The main cause of Northwest floods is the moist air masses that regularly move over the region in the winter. In Wasco County, the weather that produces the most serious flooding events is extensive wet conditions that follow a period of mid and high elevation ice and snowpack development. Riverine and flash floods may both occur in Wasco County. Riverine floods happen when the amount of water flowing through a river channel exceeds that channel's capacity. 

Riverine floods are the most common type of flooding. Flash flooding occurs during sudden rainstorms when a large amount of rain falls in a short period of time. These happen in steeply sloping valleys and small waterways. 

A secondary category of a flood is the stormwater or urban flood. Stormwater flooding occurs when runoff from rainfall concentrates in developed areas, drainage, and low lying areas. Poor drainage, elevated groundwater levels, and ponding are all symptoms of stormwater flooding that can damage property. 

Stormwater flooding should be a concern in Wasco County because of rapid development. In the February 1996 flooding, a surprising number of properties were impacted that were not near a tributary. Instead, these properties were in poorly drained areas where ponding and runoff patterns caused basements to flood and other types of water damage. Not all of this is due to development. Natural soil conditions and geological features often determine drainage patterns.

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