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Impacts of seasonal flooding disturbance on floodplain habitat dynamics

Date: July 16, 2019

Floodplain habitat composition and abundance are influenced not only by simple inundation, but also by the creation of new and different habitats that differ in whether they are sensitive or resistant to flooding disturbance


Clark Fork River hydrograph for the study reach in 2014. Black dots represents sample dates when aerial imagery was collected.
Clark Fork River hydrograph for the study reach in 2014. Black dots represents sample dates when aerial imagery was collected. Credit goes to: Katelyn P. Driscoll
Seasonal bankfull flooding is a common disturbance in floodplain landscapes. Along with other processes, regular flooding generates shifts in the location of diverse aquatic and terrestrial floodplain habitat patches. Despite this spatial dynamism, the habitat types and their relative abundance remain relatively constant in the long-term. This phenomenon, known as the shifting habitat mosaic, is well established and is a critical process in riverine systems. However short-term patterns and processes that occur during seasonal flooding are less understood. It is possible that on the short timeframe of a bankfull flood disturbance, the floodplain landscape does not follow the shifting mosaic model and habitats destroyed in one location are not necessarily created in another. Understanding the link between changes in discharge and landscape patterns during a seasonal flood provides insight into the annual flux and spatial change in habitats, as well as the long-term structure and function of floodplains.

To improve understanding of floodplain habitat dynamics during disturbance, we collected aerial imagery of the Clark Fork River in western Montana on six dates in 2014 that spanned the seasonal flood. We georectified raw imagery to create nearly seamless digital images for each sample date. We used imagery from base flow conditions prior to the flood to identify three zones: the main channel, the parafluvial zone with recently scoured and deposited cobble and early stage vegetation present, and the orthofluvial zone with gallery forests, old channels, and herbaceous surfaces. We established permanent plots within these zones and the plot and zone boundaries were held constant throughout the flood pulse. We identified 17 different aquatic and terrestrial habitats from the photos using spectral signatures. We measured and mapped changes in the areal abundance of each habitat type within each floodplain zone from April through September. We tracked transitions in habitat type within each plot using alluvial diagrams to understand the development of individual habitat types during flood disturbance. Finally, we calculated two measures of habitat diversity (α and β) to understand how short-term floodplain complexity is impacted by seasonal flooding.

Example of habitat classification using spectral reflectance for two plots in the orthofluvial zone.
Example of habitat classification using spectral reflectance for two plots in the orthofluvial zone. Credit goes to: Katelyn P. Driscoll, Andrew Hauer, Ric Hauer

  • Floodplain habitat types differ in sensitivity to flooding disturbance. For example, in this study the total area of pools and large woody debris did not change because bankfull flooding simultaneously destroyed and created these habitat types. There were also no significant changes in the abundance of mature vegetation, orthofluvial ponds, or orthofluvial springbrooks. This was due to the location of these habitats in the orthofluvial zone, where past studies have shown habitat turnover is uncommon. Relatively stable and consistently abundant habitat features that do not change during seasonal flooding can influence ecosystem function by retaining organic matter and providing habitat refugia for aquatic species.
  • The total areas of shallow shores, riffles, runs, overbank flow, cobble bars, and early successional vegetation were variable during the seasonal flooding disturbance in this study. These habitats did not follow the shifting habitat mosaic model, meaning more habitat was created than destroyed and abundance increased or more habitat was destroyed than created and abundance decreased. Additionally, backwaters, parafluvial ponds, and parafluvial springbooks were observed to be small, highly transient, and extremely sensitive to small changes in discharge. 
  • Habitat change during seasonal flooding is highly influenced by location of the habitat on the floodplain. For example, the areal abundance of springbrooks and ponds was either resistant or extremely sensitive to disturbance depending on their location in the orthofluvial or parafluvial zone.
  • The creation and increase in abundance of new habitat types in the parafluvial zone during intermediate flows on the rising and falling limb of the hydrograph generated significantly higher α habitat diversity than the main channel or orthofluvial zones. Similarly, new habitat types were created in the orthofluvial zone during peak flow, producing significantly higher α habitat diversity than the main channel.
  • β habitat diversity significantly changed during flooding within all three floodplain zones. This result highlights the importance of habitats that are highly transient or have relatively low abundance. For example, when the diversity of the main channel is low during flooding, the increased number of habitats in the parafluvial and orthofluvial zones, including those that are small, short-lived, or both, can provide critical habitat with different water temperatures, velocities, organic matter retention, nutrient processing, or sedimentation rates.  These habitats promote high biodiversity by preventing competitive exclusion and have the potential to leave a legacy of patches with different substrates, nutrient availability, and depth to the water table that could influence successional trajectories and the long-term structure and function of the ecosystem.

Driscoll, Katelyn P.; Hauer, F. Ric. 2019. Seasonal flooding affects habitat and landscape dynamics of a gravel-bed river floodplain. Freshwater Science 38(3):000-000.

 

Alpha habitat diversity calculated using Shannon’s diversity index for the main channel, parafluvial, and orthofluvial zone.
Alpha habitat diversity calculated using Shannon’s diversity index for the main channel, parafluvial, and orthofluvial zone. Credit goes to: Katelyn P. Driscoll
Beta habitat diversity for each sample date within the main channel, parafluvial, and orthofluvial zone. β=0 indicates a high frequency of plots with entirely different habitat composition. β=1 indicates a high frequency of plots with the same habitat com
Beta habitat diversity for each sample date within the main channel, parafluvial, and orthofluvial zone. β=0 indicates a high frequency of plots with entirely different habitat composition. β=1 indicates a high frequency of plots with the same habitat com

 



Principal Investigators: 
Principal Investigators - External: 
Rick Hauer - Univeristy of Montana
Research Location: 
Clark Fork River, Montana