Flooding and High-Flow Conditions in Wyoming and Montana

Science Center Objects

The USGS monitors flood and high-flow conditions at more than 300 streamgages across Montana and Wyoming.  Water level and flow information are used by the National Weather Service (NWS) to make accurate flood forecasts. High-flow conditions are expressed as percentiles comparing current (within the past few hours) instantaneous flow to historical daily mean flow for all days of the year.

A percentile is a value on a scale of one hundred that indicates the percent of a distribution that is equal to or below it.

  • "<95" indicates that the estimated streamflow is less than the 95th percentile for all days of the year,
  • "95-98" indicates that the estimated streamflow is between the 95th and 98th percentiles for all days of the year,
  • ">=99" indicates that estimated streamflow is greater than the 99th percentile, and
  • "Not ranked" indicates that a flow category has not been computed. Common reasons for a "Not ranked" category are insufficient historical data or no current streamflow estimates.

Flood stages are determined by the NWS; not all USGS streamgages have flood stages.  A depiction of other streamgages at or above flood stage is available the NWS River Conditions Map.

WY FloodWatch Screen Shot

Current Flooding and High-Flow Conditions:
(Current-conditions data are provisional and subject to change)




MT FloodWatch Screen Shot



Video - 2011:  The Year of the Flood

How can a 100-year flood strike twice in 15 years? This is a downloadable poster discussing the meaning and use of probability language in flood characterization.
Poster: 100-Year Flood--It's All About Chance