PROJECT ALERT NOTICE (MT WY) Minor to moderate flooding continues in Eastern Montana and Northeastern Wyoming

Date: Sun, 24 Mar 2019 13:05:14 EDT

Summary: Minor to moderate flooding from snowmelt-runoff and ice-jams continues in northeastern Montana and northeastern Wyoming.

Gage-heights at seven (7) U.S. Geological Survey streamgages in eastern Montana are exceeding National Weather Service (NWS) minor and moderate flood stages. Lowland snowmelt-runoff and ice-jams are the cause of most of the flooding. In addition to previously reported flooding in the Milk River basin and tributaries, a new NWS flood warning were issued for the Little Bighorn River basin in southeastern Montana where lowland snowmelt-runoff and possible ice-jams have resulted in localized flooding and subsequent closure of about 50 miles of Interstate 90 from Hardin, Montana to the Wyoming-Montana state line. Threatening ice-jams on the lower Yellowstone River also have resulted in the issuance of a NWS flood warning.

In northeastern Wyoming, Belle Fourche River inflows to Keyhole Reservoir and reservoir levels triggered an internal alert by U.S. Bureau of Reclamation officials.

Annual exceedance probabilities have not been determined. Streamflows are expected to increase at many sites. In most instances, recent maximum streamflows were below those of 1978, 2011, and 2018. No streamgages have been damaged or destroyed.

One crew is deployed this weekend to complete direct measurements at sites that are part of a crest-stage gage network in north-central and northeastern Montana and to validate ice-effects at currently qualified streamgages. Even as ice jams release, submerged ice, floating ice, shore ice, and deep slush continue to affect stage-discharge relations at several streamgages.

Cooperators, stakeholders, and the general public are being kept informed by multiple methods including NWSChat, email, and social media. Photographs of hydrographers measuring flood- and high-flows with text for context and links to data are being posted at Twitter and Instagram accounts @USGS_WY and @USGS_MT.

Interactive flood map for Milk River, northeastern Montana