Upstream-downstream water quality comparisons of restored channelized streams

Abstract Although a large number of river restoration projects have been implemented around the world, quantitative outcomes of the projects are often not well documented. The main purpose of this study is to assess changes in water quality of two completed river restoration projects, which convert heavily channelized streams into more natural conditions using nature-based solutions. Another purpose is to examine factors behind changes in water quality, based on which to identify possible ways to improve the design and operation management of similar projects in the future. An upstream-downstream sampling approach was used, making it possible to directly compare changes in water quality before and after the restored sites by reducing variations due to seasonality, land use, and other uncontrollable external factors. A number of water quality parameters were tested, and statistical analysis was performed to evaluate the effects of the restored sites and the correlations of water quality parameters. General improvements in water quality were found at one of the restoration sites, but not at the other. It was found that not only water quality but also the effects of restoration on water quality is subject to seasonal variations. Correlations between certain water quality parameters point to some common sources of pollution in the urban context. Possible causes behind the performance difference of the two sites are discussed, and improvements on design and operation management practices are recommended.