and why p…
NGVD 29 vs NAVD 88: Why Lake Elevations DifferWhat a vertical datum is, how NGVD 29 and NAVD 88 differ, and why the same lake …
What Is Full Pool? Lake Levels ExplainedWhat full pool means on a reservoir, how it differs from the flood pool and inac…
Who Manages US Reservoirs? Corps, Reclamation, TVAThe main agencies and companies that run US reservoirs, what each usually manage…
Why Do Lake Levels Change? Inflow, Outflow and MoreHow rain, runoff, dam releases, water supply and evaporation raise or lower a la…
Winter Drawdown: Why Reservoirs Are LoweredWhy many reservoirs are lowered in fall and winter, how they refill in spring, a…

Gauge Height vs Streamflow: What's the Difference?Gauge height is how high the water is; streamflow is how much water passes. Lear…
How Does a River Gauge Work? USGS Streamgages ExplainedHow a USGS streamgage measures river height every 15 minutes, sends it by satell…
Provisional vs Approved USGS Data: What It MeansWhy live USGS river readings are labelled provisional, how they are reviewed and…
What Is CFS? Cubic Feet per Second ExplainedWhat cubic feet per second means for river flow, how to picture it, how to conve…

Annual Peak Flow and Record Crests ExplainedWhat a river's annual peak flow is, how the October-to-September water year work…
Flood Stages Explained: Action, Minor, Moderate, MajorWhat the National Weather Service flood categories mean, how flood stage differs…
How NWS River Forecasts Work, and Where to Find ThemHow NOAA's River Forecast Centers turn rain, snow and gauge data into official r…
What Is a 100-Year Flood? The 1% Chance ExplainedA 100-year flood has a 1% chance of happening in any year. Learn why it can happ…

How Rising and Falling Water Levels Affect FishingWhat state fisheries agencies say about fishing when lake levels and river flows…
Low Water Boating Hazards on Lakes and RiversWhat lake operators and boating agencies say about low water: submerged trees an…
Water Temperature and Fish: Cold-Water vs Warm-WaterWhy water temperature matters to fish, how cold-water, cool-water and warm-water…