EFFECTS OF RIPARIAN FOREST RECOVERY ON THE DELINEATION OF CHANNEL MIGRATION ZONES

The protection of riparian forests and rehabilitation of riparian habitat through the re-introduction of wood will increase the size and supply of wood to Washington rivers. Because the potential for a channel to avulse and migrate across its floodplain increases with the size and volume of instream wood, the area of the valley bottom potentially occupied by a channel (the channel migration zone or CMZ) will largely depend on the state of riparian forests. The return of riparian forests afforded by current land management practices will increase the volume and caliber of wood entering Washington rivers to a degree unprecedented since widespread clearing of wood from forests and rivers nearly 150 years ago. As the size of riparian trees is allowed to increase, so too will the extent of the CMZ and the hazards associated with this increase. We conducted field surveys, river gauging, and hydraulic modeling of eleven unconfined rivers in Washington State and document increases in bed- and water-surface elevations upstream of channel-spanning wood accumulations that also initiated lateral channel migration. A reasonable estimate of the time required to grow a mature riparian forest that can supply key-member trees can be obtained from a comparison of riparian growth rates, a bankfull depth typical of our unconfined rivers, and the minimum diameter of key-member trees necessary to initiate a significant channel response. By these standards, few mature riparian forests presently exist in Washington State, although they will likely become more common in the next century if they are allowed to grow. We propose conceptual guidelines for the delineation of the CMZs that include allowances for increased vertical fluctuations in channel elevation anticipated as accumulations of large woody debris become more common. Given the potential ecological gains and economic impacts resulting from increased habitat complexity, floodplain inundation, and lateral channel migration, regulatory guidelines for the delineation of CMZs (as well as flood hazard areas) that fail to consider the potential for wood-mediated fluctuations in channel elevation will offer little protection of critical forest habitat and the economic resources linked to the health of riparian forests.