Critical Areas Information

 (This information is taken from the City Code of Mercer Island Section 19.19.07)

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Areas of natural vegetation are protected in critical areas.  Permits are usually required if changes are to be made in any critical areas.

Critical areas are:

Watercourses

Watercourse corridors are areas of natural or partially altered streams which contribute to water quality and stormwater and erosion control and which provide wildlife habitats. They are characterized by year-round or seasonal flows within stream corridors that exist in an entirely native state or may exhibit a range of alteration.  The watercourse corridor extends 25 feet on both sides of the stream center channel.

  • Permits are required for any alterations that maintain or improve the watercourse channel, including depth, width, length, gradient, and hydrologic conditions; create an equivalent or improved channel bed; and restore or improve native vegetation and fish or wildlife habitat and/or
  • No power equipment shall be used within a watercourse corridor without the explicit approval of the code official
  • The city encourages the opening of previously channelized/culverted watercourses and the rehabilitation and restoration of watercourses.

Wetlands

Wetlands on Mercer Island are characterized by hydric soils, water-tolerant plants (hydrophytes), and surfaces which are either saturated or inundated with water for a minimum period of time. A wetland directly impacts water quality and stormwater control by trapping and filtering surface and ground water. Because of the difficulty in replacing these rare and valuable areas, the regulations control adjacent development and limit the amount of wetlands, which may be altered.

Shorelines

The shorelines are among the most valuable and fragile of the state’s resources.  They are under ever increasing pressure of additional uses.  The shoreline jurisdiction is geographically defined as all lands extending landward 200 feet in all directions as measured on a horizontal plane from the ordinary high water mark and all associated shorelands.

Critical slopes/geologic hazard areas

These critical areas are characterized by lot slope, soil type, geologic material, and groundwater which may combine to create problems with slope stability, erosion, and water quality during natural events such as earthquakes or excessive rain storms.