Document 40: Flood Problems
Hiram M. Chittenden, ed. The Duwamish-Puyallup Flood Problem (Seattle: Lowman and Hanford S. and P. Co., 1907), p. 10-11, 21-22.
The last of these floods, that of November 1906, was a good example. The rainfall of September was six inches on the western slope of the Cascades, 75 percent, greater than normal. For October is was 7.5 inches, 35 percent, above the normal, and the greatest for that month in 17 years. The watershed was therefore thoroughly soaked at the beginning of November, the streams were all in moderate flood state, and the conditions the worst possible to meet the emergency which followed…
The result of this combination of circumstances was that for several days the streams were swollen beyond their capacity and everywhere in the lowlands overflowed their banks. The quantity of drift transported was enormous and the channels were partially or completely blocked in hundreds of places, thus greatly increasing the amount of overflow. Highway and railroad bridges were destroyed or badly damaged; grades were washed out; many buildings were destroyed or their foundations badly shaken; hundreds of acres were cut away by the currents and many thousands of acres covered with water which left deposits of sand and silt varying from a few inches to several feet in depth. The total area of overflow in the valley between Seattle and Tacoma was not less than 50 square miles, or 32,000 acres…
As always happens where a stream overflows in this way, its energy was dissipated, it lost the power to carry along its load of drift and sediment and the channel became choked up to a greater or less extent. All along…the river the channel was completely demoralized, the banks were cut to pieces and the bed filled with sandbars and driftwood.
With the drift question disposed of, and the Black and White Rivers diverted to other outlets, the Duwamish will have to carry only the water of the Green River, estimated at 15,300 second feet in flood. As nature has accustomed this channel to carry a much larger volume, comparatively little additional work will be necessary to make it contain the flood flow of the Green. No cutoffs are recommended solely for the purpose of facilitating the flow of water. There are, however, a few bends of very sharp curvature where the danger of cutting banks, stoppage of drift and breaking of levees is so great that it is desirable to cut them out. The straightening of the river in the lower stretches of the Duwamish will be of much advantage in many ways and it ought to be done sooner or later, even if not for the purpose of carrying floods…
[Statistics illustrate] that four and one-half miles of cut will shorten the river by about ten and one-half miles and will save 269.7 acres in channel area. The gain in land area would not mean much until the old channels are filled up, but it is quite probable that this will result along the lower stretch of the Duwamish at a comparatively early period owing to the contracted area of the valley and the value of the land…