Restoration of Fish Habitat in Relocated Streams
CONSTRUCTION OF RELOCATED CHANNELS WITH HABITAT STRUCTURES
When a highway is built near a stream there is always a risk of ecological damage to the stream,
even if the highway does not encroach on the stream channel or its flood plain. Most of this
damage is from sediment. Runoff from the newly graded slopes and ditches carries clay and silt
into the stream, where it can cause turbidity dangerous to fish and smother aquatic vegetation and
invertebrate life. Sediment can also fill pools and cover spawning areas.
Minimizing Sediment Damage
A good soil erosion and sediment control plan will prevent much of the sedimentation caused by
road construction. Such a plan will provide for prompt seeding and mulching of the newly-graded slopes, and for control of run-off by temporary berms, interceptor ditches, hay check dams
and settling ponds. For best results, the main features of the plan should be developed by the
highway agency concurrently with the road design. The specifications should give the project
engineer and contractor considerable latitude to adapt the plan to local conditions in the field.
New channels for stream locations should be excavated in the dry if possible. Where work must
be done in the existing channel, as for example when starting a retaining wall or riprap, work
should be scheduled during extreme low water periods, and the stream diverted, possibly through
a temporary culvert. The effluent from operations which generate large amounts of sediment,
such as gravel mining and washing, should be diverted into settling ponds before release into
Protection of Streamside Vegetation
No streamside vegetation should be unnecessarily destroyed. The project engineer should mark
the construction limits for the highway, any necessary channel relocations, and all permissible
haul roads. The contractor should confine his operations within these limits. Trees that are to be
saved should be clearly marked and protected from damage. Small trees and other usable plant
material within construction limits can often be removed in advance of excavation and saved for
revegetating the new channel slopes, haul roads and disturbed areas. Where the toes of
embankments are near streams care should be taken to avoid damage to bank-side vegetation.
Seasonal Work Schedules
Sediment greatly reduces the chances for survival of fish eggs in the spawning beds.
Construction operations which produce sediment should be coordinated with state wildlife
agencies and, if necessary, suspended or curtailed during the spawning season.
Stakeout for Construction
In the design process, stream relocations may be divided into fairly long reaches of uniform
average gradient. However, they do not have to be built this way, and, in fact, a smooth, uniform
gradient is rarely found in natural streams. Furthermore, a natural stream may have a variable
width and side slopes, depending on the resistance of the bed and bank materials to erosion. The
stakeout of such an irregular channel should be a flexible process, and the project engineer
should have considerable freedom to make minor changes in alignment, gradient, width or bank
slopes. For example, where the new channel will be in hard materials such as boulders or large
gravel, the side slopes may be staked vertical. Where the banks will be of finer materials, flatter
slopes may be used for stability. The gradient may be varied slightly from the average gradient
for short distances without seriously affecting discharge capacity. The staked channel should be
reviewed by the hydraulic engineer before construction begins.
Placement of Habitat Structures
A newly excavated stream channel lacks most of the features that make for good fish habitat.
However, by adding check dams, current deflectors and large rocks, the erosive power of the
stream can be harnessed to create scour holes, plunge pools, undercut banks, spawning beds, and
other features of a good fishing stream.
Check dams and deflectors must be embedded in the bed and banks of the channel as shown in
Chapter 4 to prevent undermining and washout. They must therefore be constructed before the
stream is turned into the new channel, and there is no opportunity to vary their locations
experimentally to produce the best habitat effects. Check dams required for gradient control
must be built where the designer has shown them on the plans. The designer should also indicate
the approximate locations of check dams and deflectors intended primarily to enhance habitat.
However, the field engineer should have some flexibility to move these to fit the as built channel.
Ideally, the best time to place large habitat rocks would be after the channel has experienced one
or two annual floods. This would give the stream time to adjust to the new channel and develop
some armor. Also, if the rocks are placed while the stream is running, their effect can be
immediately observed and they can be arranged more effectively. However, in practice it may be
difficult to defer the placement of large rocks. Most stream relocations are constructed by
contract, and it may be very costly to bring the contractor back a year later to place rocks. It
would also increase their cost to stockpile the rocks and place them later by the maintenance
forces or by separate contract. Also, unless the stream relocation is easily reached from a road,
the placement of the rocks might involve the destruction of much bank vegetation.
Generally, the most practical solution will be for the designer to estimate a suitable number of
large habitat rocks in each reach of the new channel, and leave their placement to the discretion
of the field forces. These rocks can most easily be placed before the stream is admitted to the
channel, when the channel bed is available as a haul road. Some oversight from a hydraulic
engineer, geologist or other person familiar with stream processes may be desirable.