Document Type

Thesis

Date of Degree Completion

Summer 2022

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Lisa Ely

Second Committee Member

Carey Gazis

Third Committee Member

Jennifer Lipton

Abstract

The importance of large wood (LW) in creating channel complexity is widely recognized; however, few LW projects have been in place long enough to track meaningful channel changes on a decadal timescale. Taneum Creek, located in central Washington, is one of the earliest LW restoration areas (2008) in the Yakima River Basin and the central Cascade Mountains. The flood in 2011, with an estimated discharge of 69 m3/s (2,400-2,800 cfs), provided further channel change by mobilizing LW and channel sediments. Three reaches with similar channel characteristics and LW additions were compared with a control reach without LW additions to document this annual channel change. The effect of LW on annual floodplain connectivity was further assessed using a normalized difference vegetation index (NDVI), which represents density of greenness and plant health. This index is used as a proxy for floodplain ‘greenness’, which will help illustrate floodplain connectivity. In response to the large flood of 2011, LW created new channel complexity, such as significant increases in multi-threaded channels in each of the LW study reaches, except the control reach, as well as side-channel formation which allowed for beaver dam construction. Of the side channels that formed in the LW reaches, 50% or more formed 10 m downstream of LW jams. Sinuosity increases were not uniform among the different reaches with fluctuating increases and decreases. The reaches with increased channel complexity related to the LW and large flood also increased in floodplain greenness and connectivity. This increase is likely a result of the floodplain inundation that increased delivery of water to side channels and beaver ponds and perhaps raising the local groundwater table. The results of the study indicate that the reaches with LW additions increased in channel complexity and groundwater-floodplain connectivity following the large flood, which is important for maintaining diverse aquatic and riparian species and possible aquifer recharge.

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