The western portion of the United States is relatively young geologically. Erosion and deposition are rapid and alter streams, flood conditions and the effectiveness of flood hazard mitigation designs.

We also have oceanic weather conditions, which can lead to widely varying runoff.

The Puget Sound Basin is extremely young geologically. The topography was greatly altered by glaciers that covered all of the Puget Sound Basin with ice up to 5000 feet thick in places. The glaciers carved deep valleys, and let deposits of sands and gravels in higher areas, and sandy and silty lake bed deposits in other areas. Some these deposits are easily eroded.

The glaciation ended about 10,000 years ago. Shortly thereafter, all of the Duwamish - Green River - Kent - Puyallup valley, and the Snohomish River Valley below Carnation, were deep salt water filled fiords.

Infill of the fiords has been rapid, about 1/4 mile per 100 years. For the last 90 or so years the effect or this rapid deposition has been obscured by dredging and channel realignment. The high rate of sediment transport and channel filling results in a continuing need to dredge and raise levees. The Duwamish River from approximately Boeing Field to Elliot Bay is dredged periodically by the US Army Corps of Engineers. The Cedar River has been dredged regularly from its mouth at Lake Washington upstream for about 1 mile. The Corps plans to dredge the lowest approximately one mile of the Cedar River in 1998. The average dredge depth will be about four feet. The Snohomish River has also been regularly dredged in an effort to reduce flood hazard.

Dredging has been significantly reduced in the past 10 years, compared to the dredging done earlier, because of insufficient budgets, and concern about the potential harm of dredging on fisheries.

There is other evidence of continuing rapid infill. Harbor Island and the SODO area in Seattle, and most of the Port of Everett, Sand Island and much of the land at the mouth of the Snohomish river did not exist or were only tide flat and marsh in 1900.

There appear to be some unintended benefits from channelization of the rivers and the straightening and dredging. There now are gravel bars in portions of the Green River in the Kent Valley. These gravel bars are used by Salmon for spawning. The gravel bars indicate that river velocities are higher than would be normal for an unrestrained river in an alluvial valley. Higher river velocity is consistent with channelization of the river and steeper river gradients. Increased gradients indicate channel filling and reduced flood carrying capacity, particularly neat the point where the river first enters the valley.

These severe and extremely rapid geologic changes greatly effect the performance of flood hazard mitigation designs, and need to be given proper weight in evaluating flood control designs and fish habitat enhancements.

by: Roger A. Lowe (rlowepe@main.net)