4.2 Connectivity Regimes: Scaling from Site to System Scales
Aggregating target site specific dynamics to river-floodplain system
scale behavior is critical for understanding how river connectivity in
floodplains impacts broader landscapes processes. While our analyses
here are limited due to our relatively small sample size of sites,
aggregation of site behavior did reveal important distinctions between
mean system behavior and spatially distributed behavior. As
conceptualized in the flood and flow pulse concepts (Junk et al., 1989;
Tockner et al., 2000), mean connectivity across the river-floodplain
system rose as streamflow increased (Figure 7c). Thus, while
threshold-like behavior was observed at many individual sites, mean
system behavior followed a continuous gradient because the connectivity
thresholds were highly variable among sites. However, it is also clear
that the mean is a poor descriptor of the spatially aggregated behavior,
particularly at lower river flows when connectivity strength values
across the floodplain had a bimodal distribution with some sites
maintaining relatively high connectivity while the majority were
disconnected. This has important implications for scaling many spatially
distributed biogeochemical and ecologic processes impacted by
connectivity such as carbon production and storage, nutrient retention,
and methane fluxes (Lynch et al., 2019; Roley et al., 2012; Samaritani
et al., 2011; Sutfin et al., 2016).
Aggregating site-specific behavior also demonstrated that
river-floodplain system-wide variance in connectivity was maximized at
intermediate river flows. The low variance in connectivity observed at
high river flows (Figure 7c) is consistent with the flood homogenization
theory that physical and chemical states across floodplains are more
similar at high flows (Thomaz et al., 2007). Our results also support
the idea that physio-chemical condition at individual sites in
floodplains are most different from the river at the lowest flows (low σ
values) due to isolation. However, our findings diverge from the
homogenization theory in that peak variability in connectivity dynamics
was observed at intermediate flows rather than low flows as the theory
suggests. Thus, while individual sites might be most different from each
other at lowest flows due to isolation, the distribution of connectivity
dynamics across the floodplain was most variable when river stage was
intermediate and some sites were isolated while others remained strongly
or moderately connected to the source.
4.3 Inter-annual Variability in Connectivity at Site-Specific and
River-Floodplain System Scales
In watersheds characterized by a single large snowmelt event, hydrologic
variability is often driven by inter-annual variation in snowpack
accumulation and melting that regulates the timing and magnitude of
streamflow (Hammond et al., 2018). In floodplains within these
watersheds, whether connectivity regimes are sensitive to this
inter-annual variability will depend on the interactions between
streamflow hydrographs and the physical structure of river-floodplain
connections and corresponding thresholds. As future climate predictions
indicate lower snowpack, earlier snowmelt and drier late season
conditions (Barnett et al., 2005; Stewart et al., 2005), assessing the
degree of sensitivity is important for understanding how future
hydro-climatic regimes may change connectivity in river-floodplain
systems. Our connectivity predictions for five years at intermittent
sites highlight that site-specific and river-floodplain system scale
connectivity regimes are sensitive to streamflow variability with
substantial year to year shifts in the duration of high and low
connectivity. However, within a given river-floodplain system, there
will be spatial variation in sensitivity that will be driven by the
river-floodplain physical structure and corresponding stage thresholds,
but also by the manner of changes in streamflow hydrographs. This can
observed in our dataset by comparing floodplain connectivity in two low
flow years: 2018 and 2020. In 2018, we observed the lowest peak flows at
Inflow in the five year dataset but 2018 had a longer duration of medium
to high flows than was observed in 2020 (Figures 8 & S5). As a result,
a majority of sites remained highly connected for longer in 2018 than
2020, while durations of intermediate connectivity were highest in 2020.
As such, efforts to understand how climate change will alter floodplain
function in snowmelt watersheds will need to consider both changes to
flow magnitudes and to flow durations generated by changing climatic
conditions.