2.2.3. Mapping Inundation
For each survey, we mapped inundation by digitizing a polygon around the
wetted edge visible in the aerial imagery. We inferred visible
boundaries where vegetation or shadows obscured the water’s edge. We
estimated inundation area uncertainty for each survey based on the
resolution of the imagery used to digitize survey features. This
procedure resulted in two separate buffered polygons representing the
upper and lower bounds of maximum and minimum inundation within the
valley bottom.
We delineated each inundation survey polygon into three flow type
classes on a continuum from more lotic (free-flowing) to more lentic
(ponded, but still flowing). We defined these classes as follows: 1)
free-flowing – not obstructed by river channel or by a channel-spanning
structural element, 2) overflow – flow that is spilling over
active channel boundaries and onto the floodplain or otherwise exposed
in channel surfaces (e.g., bars, benches and/or ledges) and 3) ponded– structurally forced backwater creating a pond or pool
upstream of a channel-spanning structural element (e.g., a beaver dam;
Wheaton et al., 2015). Once inundation types were classified, we used
these data to derive the total area of each inundation type. We then
divided inundated area by the valley bottom area which gave us the
percent of both total inundation and each inundation type, allowing for
comparison of inundation across reaches. We also estimated the
integrated wetted width by dividing the total inundated area by the
valley bottom length.
To characterize the diversity of inundation types (a proxy for habitat
complexity), we used the Shannon’s Evenness Index (also referred to as
Shannon Equitability or Shannon Evenness) to calculate a value for each
site and survey, a metric frequently used to describe spatial
heterogeneity (e.g., Laurel & Wohl, 2019; Wyrick & Pasternack, 2014).
The Shannon’s Evenness Index value is calculated as follows:
\begin{equation}
SHEI=\ \frac{-\sum_{i=1}^{m}{(P_{i}*lnP_{i})}}{\ln v}\nonumber \\
\end{equation}where Pi is equal to the proportion of the valley
bottom occupied by each inundation type i and v is equal
to the number of inundation types present in the valley bottom.
In our study, v was equal
to four to include the three inundation types (free-flowing, ponded,
overflow) and dry conditions.