Interactions of wood accumulations, channel dynamics, and geomorphic heterogeneity within a river corridor

Natural rivers are inherently dynamic. Spatial and temporal variations in water, sediment, and wood fluxes both cause and respond to an increase in geomorphic heterogeneity within the river corridor. We analyze 16 two-kilometer river corridor segments of the Swan River in Montana, USA to examine relationships between wood accumulations (wood accumulation distribution density, count, and persistence), channel dynamism (total sinuosity and average channel migration), and geomorphic heterogeneity (density, aggregation, interspersion, and evenness of patches in the river corridor). We hypothesize that i) more dynamic river segments correlate with a greater presence, persistence, and distribution of wood accumulations; ii) years with higher peak discharge correspond with greater channel dynamism and wood accumulations; and iii) all river corridor variables analyzed play a role in explaining river corridor spatial heterogeneity. Our results suggest that decadal-scale channel dynamism, as reflected in total sinuosity, corresponds to greater numbers of wood accumulations per surface area and greater persistence of these wood accumulations through time. Second, higher peak discharges correspond to greater values of wood distribution density, but not to greater channel dynamism. Third, persistent values of geomorphic heterogeneity, as reflected in the heterogeneity metrics of aggregation, interspersion, patch density, and evenness, are explained by potential predictor variables analyzed here. Our results reflect the complex interactions of water, sediment, and large wood in river corridors; the difficulties of interpreting causal relationships among these variables through time; and the importance of spatial and temporal analyses of past and present river processes to understand future river conditions

Methods
This data was collected using field and remote sensing methods. 

To provide spatial context for the measurements of wood distributions, geomorphic heterogeneity, and channel dynamism along our 32-km study reach, we segmented the study reach at uniform 2-km intervals prior to data collection. The downstream-most 8 segments were selected based on the naturalness of the river corridor and the presence of abundant large wood accumulations in the active channel(s). We focused on these segments for ground-based measurements. We subsequently expanded analyses to include an additional eight upstream segments. These segments were included because of anecdotal evidence of at least localized timber harvest in the river corridor, bank stabilization, and large wood removal from the active channel. We included these sites to provide a greater range of values within some of the variables analyzed and thus potentially increase the power of our statistical analyses.

Wood accumulations and beaver modifications

We conducted aerial wood accumulation surveys using available Google Earth imagery between 2013 and 2022 (four years of available imagery: 2013, 2016, 2020, 2022). We mapped all logjams that could be detected via the aerial imagery. Wood accumulations that were under canopy, too small for the spatial resolution of imagery, not interacting with base flows, or containing less than three visible wood pieces were not included. We recorded the number of wood accumulations per 2-km segment for each available imagery year as a minimum wood-accumulations count and divided the wood count by floodplain area for each segment to get the wood distribution density. We also noted the occurrence of persistent wood accumulations that were continually present in the Google Earth imagery, in what we refer to as “sticky sites”. GPS coordinates of wood accumulations were collected in the field during August 2022 to verify imagery identification.

We also manually identified active and remnant beaver meadows using Google Earth. Similar to large wood, American beaver (Castor canadensis) both respond to spatial heterogeneity in the river corridor (e.g., preferentially damming secondary channels) and create spatial heterogeneity through their ecosystem modifications. Beaver-modified portions of the river corridor (beaver meadows) were identified based on presence of standing water in ponds with a visible berm (beaver dam); different vegetation (wetland vegetation including rushes, sedges, and willow carrs that appear as a lighter green color in imagery) than adjacent floodplain areas; and detectable active or relict beaver dams (linear berms with different vegetation than adjacent areas). Several of the sites identified in imagery were also visited in the field to verify identification.

Channel dynamism and annual peak discharge

Channel dynamism was quantified using metrics of active channel migration and total sinuosity over time. To measure active channel migration, we developed a semi-automated approach to map surface water extent and planimetric centerline movement, which are commonly used to understand morphological evolution in rivers. We followed existing methodologies using base flow conditions as a conservative delineation of planimetric change given our goal of looking at relative channel change over time to understand which segments of our study area were the most dynamic.

Surface water extent was delineated for 2013, 2016, 2020, and 2022 to keep the timestep consistent with our wood surveys. Imagery collected for the National Agriculture Imagery Program (NAIP) was used when available (2013 and 2016). For 2020 and 2022, cloud-free multispectral composite images were created in Google Earth Engine (GEE) from Sentinel-2 imagery from average baseflow months (August-October). Surface water was classified using the normalized difference water index (NDWI) (Gao, 1996) for NAIP imagery, and modified normalized difference water index (MNDWI) in Sentinel-2 imagery. A unique threshold was empirically determined for each year to optimize the identification of the river surface while minimizing false-positive water identification, resulting in binary water and non-water masks for each year. Gaps and voids in the Sentinel-2 derived water masks (from shadow-covered areas, thin river segments, or mixed pixels along the river edge) were filled by sequentially buffering the water areas outwards by 30 meters (three pixels) and then inwards by 15 m.  Similarly, gaps and voids in NAIP-derived water masks were filled using a sequential 20 m outwards then inwards buffer. The resulting binary water masks were imported into ArcGIS Pro and vectorized. Manual adjustments were made to remove any remaining misclassified areas and join disconnected segments.

We delineated centerlines of our channel masks using the ArcGIS Pro Polygon to Centerline tool. When multiple channels were present, the dominant channel branch was chosen for the channel centerline. Consequently, our analysis represents a minimum value of channel migration during each time step because it does not include secondary channel movements. The Feature to Polygon tool was used to extract area differences between two centerlines at each segment. Areas between the centerlines for each segment were divided by centerline length to get a horizontal change distance.

We measured total sinuosity in each 2-km segment for 2013, 2016, 2020, and 2022 using Google Earth imagery and the built-in Measure tool in Google Earth. We measured total sinuosity as the ratio of total channel length of all active channels/valley length.

We obtained annual peak discharge from the nearest US Geological Survey gauge (12370000, Swan River near Bigfork, MT). This site is below Swan Lake, a natural lake, into which the Swan River in our study area flows. Consequently, the gauge records reflect relative inter-annual fluctuations in peak discharge, but not actual discharge at the study site. We used annual peak discharge for the same time intervals used for analyzing channel position. 

Geomorphic heterogeneity 

We performed an unsupervised remote sensing classification on a stack of data containing a 2022 Sentinel-2 imagery mosaic prepared in GEE, and normalized difference vegetation index (NDVI) and normalized difference moisture index (NDMI) rasters calculated from the Sentinel-2 mosaic in ArcGIS Pro. The Sentinel mosaic was prepared for the approximate growing season in Montana, USA, (June 1 to October 31) based on annual phenology activity curves (2018-2022) of the existence of leaves or needles on flowering plants. The unsupervised classification was completed on the floodplain extent of the Swan, delineated manually in ArcGIS Pro using the 10-m 3DEP DEM, hillshade prepared from the DEM, Sentinel-2 imagery, and ArcGIS Pro Imagery basemap as visual references.

Although the classification is unsupervised, the classes were intended to represent distinct types of habitats within the river corridor that blend geomorphic features and vegetation communities as observed in the field, including, but not limited to: active channels, secondary channels, accretionary bars, backswamps, natural levees, old-growth forest, wetlands, and beaver meadows. The ISO Cluster Unsupervised Classification ArcGIS Pro tool was used to perform the classification. Inputs to the tool were a maximum of 10 classes, a minimum class size of 20 pixels (tool default), and a sample interval of 10 pixels (tool default). The entire reach was classified once, and then clipped into individual 2-km segments. The classified Swan raster was brought into R for statistical analysis of heterogeneity metrics. Data were visualized using the tidyverse and terra packages. All heterogeneity metrics were calculated using the landscapemetrics package using the Queen’s case.

Statistical analyses

Statistical analyses were conducted in R. The data we collected span different time intervals, and we conduct our statistical analyses to match the temporal and spatial scales of data we have for each of our hypotheses. We used an alpha (probability of rejecting the null hypothesis when the null hypothesis is true) of 0.05 in all statistical analyses.

To understand the influence of time on our river corridor variables, we examined whether there was significant variation in the medians of river corridor variables between timesteps using a Kruskal-Wallis Rank Sum test. For any variable where there was a significant change between timesteps, we used a Dunn Test to determine exactly which groups were different. We also conducted the same exploratory statistical analysis to understand whether there was any significant variation in medians for each variable between segments. 

To understand the predictors of channel dynamism (hypothesis i), we created mixed-effect models with total sinuosity or average channel migration as response variables and logjam distribution density, count, persistence, and flow as fixed effects and segment as a random effect. We performed an AICc model selection, corrected for small sample size to provide a relative indication of the quality of statistical model for our given set of data. We included segment as a random effect in our mixed effect models to account for any potential spatial autocorrelation between segments. 

To address hypotheses i and ii, we calculated both Pearson (r) and Kendall (τ) correlation coefficients. Given our small sample, we report both r and τ values. All correlation coefficients were calculated using the cor.test() function in base R. Hypothesis iii was further addressed through mixed effect models and AICc model selection to determine whether proportion of beaver meadows, sticky sites, total sinuosity, channel migration, wood distribution density, or wood count are clear predictors of patch density as a spatial heterogeneity metric (response variable). We only used data from 2022 to keep a consistent timestep across all variables. We included segment as a random effect in our mixed effect models to account for any potential spatial autocorrelation between segments.