Performance of Hydraulic Models Compared to Geomorphological and Botanical Approaches to Delineate the Ordinary High Water Mark for Small Rivers

ABSTRACT The ordinary high water mark (OHWM) delineates the water environment from the land environment. The preferred method for the OHWM delineation in Québec, Canada, is a method based solely on bank vegetation characteristics, positioning the botanical OHWM (BOHWM) at the transition from hydrophytic to terrestrial vegetation. However, in anthropized landscapes, the implementation of this botanical method is problematic due to vegetation disturbance on the shoreline or riverbanks. The hydraulic and geomorphological methods position the OHWM at the 2‐year flood line and at the bankfull level of the river, respectively. The aim of this study was to compare the position of the 2‐year flood line to the BOHWM and to the geomorphological bankfull level for rivers located in agricultural, urban and forest areas. Results showed that the best concordance between the 2‐year flood line and the BOHWM was in agricultural areas (mean distance = 1.31 m) and the least in forest areas (mean distance = 3.31 m). The best agreement between the 2‐year flood line and the geomorphological method was found in agricultural (mean distance = 1.27 m) and urban (mean distance = 1.67 m) areas, while these two methods disagreed the most in forest areas (mean distance = 3.55 m).