Hydrodynamic connectivity between a river and its floodplain is critically important to floodplain ecosystems. Previous research has shown that altered flow conditions and geomorphically disconnected floodplains are detrimental to native biota. Because previous metrics often focus on habitat type, there has been a call for quantification techniques that focus on hydrogeomorphic processes important to broader ecosystem health. The objective of this study was to quantify changes in floodplain dynamics due to hydrologic alteration. Steady-state hydrodynamic modeling was conducted for two river systems in New Mexico, the Gila River and Rio Grande. Scripted algorithms were developed to extract model outputs and combine outputs with long-term mean daily flow data of baseline and altered flow conditions for each of the rivers. Selection of these rivers allowed for the comparison of two systems that differed in size as well as form and flow alteration. Floodplain connectivity was assessed with a focus on spatial and temporal inundation dynamics and channel-floodplain interface dynamics. Analysis of interface dynamics focused on quantification of mass and momentum flux, as the flux of water and associated materials is critical to many ecological and geomorphic processes.
Results/Conclusions
Floodplain inundation frequency results show impacts on both river systems. In the geomorphically unaltered Gila River, small hydrologic alterations lead to reduced inundation frequency of high-flow side channels, while in the heavily manipulated Rio Grande, large-scale flood protection severely limits inundation of elevated floodplains along incised portions of the reach. Temporal analysis of cumulative inundation under altered flow conditions showed similar trends in both systems although at different scales due to the size of the systems. Spatial examination of channel-floodplain interface mass flux displays important areas of floodplain connectivity. The degree of change in cumulative mass and momentum transfer differed between the two systems, likely because of extreme geomorphic manipulation along the Rio Grande and the removal of many peak flow events. The work done in this research provides a novel approach to spatiotemporal investigations of floodplain connectivity.