I went back to the PDF. I think they are stating 16 feet as the normal level of flooding?
The Columbia River has a long history of flooding. Flood stage for the river, as routinely measured near the Port of Vancouver, is 16 feet. It was recently exceeded in May‐June 2011.
The following are the 20 highest historical crests of the Columbia, as well as what you can expect when the river reaches certain levels.
All information is courtesy of the National Weather Service.
1) June 13, 1948 31.00 feet
2) June 1, 1948 30.80 feet
3) Dec. 25, 1964 27.70 feet
4) June 4, 1956 27.60 feet
5) Feb. 9, 1996 27.20 feet
6) June 19, 1933 26.30 feet
7) May 31, 1928 26.20 feet
08) June 12, 1921 26.00 feet
9) June 26, 1950 25.90 feet
10) June 16, 1903 25.60 feet
11) Jan. 19, 1965 25.44 feet
12) June 22, 1917 25.30 feet
13) June 3, 1916 25.30 feet
14) June 13, 1913 25.20 feet
15) Jan. 3, 1997 22.55 feet
16) June 12, 1972 21.50 feet
17) June 22, 1974 21.10 feet
18) Jan. 24, 1970 19.54 feet
19) June 5, 1997 19.03 feet
20) Dec. 1, 1995 18.50 feet
Above 16 feet
Expect some flooding of islands and low areas, with minor impacts for parks and trails along the Columbia River.
Above 17 feet
Expect flooding of some lowland access roads, parking areas and trails on Sauvie Island, around Vancouver Lake and near downtown Vancouver. Access to some houseboat communities will be affected, along with recreation and camping areas at Cottonwood Beach near Washougal and on Government Island.
Above 18 feet
Expect flooding of undiked lowland, pasture and farmland to begin. Historically, the main areas affected have been from Sauvie Island downstream.