3.3. Valley Plug Inundation
The valley plug reach of river measured as 5.5 km2(7.36 km) with a slope of .0016 (Figure 5; Table 2). We collected and analyzed imagery from 2009 (pre-valley plug) which showed that at moderate flow (8.07 cms), the SRR’s inundation was contained entirely within a single free-flowing primary channel (integrated wetted width = 15.8 m). The inundated area was measured to be 116,292 m2 or 2.1% of the valley bottom.
We collected imagery for spring of 2015 from Google Earth during a low flow period (0.26 cms). Inundation increased from 2.1% to 26% (1,428,893 m2) of the valley bottom (Figure 6A). Of that total inundated area, 7.1% (101,889 m2) was free-flowing and 92.9% (1,327,004 m2) was overflowing. Integrated wetted width increased to 194.1 m from 15.8 m. In analysis of the 2021 drone-collected imagery when flows were at 0.40 cms, we observed a 45% (1,997,523 m2) increase in total inundated area, with 62.3% (>3,426,416 m2) of the valley bottom inundated. Of this 62.3%, 3.2% (>110,528 m2) of inundation was free-flowing, 96.3% (3,300,537 m2) was overflowing and 0.5% (>15,350 m2) was ponded. We observed a 700% increase in the number of active channels, and observed a visible increase in beaver activity with eight new and intact beaver dams actively ponding water.
Although the total surface area of free-flowing inundation decreased from 2009 to 2021, flows were dramatically different between these two time periods (8.07 cms in 2009 versus 0.40 cms in 2021), and a decrease in inundation was expected. Even with these dramatic differences in flow, we observed that ponded and overflow area increased over 11 years by 2,846.4%. We also observed that changes to the diversity of inundation types were reflected in an increase of the average Shannon’s Evenness Index value from an average of 0.007 in 2009 to 0.057 in 2021 (Figure 6B). All measured metrics of change from 2009 to 2021, along with site-specific constants are summarized in greater detail in Table 2.