Re-introduction of large woody debris to New England rivers by Tropical Storm Irene

Woody debris is a critical geomorphic and ecologic element of rivers that provides habitat complexity, sediment storage, and downstream flood abatement. However, the pre-Anthropocene role of wood in Northeast US rivers is poorly understood due to centuries of wood removal and reduced wood sources from logging and flood control, which reduces flows that would otherwise strip wood from channel margins. Studies of present day wood loading in Northeast rivers generally indicate small amounts of wood and few channel-spanning logs or logjams. Hence, wood has limited geomorphic effects in Northeast rivers, especially compared to rivers in other temperate forests, such as in the Pacific Northwest. Here we examine the effects of flooding during Tropical Storm Irene in 2011 on wood and the post-flood human response in seven rivers in the Deerfield watershed in western Massachusetts using repeat aerial photos, interviews, and field surveys. Woody debris accumulations of 25 to >200 logs, which we called `megajams' were observed on each river following Tropical Storm Irene, with a frequency of up to 1.2 megajams per river km. Landslides and channel margins were wood source areas. Approximately 17% of megajams were deposited directly at the toe of landslides, and 83% were fluvially deposited at river bends. Each megajam caused local geomorphic effects along 50 to 150 m of channel length by storing sediment, redirecting flow, narrowing channels, increasing local channel slope, and creating pools. These local effects created complexity along up to 10% or the total channel length of some rivers. The prevalence and effects of megajams were unexpected compared to previous studies in the region, and suggest that wood played a much larger geomorphic role in Northeast rivers prior to the arrival of Europeans. Within three years following the flood, up to 75% of the megajams were removed or depleted by humans on some rivers, demonstrating the ability of humans to rapidly deplete wood loading. The remaining megajams showed little to no decay or depletion five years after the flood, suggesting these features are persistent in the absence of human modification. Furthermore, the prevalence and beneficial function of the megajams signals that re-creation of these large woody features should be considered in river restoration projects.