Opportunities and challenges for restoration of the Merced River through Yosemite Valley, Yosemite National Park, USA

Abstract

Successful river restoration requires understanding and integration of multiple disciplinary perspectives, including evaluations of past and ongoing watershed processes, local geomorphic response, and impacts unique to human activity. Nowhere is this more apparent than along the Merced River in Yosemite National Park, USA, where both an outstanding natural landscape and the consequences of over a century of human disturbances continue to interact. An intact upstream watershed highlights the importance here of local impacts on geomorphic response. Incision and the resulting decoupling of the channel from its adjacent late‐Holocene floodplain are consequences of reduced channel roughness, likely from de‐snagging the river, and instream gravel mining in the 19th and early 20th century. Riparian‐zone disturbance by visitor use has damaged riparian vegetation and soils, inducing channel widening. Revetments and channel‐spanning bridges, the latter being visible and oft‐cited impacts to fluvial processes, have distorted the natural evolution of meanders and induced local channel narrowing. The historical rate of sediment export from Yosemite Valley has greatly exceeded replenishment from upstream and lateral sources, creating a deficit that now inhibits recovery via passive restoration of more natural channel form and function. Climate change may amplify now‐diminished fluvial processes but also exacerbate the rate of sediment export. These conditions, reflecting a complex intersection of geologic history, modern geomorphic processes, and human interactions, demonstrate how a limited influx of sediment coupled with intensive human use can have long‐term consequences for riverine conditions, restoration opportunities, and social engagement with the riverine landscape.

Rediscovering, reevaluating, and restoring Entiatqua: Identifying pre‐Anthropocene valleys in North Cascadia, USA

“…In this, our findings are comparable to those of Booth et al (2020), who concluded that human activities such as those known to have impacted the ERV caused the Merced River to incise and partially disconnect from its historical floodplain. Specifically, they found that alluvial deposits 2–2.5 m above baseflow are now activated only half as frequently as they were a century ago.…”

“…Our investigation of the ERV and more parsimonious analyses in other valleys suggests that GGL-REMs can be a valuable tool in revealing the history, valley process domains, and full restoration potential of these systems. For example, GGL-REMs correspond with surficial geologic mapping of the Yosemite Valley at a resolution comparable to that reported here for the ERV (Booth et al, 2020;Figure 10a), while a similar agreement has been found with coarser resolution geologic mapping of the Methow and Twisp River Valleys (USBR, 2009; Figure 10b), and even with General Land Office mapping of the Willamette River (Sedell & Froggatt, 1984; Figure 10c).…”

“…GGL‐REMs compared with (a) surficial geology of the Merced River Valley, Yosemite, CA (Booth et al, 2020). (b) Coarser geologic mapping of the Methow and Twisp River Valley, WA (USBR, 2009).…”

“…Effective implementation of process‐based restoration requires robust conceptual and analytical frameworks and analysis of the governing processes at multiple spatial and temporal scales (Beechie et al, 2010; Cluer & Thorne, 2014). Moreover, successful outcomes require integrated analysis of biophysical and historical data to interpret past and ongoing watershed processes and trajectories and the signatures of human activities (Booth et al, 2020; Woelfle‐Erskine, Wilcox, & Moore, 2012). In the context of process‐based restoration, acquiring a sound understanding of geologic, historic, and current channel and valley floor forms and functions is critical to restoration design (Walter & Merritts, 2008).…”