Streambank erosion and related geomorphic change in Tuolumne Meadows, Yosemite National Park, California

Landscape change in Tuolumne Meadows, Yosemite National Park, California, was characterized using data derived from four lidar surveys: one airborne survey in 2006 and three terrestrial surveys in 2016, 2017, and 2018. These surveys were used to generate a better quantitative understanding of changes associated with fluvial processes along the reach of the Tuolumne River within Tuolumne Meadows. This research was performed to provide a scientific basis for restoration and management decisions made by the National Park Service in accordance with the Tuolumne Wild and Scenic River Final Comprehensive Management Plan. A total of 15 reaches of the streambanks along the Tuolumne River in Tuolumne Meadows were subject to measurable streambank erosion between 2006 and 2018. In these areas, streambank retreat rates ranged between 0 and 2.7 meters per year (m/yr), recorded as an average retreat distance along the length of changing streambank position, with most retreat rates being less than 0.50 m/yr. The highest streambank retreat rates are associated with a year of high spring streamflow in 2017. Based on the data available, it was concluded that deposition on channel and point bars balances streambank erosion over a period of 12 years along the Tuolumne River in Tuolumne Meadows. As such, the river could be considered to be in a state of dynamic equilibrium during this period; erosion and sedimentation occur in distinct pulses in response to hydrological forcing but it is not clear that there is a trend towards sediment accumulation or removal in Tuolumne Meadows nor is there an obvious trend toward channel widening or narrowing. The existence of visible paleochannels in the meadow are an indication that more dramatic channel planform geometry changes have occurred in Tuolumne Meadows over an undetermined period and may occur again in the future. Geomorphic change rates relate to hydrology; during the study period, the high water in 2017 led to the highest rates of geomorphic change. Land managers should anticipate that floods with discharge rates greater than the peak flow in 2017 may cause more substantial landscape change than what was observed in this study, but erosion resulting from these events may be balanced by channel and point-bar deposition over a period of years.