Disturbed stream banks in many portions of Big River are infested with reed canary grass (Phalaris arundinacea) that has altered channel and floodplain interactions. Japanese knotweed (Polygonum cuspidatum) and Giant knotweed (Polygonum sachalinense) are rapidly colonizing portions of the lower mainstem. These non-native invasive plants are competing with native riparian plant colonization of stream banks and floodplains, which can alter floodplain and channel migration dynamics (e.g., floodplain roughness and sediment filtering efficiency; bank stability and erosion rates; and future LWD recruitment).
Colonization of native and non-native vegetation along the lake shoreline may influence sediment particle size distribution along the spawning beaches. Ritchie (2005) and Herrera (2005, 2006) both found increases in shoreline vegetation during the last 50 years. Herrera (2005) determined that vegetation has substantially encroached along the lake shoreline as a result of the lowering of both winter and summer lake levels following large scale wood removal operations in the Ozette River. Vegetation colonization of the spawning gravels can decrease wave energy in and around the spawning gravels, which can result in increased fine sediment deposition. A positive feedback loop can develop between vegetation colonization and sediment deposition: increased sediment improves vegetation colonization, and increased vegetation further increases sediment deposition. The dense root networks of vegetation submerged in the winter act as excellent filters for trapping fine sediment and building up a soil layer over previous gravel bed surfaces. Furthermore, vegetation colonization that blocks or limits wave energy and wave-driven currents can negatively affect sockeye spawning in lake shore areas where sockeye are dependent on wave-driven currents for egg oxygenation.
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