Seasonal flooding affects habitat and landscape dynamics of a gravel-bed river floodplain

Floodplains are comprised of aquatic and terrestrial habitats that are reshaped frequently by hydrologic processes that operate at multiple spatial and temporal scales. It is well established that hydrologic and geomorphic dynamics are the primary drivers of habitat change in river floodplains over extended time periods. However, the effect of fluctuating discharge on floodplain habitat structure during seasonal flooding is less well understood. We collected ultra-high resolution digital multispectral imagery of a gravel-bed river floodplain in western Montana on 6 dates during a typical seasonal flood pulse and used it to quantify changes in habitat abundance and diversity associated with annual flooding. We observed significant changes in areal abundance of many habitat types, such as riffles, runs, shallow shorelines, and overbank flow. However, the relative abundance of some habitats, such as backwaters, springbrooks, pools, and ponds, changed very little. We also examined habitat transition patterns throughout the flood pulse. Few habitat transitions occurred in the main channel, which was dominated by riffle and run habitat. In contrast, in the near-channel, scoured habitats of the floodplain were dominated by cobble bars at low flows but transitioned to isolated flood channels at moderate discharge. The areal abundance of habitat types in near-channel floodplain areas varied as discharge fluctuated. These areas were most diverse and dissimilar in habitat composition at intermediate flows during both the rising and falling limbs of the hydrograph. The geomorphically stable areas of the floodplain were dominated by herbaceous habitat, and we observed fewer transitions from one habitat type to another during the seasonal flood pulse. However, the largest habitat diversity occurred during peak flow, which highlights the influence of flooding on the development and expansion of a broad array of aquatic habitats during the annual cycle of flooding. We conclude that the response of floodplain habitat abundance and diversity is strongly influenced by location on the riverscape.