FOREST INFLUENCE ON FLOODPLAIN DEVELOPMENT AND CHANNEL MIGRATION ZONES

Floodplain development and channel migration within a 30 km section of the Hoh River in northwest Washington is significantly influenced by forest conditions, particularly tree size. Channel migration rates were found to be significantly lower for forested areas where tree diameters exceeded 53 cm than those areas with smaller or no trees. Valley reaches in Olympic National Park where old growth riparian forests remained largely intact experienced no significant change in the width of active channel migration between 1939 and 2002. However in areas where old growth forests were removed outside the park, the width of active channel migration increased by up to 25% during the same time period. Erosion between 1939 and 2002 was almost 4 times greater outside the park, averaging 5.84 hectares/km versus 1.57 hectares/km inside the park. The percentage of non-alluvial bank increases from 4% within the park to 28% outside the park (excluding armored banks), a change that may reflect expansion of the channel migration zone into bedrock or glacial deposits within which the valley is situated. Despite the high sediment supply, actively migrating channel, and narrow valley of the Hoh River, its banks are composed primarily of river alluvium within the park. Additional results of this study suggest that periodic input of large trees into the river create flow obstructions that gradually constrain the zone of channel migration within the valley bottom. Snags and logjams create areas of floodplain refugia that can eventually lead to development of aggradational terraces along the margins of a valley. Removal of mature forests can reverse this process and increase the zone of channel migration within a valley over relatively short time scales. Results of this study have important implications to both land management and geologic interpretations of alluvial deposits.