Sediment mobility and river corridor assessment for a 140-km segment of the mainstem Klamath River below Iron Gate Dam, CA - vegetation mapping 2016

This report documents river response to hydrologic disturbance along a 140-km segment of the mainstem Klamath River below Iron Gate Dam, CA by assessing fundamental indicators of active sediment transport and dynamic changes in riparian vegetation. A combination of field and remote sensing methods were used to document river response to natural rainfall-runoff events (e.g., 2006 and 2017) and managed flow releases (e.g., 2016 to 2019). Field data provide direct evidence of near-bed sediment transport, episodic scour and fill, and surface flushing of alluvial margins. Remote-sensing data provide spatially-continuous summaries of alluvial features and dynamic vegetation changes. These physical datasets validate channel maintenance flows proposed by Shea and others (2016). Sediment mobility thresholds defined at two USGS gaging stations were 5,210 ft3/s at Iron Gate Dam and 8,810 ft3/s at Seiad Valley. Peak flows during water years 2006 and 2017 resulted in 1 to 2 feet of aggradation of the channel bed that was followed by scour during the intervening dry years. Episodic scour and fill resulted in no long-term trend in the mean bed elevations. Surface fines along alluvial margins were flushed by high flows in 2017 with a recurrence interval 5 years. From 2005 to 2009, riparian vegetation was scoured by high flows in 2006 with recurrence intervals of 5 to 10 years, which was followed by vegetation encroachment from 2009 to 2016. Dynamic channel changes caused by high flows with recurrence intervals of 5-years or more initiated dynamic channel changes. The study results will be useful for interpreting linkages among physical and biological processes and evaluating the effectiveness of flow management decisions that impact endangered salmonid populations.

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