FLUVIAL SYSTEM CHANGES IN TUOLUMNE MEADOWS, YOSEMITE NATIONAL PARK, CALIFORNIA, USA, FROM 2006 TO 2018

Tuolumne Meadows, in the high country of Yosemite National Park, is a valuable ecological, hydrologic, and recreational resource which may be affected by visitor activity, land use, and climate change. We report on landscape change along the reach of the Tuolumne River as it traverses Tuolumne Meadows 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 better quantitative understanding of changes associated with fluvial system process and allow for measurement of both streambank retreat and sediment deposition on channel and point bars where they are above water level. Fifteen reaches of the streambanks within Tuolumne Meadows were the loci of erosion between 2006 and 2018. In these areas, streambank retreat rates ranged between 0 and 2.7 m/yr, though most retreat rates are below 0.50 m/yr. From 2016 to 2018, the highest streambank retreat rates were associated with high discharge in spring 2017. However, based on our data, we conclude that deposition on bars balances streambank erosion in Tuolumne Meadows over a period of twelve years. This may indicate that the Tuolumne River in Tuolumne Meadows is in a state of dynamic equilibrium. The existence of paleochannels across Tuolumne Meadows are an indication that channel planform geometry changes have occurred in the past, raising the prospect of rapid channel avulsion from future floods. Based on meadow topography adjacent to the river, there is no indication that channel avulsion, rather than quasi-steady channel migration, is imminent. Nonetheless, land managers should anticipate that floods with discharge greater than peak flow in 2017 may lead to more significant landscape change than was observed in this study.