7720

• UCREFRP Catalog Number: 7720
• Author: Sinning, J. A. and J. W. Andrew
• Title: Habitat Enhancement for Colorado Squawfish in the Yampa River in Conjunction with Railroad Construction
• USFW Year: 1979
• USFW - Doc Type: Mitigation Symposium
• Copyright Material: YES

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a biological standpoint, such a backwater
design cannot be done with absolute certainty
for the Colorado squawfish because the rearing
habitat requirements are incompletely known.
Since squawfish larvae have been found in
relatively shallow backwaters which are
largely dry during late fall and winter low
flows, it was decided to try and duplicate
these parameters in the artificial backwaters.

Natural backwaters in which squawfish
have been found are usually open to the main
channel sufficiently that a small water
circulation (from percolation at the up-
stream end) prevents stagnation. While they
may have somewhat irregular bottom profiles,
it was felt desirable to construct the arti-
ficial backwaters with a sufficiently
regular, U-shaped profile to allow easy use
of management techniques such as seining or
block netting the open end of the channel.
Since natural backwater formation processes
could not be duplicated, percolation of
~~~pr through the upstream end was designed
into the upstream-end darn between the main
channel and the backwater.

Another aspect of the backwater design
was that they should be easily removeable if
they proved unuseable for squawfish rearing
either because of de facto operational
characteristics or because they encouraged
the proliferation of exotic competitive
species such as reds ide shiners. This can
be accomplished, based on the premise that
the darns could be physically breached with
relatively Ii ttIe effort and equipment by
allowing natural sedimentation processes to
return the backwater to a close approxi-
mation of its original condition.

Since the possibility of constructing
backwater habitat enhancement areas was
recognized relatively early in the railroad
design phase (just after route selection),
incorporating the backwater designs into the
overall design \o,'as not a difficult task.
Potential sites were chosen from stereo
pairs of aerial photos. The choice criteria
were the presence, configuration, and size
of presently non-flowing side channels.
These areas were chosen so that construction
activity (and thus ecological disturbance)
could be minimized. Another advantage of
choosing old side channels was that since
flow at one time created the channels,
redirection of in-channel flows would be
less, and backwater stability would likely
be enhanced. Sediment data and river bed
profiles were obtained during a field visit
at which time the two best locations for
backwater development were chosen based on
professional judgement. At each location,
four profile transe~ts were measured across
the main and side channels.

Geomorphic changes in the channel were
determined, based on sediment load and size
distribution and channel geometry, for the
mean annual hydrograph and the 1 in 25 year
flood hydro graph . The model used provided
one-dimensional sediment routing, uncoupled
from water routing, for each requested
discharge. The model was operated with both
natural (existing) conditions and with the
porous dams in place, and the results with and
without the dams were compared.

The maximum change in water surface
elevation with the dams in place was less than
one foot for the 1 in 25 year discharge of
15,250 cubic feet per second. The change in
channel geometry (deepening) as a result of
the backwater dams was also less than one
foot. The model also indicated that water
surface elevations during the average annual
peak flow of 7100 cubic feet per second would
be about two feet over the top of the upstream
dam. Since the dam construction was specified
of material of sufficient size; this over-
topping would not cause loss of the dam.
Because the adjacent land area (islands)
forming the backwater are also overtopped
during these peak flows, raising the dam
height would not prevent annual peak flows
from entering the backwater area. By allowing
the peak flow to pass over the backwater dam
and through t~ backwater, some sediment
transport will occur through the backwater and
more natural conditions will result. The
downstream dam will not be overtopped by the
average peak flow, but a flow of 1 in 25 years
will overtop the dam, Thus during most years,
sediment deposition at the lower end of the
backwater is likely. This indicated that more
maintenance would be likely at the downstream
backwater area.

Based on the predicted water surface
elevations, some erosion potential of the
islands at the ends of the dams was predicted,
and riprap protection was recommended. Ear
formation and some channel agradation wi thin
the backwater areas was also predicted to
occur during the descending limb of the annual
hydrograph. This is expected to be primarily
at the downstream end of the backwater area
and will require periodic (though not neces-
sarily annual) maintenance. Maintenance can
be accomplished with small size earth moving
equipment during low flows and is not likely
to result in significant disturbance to the
ecology of the area.

As is usual for new mine construction and
operation, a variety of permits and approvals
are necessary. The agency with primary
approval responsibility for railroad con-
struction was the ELM. Since bridge con-
struction and channel encroacl~ent of
navigable waters were involved in the con-

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