Initial flooding begins when the Willamette reaches 16 feet and the Wheatland Ferry, which links Marion County with Yamhill and Polk counties, ceases operation. At 26 feet, roads bordering the river will flood and at 28 feet there is widespread flooding in the lowlands. Lowland agricultural areas are under water at 30 feet and the Willow Lake Wastewater Treatment Plant is threatened by the rising water. Above 35 feet, the Willamette backs up tributaries and floods residential areas in the Salem and Keizer areas.
Eugene generally experiences flooding in December and January, due to extensive rains and snowmelt. Several reservoirs upstream attempt to protect the city from high water. Danger for Eugene begins once the river reaches 21 feet and is brimming the banks, with possible overflowing in some areas. At 23 feet, the river has overflowed its banks and low-lying roads and property will be flooded. At 24 feet, Coburg Road will be closed and flooding will be widespread. When the river reaches 26 feet, flooding from the Ferry Street bridge to Belt Line Road is likely.
Just below Willamette Falls in Oregon City, there is danger of flooding when the river reaches 27 feet when low-lying areas begin to flood downstream from Oregon City to the town of Lake Oswego. In the past, the areas most effected have been the River Green Golf Course and City Park at Gladstone. Above 32 feet, river islands are threatened from Oregon City to Portland; lowland flooding is more widespread also. At 40 feet, experts at the National Weather Service consider the area at a major flood stage with widespread flooding in lowland areas.
Albany lies along I-5 in the Willamette River Valley between Eugene and Salem. When the Willamette River reaches 25 feet, experts consider the area to be in a flood stage. At this level, expect widespread flooding of low-lying agricultural land; rural and secondary roads are also at risk. Bryant Park is in danger when water levels reach 27 feet and South Albany is also in danger of flooding. At 34 feet and above, the Calapooia River is likely to fill with backwater from the Willamette, flooding residential areas in Southwest Albany.