DISCHARGE INTERACTIONS AND CONTROLS ON THE RIBBON OF GREEN: RELATING DISCHARGE TO WOODY RIPARIAN VEGETATION TYPE, SAN JUAN RIVER SE UTAH

While many studies have addressed the hydrologic characteristics necessary for the establishment of native woody riparian vegetation, there has been comparatively little research on how variable discharge impacts novel and rapidly-changing forests of mixed native and invasive riparian vegetation. Here we document a model and floodplain inundation at different discharges on a long multi-thread alluvial reach of the partially regulated San Juan River near Bluff, Utah. This is especially important in this area as the increase in vegetation is reducing channel complexity and thus habitat availability for endangered and threatened fish.

Floodplain inundation in this channel/island complex at different discharges is being constrained using (a) one-dimensional hydraulic modeling derived from field-surveyed channel cross sections supplemented with interpolated bathymetric data from lidar point clouds, (b) GIS measurements of inundation extent at different discharges for the last two years from daily Planet Labs satellite data, and (c) GIS measurement of inundation extent derived from airborne lidar at a known discharge. The spatial extent of woody vegetation, and where possible species-level classification are derived from NAIP and 2019 Google Earth imagery and ground-truthed in the field. Overall, we find from the Google Earth imagery and NAIP imagery that there is a mix of riparian vegetation. Russian Olive are found along the banks and channels typically 0.95-1.66 meters above the 1920 cfs discharge level and extend distally ~ 10 m from the water’s edge. Mature cottonwoods are found at higher stages, about 4 meters above the water surface and typically extend ~15m onto the floodplain. Tamarisk extends onto the floodplain roughly 30 meters from the water’s edge at 1920 cfs, whereas willows extend only 3-12m.

This work has broad geographic significance as it relates river discharge and riparian vegetation in managed systems, which may serve as a model for vegetation management on other regulated river systems across the American west.