The flood-carrying capacity of the Puyallup River 's leveed channel from its mouth to the city of Puyallup exceeds the 100-yr-flood discharge at most locations. Upstream from the city of Puyallup the flood-carrying capacity also is less than the 100-yr flood at many locations. In King County the flood-carrying capacity would be greater than the 100-year flood nearly everywhere if the levees were in a good state of repair. The flood-carrying capacity of the leveed Carbon River channel is less than the 100-yr flood at numerous locations. Changes between 1976-77 and 1984 in average channel cross-section elevations for the Puyallup and Carbon Rivers were less than 1 ft at more than one-half of 90 surveyed cross sections and were more than 2 ft at only 5 of them. Changes in average elevations for the White River downstream of the inflow from Lake Tapps rose approximately 2 ft or more at 5 of the 10 surveyed cross sections, but decreased 2 ft or more at nearly one-half of 29 cross sections upstream of the reach. Differences between computed 100-yr floodwater-surface elevations for 1976-77 and 1984 channels are similar to changes in average cross-section elevations. Dense growths of streambank vegetation could increase 100-yr floodwater surface elevations by 1.2 ft. (USGS)
