PROJECT ALERT NOTICE (SD) cdlaveau@usgs.gov

Date: Tue, 19 Mar 2019 19:06:32 EDT

Summary: USGS continues response to flooding in SD.

Peaks with a 0.04 AEP or ("25-year recurrence interval") in the lower reaches of Split Rock Creek, the Big Sioux, Vermillion, and James River Basins caused by rain (on March 13-14) and melting snow have passed. Most sites in the lower basin are on the receding limb, but melting snow in the upper reaches of the basins will result in a second peak. The second peak is expected to exceed the first peak at many locations with flood conditions more widespread. Flooding is also progressing westward as snowmelt runoff causes flood stages to be reached in western SD on the White and Cheyenne Rivers.

Currently, 12 sites are at or forecasted to be at major flood stage on Split Rock Creek, the Big Sioux, Vermillion, and James River Basins. POR was reached at 4 sites on the initial peak, and POR is forecasted for additional sites of which the following are notable for their length of record:

James River nr Scotland (87 years of record) https://waterwatch.usgs.gov/?id=wwsa&site_no=06478500
Big Sioux River near Dell Rapids (69 years of record)
https://waterwatch.usgs.gov/?id=wwsa&site_no=06481000

There are an additional 11 sites at or forecasted to be at moderate flood stage and 3 at minor flood stage. The second peak should begin arriving at stations this weekend and continue through at least next week. Cities in eastern SD are expected to be particularly impacted:

https://www.ksfy.com/content/news/Sioux-Falls-city-officials-preparing-for-weekend-flooding-507323231.html
https://www.ksfy.com/content/news/Sioux-Falls-city-leaders-in-preparation-mode-for-next-round-of-flooding-507332321.html

The MBRFC has requested that ratings be extended at nine sites operated by the Dakota WSC on the Big Sioux, James, Moreau, and White Rivers. These extensions will be addressed in the next couple of days.

Seven USGS crews were deployed in SD today collecting discharge measurements, repairing streamgages, and verifying sites were ready for open water conditions to develop. Additional crews from ND are expected to deployed as the second peaks approach. Efforts to measure efficiently are being hindered by ice/debris flow in the channel and extensive overland flooding. Many streamgages are still not displaying discharge to the public due to ice in the channel

Stakeholders are being informed of our activities through updates to NWISWeb and use of NWSChat.