THE ROLE OF VEGETATION AND BED-LEVEL FLUCTUATIONS IN THE PROCESS OF CHANNEL NARROWING

A catastrophic flood in 1965 on Plum Creek, a perennial sandbed stream in the western Great Plains, removed most of the bottomland vegetatio n and transformed the single-thalweg stream into a wider, braided chan nel. Following eight years of further widening associated with minor h igh hows, a process of channel narrowing began in 1973; narrowing cont inues today. The history of channel narrowing was reconstructed by cou nting the annual rings of 129 trees and shrubs along a 5-km reach of P lum Creek near Louviers, Colorado. Sixty-three of these plants were ex cavated in order to determine the age and elevation of the germination point. The reconstructed record of channel change was verified from h istorical aerial photographs, and then compared to sediment stratigrap hy and records of discharge and bed elevation from a streamflow gaging station in the study reach. Channel narrowing at Plum Creek occurs in two ways. First, during periods of high flow, sand and fine gravel ar e delivered to the channel, temporarily raising the general bed-level. Subsequently, several years of unintermpted low flows incise a narrow er channel. Second, during years of low Bow, vegetation becomes establ ished on the subaerial part of the present channel bed. In both cases, surfaces stabilize as a result of vegetation growth and vertical accr etion of sediment.