Channel migration zones (CMZs)
Shoreline Master Program
(SMP) Guidelines include requirements for
new shoreline master programs to address channel migration zones (CMZs).
Requirements in the Guidelines to address channel migration do not change
the statutorily defined shoreline jurisdiction. Areas that could
legitimately be considered within the CMZ of a river
may extend beyond shoreline jurisdiction. (Channel
What is a channel migration zone?
Dynamic physical processes of rivers can cause channels in some areas
to move laterally, or "migrate," over time. The area within which a river
channel is likely to move over a period of time is referred to as the
channel migration zone.
Channel migration can occur gradually, as a river erodes one
bank and deposits sediment along the other. The natural meander patterns
of stream channels are the result of the dissipation of energy of flowing
water and the transportation of sediment. Channel migration also can occur
abruptly, as the river channel shifts (or "avulses") to a new
location. Avulsions are usually unpredictable events that occur during
high flood flows when the existing channel cannot transport all of the
water and sediment supplied to it. The highest rates of channel migration
generally occur where steep rivers flow out of foothills onto flatter
Why address channel migration in Shoreline Master Programs (SMPs)?
Interference with the natural process of channel migration often has
unintended consequences for human users of the river and its valley
such as increased or changed flood, sedimentation and erosion patterns. It
also has adverse effects on fish and wildlife through loss of
critical habitat for river and riparian dependent species. Failing to
recognize the process often leads to damage to, or loss of, structures and
threats to life safety.
The general requirement established in the Guidelines is for local SMPs to
include provisions that limit development and shoreline modifications that
would result in interference with the process of channel migration that
may cause significant adverse impacts to property or public improvements
and/or result in a net loss of ecological functions associated with the
rivers and streams (see specific policies below).
How should local governments identify CMZs
under Ecology's SMP Guidelines?
CMZs are defined specifically in the Guidelines as
"the area along a river within which the channel(s) can be reasonably
predicted to migrate over time as a result of natural and normally
occurring hydrological and related processes when considered with the
characteristics of the river and its surroundings."
The Guidelines require CMZs be established "to
identify those areas with a high probability of being subject to channel
movement based on the historic record, geologic character and evidence of
past migration." The Guidelines include the following considerations in
identifying these areas:
- For management purposes, the extent of likely migration along a
stream reach can be identified using evidence of active stream
channel movement over the past one hundred years. Evidence of active
movement can be provided from historic and current aerial photos and
maps and may require field analysis of specific channel and valley
bottom characteristics in some cases. A time frame of one hundred years
was chosen because aerial photos, maps and field evidence can be used to
evaluate movement in this time frame. It should also be recognized that
past action is not a perfect predictor of the future and that human and
natural changes may alter migration patterns. Consideration should be
given to such changes that may have occurred and their effect on future
- In some cases, river channels are prevented from normal or historic
migration by human-made structures or other shoreline modifications. The
definition of CMZ indicates that in defining the extent of one, local governments should take into account the river's
characteristics and its surroundings. Unless otherwise demonstrated
through scientific and technical information, the following
characteristics should be considered when establishing the extent of the
CMZ for management purposes:
• Within incorporated municipalities and urban growth areas,
areas separated from the active river channel by legally existing
artificial channel constraints that limit channel movement should not be
considered within the CMZ.
• All areas separated from the active channel by a legally existing
artificial structure(s) that is likely to restrain channel migration
and is built above or constructed to
remain intact through the one hundred-year flood, should not be
considered to be in the CMZ.
• In areas outside incorporated municipalities and urban growth areas,
channel constraints and flood control structures built below the one
hundred-year flood elevation do not necessarily restrict channel
migration and should not be considered to limit the CMZ unless demonstrated otherwise using scientific and technical
SMP Guidelines references to CMZs
The following is a general guide to locating specific references to
CMZs in the SMP Guidelines. All
requirements must also be read in the context of the entire section and
the overall principles established in the guidelines. CMZ provisions apply
to the extent that identified CMZ fall within Shoreline Management Act (SMA)
Definition and identification
Inventory and characterization
- Critical area provisions:
- Flood hazard provisions:
173-26-221(3)(b): Establishing general principle that SMP should limit
development and shoreline modifications that would result in
interference with the process of channel migration that may cause
significant adverse impacts to property or public improvements and or
result in a net loss of ecological functions associated with the
rivers and streams.
173-26-221(3)(b)(i) - (vii): Describes more specific flood hazard
prevention principles, including
encouragement to plan for and facilitate removal of artificial
restrictions to natural channel migration.
173-26-221(3)(c)(i): Standard generally prohibiting new development in
shoreline jurisdiction where it would require new dikes or levees
within the CMZ. Includes list of specific developments that may be
appropriate exceptions to the standard.
- Modifications and Use provisions:
For more information
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