Merced River Restoration - Technical Findings

 
 

The modern landforms of the Yosemite Valley floor are primarily created through two processes—the lateral delivery of sediment from the steep bedrock walls, contributed primarily by episodic rockfalls and debris-flow fans built by the tributary channels, and the downvalley transport and deposition of sediment by the Merced River. The river emerges from the primary influence of coarse-grained glacial-age sedimentary deposits in the reach between Clarks Bridge and Sugar Pine Bridge, and it continues downvalley largely encased by its own floodplain deposits except where locally impinged upon by valley-wall deposits.

The most prominent, widespread valley-bottom feature is a terrace of fluvial sand and gravel with a surface typically 2-2.5 m above the modern summertime baseflow of the river through the Study Area, and on which most of the Valley’s iconic meadows have developed. This surface is still a zone of active deposition, at least episodically.

 
Map showing surficial geomorphic units by color and key in part of Yosemite Valley.
A portion of the geologic map of Yosemite Valley in the vicinity of Ahwahnee Bridge (in the center of the map) and Sugar Pine Bridge.
 

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.

 
An index map of restoration concept design reaches.
Index map of restoration concept design reaches.

D. Boothe

 

Channel Sediment Characterization

This diagram displays the median sediment diameter (D50) found in the channel bed at points along the Merced River, displaying the monotonic reduction in sediment size as the river enters Yosemite Valley above Happy Isle Bridge and declines in gradient as it reaches the valley floor.

 
A Map showing distribution of bed sediment sizes through the project area.
Distribution of bed sediment sizes through the project area.

D. Boothe

 

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 have used a meander migration simulation model to investigate such tendencies, and to identify areas along the river where natural geomorphic processes are particularly susceptible to the influence of artificial bank-hardening structures.

The following image shows the types of output that the meander migration model provides for the study reach on the Merced River.

 
Meander migration patterns conceptual image; shows a conceptual meander migration pattern over 35 years with no restraints considered
Meander migration patterns conceptual image, plotting river centerline positions over 35 years of migration. Blue = original position; red = final position. This displays the projected migration pattern under an assumption of no artificial bank armoring.

E. Larsen

Last updated: October 20, 2020

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