Restoration efforts along the Merced River are guided by the best available science. A number of research studies are ongoing to support the restoration work and inform future decisions concerning the most effective methods and locations for restoration work as well as the management of bridges and infrastructure along the river. Through these studies both park managers and the public will gain a better understanding of the ecological condition of the river, how the floor of Yosemite Valley was formed, and how the river naturally moves and shapes its channel.
Restoration of the Merced River - Technical Findings and Work in Progress
We document an initial period of relative landscape stability following the retreat of the valley glacier, characterized by valley-bottom aggradation of glacial till, fluvial and aeolian sediments, and lacustrine silts, as well as valley-margin accumulation of talus and fan alluvium. Recessional moraines, episodically emplaced rock avalanches, and alluvial fans altered the free surface flow of the river and controlled its local base level(s). This predominantly aggradational regime then shifted to incision, likely in the earliest Holocene (about 10,000 years ago) due to a diminishing supply of glacial sediment, which left behind a flight of abandoned fluvial terraces, with the active floodplains inset by up to ~9 m.
Cosmogenic 10Be exposure ages from rock avalanche boulders and 14C charcoal ages from deltaic sequences and inset fluvial gravels suggest variable rates of river aggradation and incision throughout the Holocene. Although some incision events likely record local base level changes at the El Capitan LGM recessional moraine, influenced by changes in both natural sediment supply and human activity, the presence of perched, well-developed outwash terraces downstream of the moraine indicates a more regional climatic forcing has likely been active as well in determining the stable gradient of the river over time.
Channel Margin Conditions
Initial studies conducted to support river restoration efforts included a physical survey of existing conditions in the Merced River channel. This census produced initial recommendations on potential opportunities and restoration concepts that holistically addressed the health of the river and riparian corridor.
Channel Sediment Characterization
This diagram displays the median sediment diameter (D50) found in the channel bed at points along the Merced River.
Restoration Concept Designs, March 2016. Folio of Riparian Conditions and Prospective Actions [2.85 MB PDF]
Channel Migration Modeling
The Merced River, like all meandering sinuous channels, tends to erode on the outside of bends, and tends to deposit on the inside of bends. In many places, such dynamic activity is restrained, and yet the tendency remains, creating forces on the banks and any infrastructure that might be there. Planning on the Merced will benefit greatly from understanding the inherent tendencies over time. We are using a meander migration model to investigate such tendencies.
The following images give the conceptual types of output that the meander migration model will provide for the study reach on the Merced River.
The patterns of migration over time as shown can be related to the shear stresses on the banks. Our modeling can also model the shear stress at any one point in time. The following figure gives an example of such modeling. This shows a conceptual mapping of bank shear stress. The darker the colors—deviating from white—the larger the conceptual value of the bank shear stress.
River high-flow measurements
During periods of high flow in the Merced River, researchers collect river velocity measurements and map the extent of the area which the river floods. This work is done to measure how bridges change the flow paths and speed of the river, as well as to validate digital models of the river that are created to learn more about how the river flows and behaves.
Last updated: December 3, 2018