Hook, Abigail. 2006. The recent history of floodplain dynamics in the North Fork Skykomish River, Washington. M.S.
Read the full thesis here. (1.4MB pdf)
In the 1998 Skyforks Watershed Analysis, the morphology of the midsection of the North Fork Skykomish was identified as a data gap. Researchers suspected that there had been a change in the river dynamics, as there were reports of the mainstem widening close to 30% between the 1960s and 1990s, but did not know the cause. The change in channel character was of particular concern because the reach in question supports runs of Chinook (Oncorhynchus tshawtscha), coho (Oncorhynchus kisutch), chum (Oncorhynchus keta) and pink (Oncorhynchus gorbuscha) salmon as well as winter steelhead (Oncorhynchus mykiss). Additionally, landowners with private inholdings on the river had lost parts of their properties to bank erosion and in some cases, had been forced to move their homes away from the river.
The lower and midsection of the North Fork Skykomish is a series of transport and floodplain reaches (river kilometer 0 to 10.6). The ability of water and sediment to move through the system is influenced by instream woody debris, channel pattern, slope, and bank vegetation. There is a feedback cycle between the morphological forcing factors (water, sediment, and woody debris) and the channel morphology and patterns. In order to understand this feedback relationship, it is necessary to examine as many of these variables as possible as they change through time. In the case of the North Fork Skykomish, I compared site specific data to existing floodplain dynamic theories to better understand how the system has evolved over the last 50 years.
The purpose of this study is to:
1) identify floodplain morphological and channel pattern adjustments from 1962 – 2003, and
2) examine how forcing factors (water and large woody debris) in 3 different floodplain reaches in the North Fork Skykomish River have changed over the same period through reconstructing flows, and recording changes in large woody debris loading.
Though the North Fork Skykomish is currently relatively unmanaged, it has a history of logging and in-channel woody debris removal (USFS, 1998). More recently, the system has experienced several large flood events (USFS, 1998). Though the debris removal and floods have been recorded, there has been no attempt to explain how the magnitude of each disturbance and timing with relation to one another has changed the channel character over time. By relating changes in flood events and wood loading and their timing relative to one another to floodplain evolution in the North Fork Skykomish River, land managers and researchers can better understand how the effects of modifying floodplain conditions or changing flow regimes influence similar Pacific Northwest montane river and floodplain systems.
Reference
United States Forest Service. 1998. Skyforks Watershed Analysis. Skykomish, WA.