The closest shuttle stop is E5, the Four Mile Trail Head
on the El Capitan Shuttle. This route runs only during the
summer sesason.
Many of the geologic features commonly associated with Yosemite
Valley were formed hundreds of thousands to millions of years ago.
However, the story of Yosemite Valley did not stop. Geologic
processes continue to reshape the landscape of Yosemite. The Merced
River continues to flow through the valley causing erosion and
deposition in various locations and at different times. This
EarthCache examines the formation of
river meanders in the Merced River as it flows
across the valley floor.
But the current geologic processes must work on the landscape
that has its origins in the past glaciations of the valley. Various
glaciations carved the steep-walled Yosemite Valley. Following the
last glaciations, an end moraine formed across the valley from
about El Capitan to Bridalveil Falls. This moraine dammed the
Merced River forming a lake that extended 5.5 miles up the valley.
While this lake existed for only a few thousand years (very short
in geologic time), between a few hundred feet to almost 2000 feet
of sediment was washed into the valley filling in the floor. This
lake sediment creates the relatively flat floor of the valley,
which is actually not characteristic of a U-shaped glacial
valley.
It is on this flat topography that the
Merced River slows and begins to meander. In areas of low
slope, the path of a river begins to wander back and forth
creating meanders, or curves. Along each curve, the flow of
the river is constantly eroding the outside bank of the
meander, while at the same time depositing material on the
inside. This is because the area of fastest flow in the river
is forced to the outside bank on the curves and the slowest
flow is on the inside. Faster water carries more sediment
causing the erosion. The slower water is unable to carry as
much sediment, so some sediment drops out of the water. The
result is a steep bank on the outer edge of a meander and a
gentle slope on the inner edge.
Over time, meanders get larger more
pronounced as the outer edge is eroded away and the inner edge
has material deposited on it. Given enough time, the meanders
could get so curvy that they bend back and touch itself
creating an oxbow lake.
An animation of river meander formation can be found at
http://www.cleo.net.uk/resources/displayframe.php?src=309/consultants_resources%2F_files%2Fmeander4.swf
In an area that floods regularly such as the meadows on the
floor of Yosemite Valley, oxbow lakes quickly fill in with sediment
to become part of surrounding meadow.
Logging requirements:
Send me a note with :
- The text "GC1QN2P Yosemite Valley River Meander" on the first
line
- The number of people in your group.
- At this location, where is the area of most erosion and why do
you think it is located there?
- How far are you from the highest velocity water flow?
- What material are you standing on and why is it located
here?
- How close to creating an oxbow lake is this meander?
The above information was compiled from the
following sources:
- All images from Prof. Stephen A. Nelson, Tulane
University.
http://www.yosemite.ca.us/library/geology_of_yosemite_valley/
Trial listing approved by
Yosemite National Park
