7 Quaternary Geology, Environmental Geology, and Geomorphology (Posters)

Monday, 20 March 2006
8:00 AM - 12:00 PM

REGIONAL CHANGES IN CHANNEL BEHAVIOR DUE TO HISTORICAL DEFORESTATION AND REFORESTATION IN THE NORTHEASTERN UNITED STATES

Paper number: 7-9
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The Northeastern United States was largely deforested in the nineteenth century and the early portion of the twentieth century due to the boom of the timber industry and increased agricultural practices. Most of these areas were reforested later in the twentieth century. As a result of these human-induced changes, the sediment loading in local streams underwent drastic changes, from a highly increased load after deforestation to a drastically reduced load following reforestation. These fluxes in sediment loading appear to have induced major changes in the erosional and aggradational patterns of these streams. A reconnaissance study of streams in the Susquehanna River and Connecticut River indicate that watersheds may be returning to their original equilibrium conditions. The evidence of these historical impacts can be found as distinct low terraces (Qa2) which are currently experiencing substantial erosion. The Qa2 surfaces are characterized by low clay contents, little soil development, and abrupt contacts with underlying older sediments (Qa1). These characteristics imply a rapid deposition and a relatively young age of the Qa2 layer. The older underlying sediments (Qa1) are usually Pleistocene clays or gravel with a buried paleosol.

The approximate ages of these low terraces were determined by dendrochronology, with buried trees approximating the onset of deposition, and with trees rooted on the low terraces approximating the end of rapid aggradation. Minimum rates of recent erosion were determined by dividing the lengths of exposed roots by the ages of trees adjacent to the channels. Some streams in the Appalachian and Berkshire Plateau regions are characterized by abundant mid-channel and point bars composed of gravel (Qa3). The source of gravel for this aggradation appears to be the low terraces, which are simultaneously being eroded. Many streams show extensive disequilibrium reaches where gravel bars have aggraded to the floodplain level or above. Major channel avulsions have created braided or anastamosing conditions as streams shift from a single channel meandering pattern to a wide and shallow pattern. The implications of these changes are extremely important in future river management issues, implying that reach-based approaches to stream restoration may be inappropriate due to the complex response of these systems.