[2] Above average snowfall was recorded in the mountains of the Columbia River watershed during the winter of 1947–48 resulting from more low pressure systems traversing the Pacific Northwest than normal.

[2] Unusually cold temperatures were recorded in Eastern Washington during the spring months, helping deep snow to remain in place into May.

During this time, the flood risk was noted by several government agencies, including the Department of Lands and Forests in British Columbia and the U.S.

[5][6] This air was generally sourced from around 60°N in latitude, which is much further north than what is expected for the time of year, driven by persistent low pressure over the Canadian Rockies.

[2][8][9] Many streamflow gauges on larger waterways like the Columbia recorded near-record high discharge rates, only being beat by major flooding in 1894 and 1876.

[10] The Snake River, which is the largest tributary to the Columbia, and its watershed did not experience widespread flooding thanks to lower snowpack in East Idaho and far western Wyoming.

Small cities in the Inland Northwest, such as Bonners Ferry, Idaho, Union, Oregon, and Libby, Montana were flooded by the rivers and creeks they sit on.

[1] Floodwaters gradually continued downstream, forcing evacuations around the Tri-Cities as early as May 27 and closing the highway between The Dalles and Cascade Locks in Oregon on the 28th.

[1] Despite frequent assurances from officials, levees protecting Vanport, a World War II era housing project, failed, completely destroying the city.

[12] Flooding in the Tri-Cities peaked on May 31 with Kennewick taking the brunt of the water because a larger portion of the city lies at a lower elevation compared to Pasco and Richland.

A joint commission involving the United States and Canada had been formed in 1944 to study improvements that could be made to the Columbia River system, and it gained further urgency following the flooding in 1948.