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Shoreline Management

Channel migration zones (CMZs)

Ecology's 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 Migration Assessment)

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 floodplains.

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 migration patterns.
  • 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 information.

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) jurisdiction.

Definition and identification

Inventory and characterization

Regulatory considerations

  • Critical area provisions:
  • Flood hazard provisions:
    • WAC 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.
    • WAC 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.
    •  WAC 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:

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