A hydrograph plots a water measurement over time. Before interpreting its shape, identify the station, parameter, unit, time zone, and approval status. A line labeled gage height does not show streamflow, even when both are available at the same station.
Look for the observation interval and the last timestamp. A smooth line may consist of 15-minute sensor readings; it does not imply continuous certainty between observations. A sudden flat line can reflect stable conditions, sensor resolution, transmission delay, or equipment trouble.
Rising and falling lines
The chart direction describes change at the observation point. A rising hydrograph does not automatically mean flooding, and a falling hydrograph does not automatically mean safe boating. Dam releases, tides, tributaries, rainfall, snowmelt, and channel controls all affect interpretation.
USGS real-time data are often provisional and may later be revised. Use official flood alerts and local guidance when decisions have safety consequences. LakeRiverNow charts include a data table so the values remain available without relying on color, hover, or the visual line alone.