The circles on this map represent US Geological Survey (USGS) stream gauges. The color codes (key at top right) demonstrate the current river level compared to historical data for today’s date at that location. For example, black dots denote river stages that are higher than 90% of all past years on today’s date. Gauges marked as unranked have insufficient historical data, or collect stream stage only.
(This information is also available from the USGS site here. Note the USGS uses a different color scheme.)