Over 10,000 residents in Minot, North Dakota had until 6 pm Tuesday night to evacuate the city.
Water from the Souris River started to spill over the city's protective levees on Wednesday, forcing authorities to sound the emergency evacuation sirens.
Officials say water levels are rising fast and are expected to exceed the devastating flood of 1969, when the Souris River reached 1,554 feet above sea level.
Crews in Minot are working furiously to raise earthen levees in an effort to protect critical infrastructure including schools, water plants, and the sewer system. On Thursday, trucks and loaders carried clay and dirt, which are being spread across the weakening levees.
Officials say parts of the city are already flooding, including a trailer park that's under several feet of water. The U.S. Army Corps of Engineers is starting to release water from the Lake Darling dam in hopes that future releases won't have to be as big.
Around 300 people are staying in shelters while the rest of evacuees found shelter with family and friends.
The Souris River loops down from Canada and through central North Dakota.
Flooding on the Canada-U.S. border is creating long detours for travellers and international truck drivers. Highway 39 around the North Portal has been closed to trucks entirely after being damaged by floodwaters. Traffic is being diverted 100 km to the west.
Several cities in southeastern Saskatchewan are completely waterlogged. The spring snowmelt combined with heavy rain has overwhelmed the watershed and has caused significant flooding.
The small village of Roche Percee, Saskatchewan is submerged in water after a dike breach last weekend. 26 communities have declared a local state of emergency and frustrations continue to rise as the situation worsens.