2.6 Multiple Year Comparison
To compare connectivity dynamics across multiple years, we made use of five years of seasonally monitored water levels (May through September) at Inflow, starting in 2016 through 2020. These stage records were collected with TruTrack Capacitance Rods and were summarized to mean daily stage. As several data gaps in the record exist, we imputed gaps using spline regressions with daily streamflow records from a nearby USGS gage at the Big Thompson River below Moraine Park, Colorado (USGS #402114105350101) (Figure S5). As seasonal hysteresis was observed in the relationship between the North St. Vrain and Big Thompson rivers, spline regression equations were fit separately for the rising limb and falling limb, which were respectively defined as prior and post peak seasonal flow at the North St Vrain.
From this five year record of Inflow stage, we used the connectivity strength models developed for each target site within the 2018 study period (described above) to predict daily connectivity strength values at each target site across all five years from 2016 to 2020 between May 01 and September 30th. To examine how much inter-annual variability was observed at target sites within the river-floodplain system, we calculated the duration of high connectivity (σm ≥ 0.6), intermediate connectivity (0.4 < σm< 0.6), and low connectivity (σm ≤ 0.4) at each target site for each year. Intermediate connectivity was chosen as between 0.4 < σm < 0.6 because analysis of the connectivity strength functions suggest that at low connectivity σm stabilized between 0.2 to 0.4, and at high connectivity between 0.6 and 0.8. To examine the effects of inter-annual streamflow variability across the river-floodplain system, we calculated the daily fraction of target sites with high, intermediate and low connectivity. Only the seven target sites with observed intermittent connectivity in 2018 were included in this portion of our analysis.
All analyses and related figures were generated in R version 3.6.1 (R Core Team, 2016).