Sensitivity of Floodplain Vegetation to Interannual Climate Variability in Southern Rocky Mountain River Networks

In unconfined mountain valleys, interactions between streams and riparian areas can generate hydrologically connected, physically complex floodplains with high seasonal water tables and extensive wetlands. However, human land use practices, river modification and the extirpation of beaver have caused streams in many of these valleys to incise into their banks, effectively disconnecting streams from their floodplains and lowering riparian groundwater tables. Here, we seek to understand the impact of this loss of river-floodplain connectivity on the sensitivity of floodplain vegetation to inter-annual climate variation. Using hydro-geomorphic floodplain mapping, we first identify the presence and distribution of wider valley segments within basins and the likelihood of hydrologic connectivity between the valley and main river channel. Making use of Landsat imagery from 1986 to 2019, we identify spatial and temporal trends in growing season NDVI across these wide valley segments in the basin and examine how these results relate to floodplain topography. We then calculate sensitivity scores by regressing NDVI with the drought metric Standardized Precipitation-Evapotranspiration Index (SPEI), comparing results from SPEI values calculated across different seasonal time windows. Our preliminary results suggest that increased hydrologic connectivity is linked to lower sensitivity of vegetation productivity to regional drought. Our results further suggest that vegetation in disconnected floodplains is more sensitive to growing season SPEI while vegetation in connected floodplains has relatively higher sensitivity to winter & spring SPEI. These results suggest that improving river-floodplain connectivity can moderate the future impact of drought on floodplain vegetation but under future climate conditions, this effect may be reduced by changes to winter & spring climate conditions.