5. Summary
In this study, we developed and applied an approach to assess the
strength of hydrologic connections between a source and target sites
within a river-floodplain system using field-based indicators. We
defined the source as the river at the upstream boundary of the
river-floodplain system with target sites both downstream within the
river channel and laterally distributed across the floodplain. Using a
field-based dataset, we generated empirical models to describe target
site-specific connectivity strength as a function of source stage, and
then predicted daily connectivity strength for five years between May
and September of 2016 through 2020. Within this approach we also tested
the use of aquatic microbiomes as a metric of hydrologic connectivity.
By examining similarity in microbial membership, we accurately assessed
the presences/absences of surface flows from the source to target sites
and found that aquatic microbiomes can provide additional information on
residence time dynamics along connected surface flow paths.
Our results demonstrate that connectivity in aquatic water bodies in the
river-floodplain system can either be stable or intermittent.
Intermittently connected target sites can differ widely in source
connection/disconnection thresholds. While some target sites demonstrate
binary connectivity behavior quickly changing between high and low
connectivity states, others display gradual behavior with substantial
durations at intermediate connectivity levels. When aggregated to the
river-floodplain system scale, mean system-scale connectivity increased
with stage due to the influence of averaging across sites with
heterogeneous conditions. However, we demonstrate that mean behavior is
a poor descriptor of river-floodplain system behavior because spatial
distributions of connectivity across all sites tend toward bimodality at
intermediate and lower flows, reflecting conditions when river stage is
sufficient to enable connectivity for only a portion of the system. As a
result, the spatial heterogeneity of connectivity state peaked at
intermediate river stage values. We also demonstrate that connectivity
regimes are sensitive to inter-annual variation in streamflow and that
while differences in the magnitude of peak flow are important so are
changes to the flow duration across the range of differing flow states.
Therefore, predictions of how river-floodplain connectivity will change
due to climate change and/or other disturbances to hydrologic regimes
need to consider the impacts across the full range of potential flows.