ASSESSMENT OF STREAM CHANNEL DYNAMICS: A PRECURSOR TO EFFECTIVE RIVER RESTORATION

Stream restoration projects in North Carolina, like the nation as a whole, have increased dramatically during the past decade. Many of these projects use in-stream structures and involve a reconfiguration of channel width, depth, slope, and/or planimetric configuration. During this investigation, cross sectional survey data collected annually as part of the North Carolina Ecosystem Enhancement Program were examined for 221 cross sections from 26 sites (27 stream-reaches). Cross sectional channel adjustments were highly variable from site to site, but tended to be large for most locations. More than 60 % of the sites underwent, on average, at least a 20 % at-a-cross section change in channel capacity. Cross sections along older projects with more than three years of monitoring data exhibited at least a 25 % net at-a-cross section change in channel capacity, and most exceeded 35 %. Net changes in channel capacity for the restored reach as a whole were often lower than those calculated for individual cross sections. Differences between these two metrics indicate that sediment eroded at some cross sections, producing increases in channel capacity, was re-deposited at others. Thus, significant re-organization in overall channel form, gradient, and planimetric configuration occurred along many sites by localized and spatially variable amounts of erosion and deposition. A hierarchal cluster analysis, combined with other statistical methods revealed that the magnitude of channel adjustment, and damage to in-stream structures were related to three factors: the stream's ability to erode and transport sediment (as defined by slope, bankfull tractive force, etc.), sediment supply, and bank cohesion. In light of these findings, it is unlikely that stable channels can be effectively designed and implemented along highly dynamic channels characterized by high stream powers, large sediment supplies, and non-cohesive banks. Thus, river reaches need to be screened early in the site selection and design process to eliminate locations which possess a high risk of geomorphic, post-construction instability. It is also clear that in many instances an enhanced natural recovery approach possesses a higher probability of success than currently used approaches based on Natural Channel Design.