GENERAL42732

• Permit No: M1976032
• IBM Index Class Name: General Documents
• Doc Date: 6/5/1996
• From: USFS
• Media Type: D
• Archive: No

The braided channel downstream from where the active bank erosion is occurring
will likely remain, until such time as the bedload is reduced to the point that
the river can transport it through these flatter reaches. Or, until such time
as a major hydrologic event reworks the channel to create a shape that will
mare efficiently move the bedload.
The vegetation occupying the now abandoned floodplain will likely change in
response to the lowering of the water table. The species most affected will be
narrow-leaf cottonwood and willows. While these species may not disappear,
they will be Less vigorous due to moisture stress. The effects on riparian
dependant vegetative species will occur for all alternatives, because the
elevation of the channel bed and, therefore, the water table, will remain
approximately where it is now.
The San Miguel channel above the confluence will continue to be a very unstable
system. In large part, this is a function of the alluvial fan the river has
created. During runoff events, when out of bank flows occur, there is no
lateral confinement to prevent the river from seeking a new course. As a
result, significant channel adjustments will continue, particularly during high
runoff years. This situation has been compounded by the removal of gravel
downstream, which has over-steepened the gradient, particularly on the lower
half of the fan where most of the channel changes have occurred. The presence
of very large boulders are preventing any signficant downcutting in the fan,
but the lateral migration will continue.
The over-all affects of the No Action Alternative would be: 1) increased
channel instability with increased erosion rates; 2) increased downstream
sedimentation rates and associated channel adjustments; 3) loss of terrestrial
(riparian) and aquatic habitats resulting on reduced populations of aquatic
organisms; 4) visual and aesthetic value impairment; 5) headword cutting of
streams far removed from the gravel operation; and 6) lowering of the water
table with gradual changes in vegetative composition of previous floodplains
and low terraces.
Alternative 2. RESTORE AQUATIC/RIPARIAN SYSTEM TO A STABLE CONDITION.
MAINSTEM RECONSTRIICTED TO A 'B' CHANNEL TYPE. (Preferred alternative)
Under this alternative, a single step-pool "B" channel type will be constructed
on the mainstem above the confluence, which will replicate the type of channels
that are associated with alluvial fans. This channel will be constructed
following the configuration of the existing channel and the capacity will be
great enough to contain annual peak flows nine out of ten years. In order to
create a single channel at the lower end of the mainstem, where multiple
channels now exist, large quantities of fill will be needed to replace the
confining terraces that have washed away.
The design of the South Fork, the mainstem above the confluence, and the
mainstem below the confluence is displayed in the attached drawings. This
alternative differs from Alternative 3, in that the mainstem, above the
confluence, will be stabilized in its present location by reconstructing the
existing channel to meet designed flow specifications. This alteration would
be accomplished by using boulders and rock weir vortices as shown in the plan
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