CHANNEL MIGRATION, ASSOCIATED EROSION RATES, AND THEIR APPLICABILITY TO RIPARIAN FOREST MANAGEMENT

Where rivers migrate laterally, they can erode adjacent forest lands on which riparian buffers are retained between rivers and logging sites.  To provide corridors of mature forest along rivers prone to lateral migration, Washington Forest Practices Rules prohibit most logging in channel migration zones (CMZs), which are defined as “the area where the active channel of a stream is prone to move and this results in a potential near-term loss of riparian function” (WAC 222-16-010).  Given an overbank area A located outboard of initial channel margins and eroded by channel migration during time t, along valley length l, the average width W, of overbank area eroded along the valley length equals A/l, and its average rate of erosion r, along the valley length equals W/t.  Erosion rates of floodplain and terrace along a 1 km section of the Greenwater River between river km 4 and 7 (drainage area about 175 km²), King and Pierce counties, Washington, were determined in this manner by mapping the extent of erosion evident on aerial photographs taken between 1962 and 1996.  Mapped patterns of erosion indicate channel migration proceeded by avulsion, as well as by bank erosion.  Between 1962 and 1996, composite erosion of floodplain and terrace occurred at an average rate of 0.5 m/yr per bank.  That rate pertains only to the progressive attrition of vegetated floodplain and terrace located outboard of the river’s banks in 1962.  It does not reflect subsequent erosion of vegetated surfaces bound by 1962 channel splays, or erosion of any area subsequent to its initial erosion by channel migration during the period. 

 

CMZs that adequately account for the progression of channel migration would enable rivers to recruit large-diameter trees from riparian forest that is allowed to mature absent disturbance by logging or channel migration.  Large pieces of wood derived from mature forest contribute to the formation of stable wood jams, which enhance the diversity of riverine habitat and promote deposition of substrate on which new forest can develop and mature.  Analyzing the historical record of erosion caused by channel migration provides an empirical basis for estimating the rate at which channel migration is likely to proceed, assuming persistence of the hydrologic and geologic conditions under which historical rates were actuated.