Authorities with Union County Emergency Management and the Union County Sheriff's Office want residents living in Dakota Dunes, Riverland Estates, Wynstone, and nearby rural residents to prepare for a voluntary evacuation.
Officials say in a statement that citizens in these areas should secure their property.
A shelter is open at West Middle School in Sioux City, Iowa.
The request comes as Missouri River water releases at Gavins Point Dam near Yankton have increased greatly over the past few days.
John Remus with the U.S. Army Corps of Engineers says the higher releases are due to more water coming from the Niobrara River in Nebraska and other tributaries they don't regulate.
"Our release decisions over the last 48 hours have been based on the inflows into Gavins Point Dam, and the ability for the dam to contain or manage those flows. The 2011 operations were not a part of our decision making process."
Union County authorities say elevations may rise up to 1,093 feet at Dakota Dunes and 1,097 feet at Wynstone. They say at these levels, Missouri River water may inundate low-lying residential properties.
Authorities say the Union County Sheriff's Office Facebook page and the county website will have updates as they are available.