Causes of Flooding in the City of Heppner

Photo: FoothillsFlooding occurs when climate (or weather patterns), geology, and hydrology combine to create conditions where river or stream waters flow outside of their usual course and “overspill” beyond their banks.  In the City of Heppner, the combination of these factors creates occasional flooding conditions in local streams.  The City of Heppner is located in an area of steep slopes where intense thunderstorms are known to occur.  These intense thunderstorms, when stalled over a creek basin, can create localized flash flooding.  Other, usually minor floods, can occur during the early spring when accumulated snow in the watershed is rapidly melted by warm rains.  This was the case with the 1979 flood.

Flooding historically has been most severe in early spring to late summer when intense thunderstorms stall over a watershed.  The City of Heppner receives approximately 13 inches of rain on average each year.  Large floods result from heavy short duration rains that exceed the capacity of the soil to absorb the precipitation.   Frozen topsoil and rain on snow can also contribute to the frequency of floods.

Riverine flooding and urban flooding are the two types of flooding that primarily affect City of Heppner. Riverine flooding is the overbank flooding of streams, the natural processes of which add sediment and nutrients to fertile floodplain areas. Urban flooding results from the conversion of land from fields or woodlands to parking lots and roads, through which the land loses its ability to absorb rainfall.