Role of tree dams in the construction of pseudo-terraces and variable geomorphic response to floods in Little River valley, Virginia

Abstract

Geomorphic response to a 1985 flood in Little River valley, northern Virginia, was different in magnitude and style from the largest historic flood in the same valley in 1949. The primary geomorphic activity during the 1985 flood was severe bank erosion and channel-gravel deposition rather than the debris flow and avalanching of the 1949 event. An unusual and widespread phenomenon of the recent flood was that large trees eroded and transported by the floodwater were aligned parallel to the river banks and, at isolated sites, were braced and stacked against trees still standing on the floodplain. Lateral barriers or dams created from these displaced trees allowed the channel to be locally aggraded above the level of the flood-plain. In these reaches, little, if any, river gravel was deposited on the floodplain, even though the adjacent channel floor was raised well above that surface. The river has now shifted around the filled segments, leaving flat, isolated surfaces, underlain by channel gravel, standing above the level of the modern floodplain. These features may be mistaken for terraces alter they become vegetated and the trees bracing the gravels decay. Interpreting these surfaces to be terrace remnants would lead to a faulty reconstruction of geomorphic history in the Little River valley and other valleys where floodplain morphology is controlled by infrequent flood events.