The relationship between the drainage area and the magnitude of the flow at bankfull stage in reference stable reaches is key to achieve a baseline for natural stream restoration design. The 1.5-year recurrence flood is considered a representative mean of bankfull discharge for many streams. However, in some cases using the 1.5-yr flood to reference geometry of new channel (restored) resulted in the overestimation in the magnitude of bankfull stage features. A study was performed to determine the recurrence interval that represents the observed discharge at bankfull stage for locations within the different physiographic regions of Tennessee. The field measured bankfull discharge for 96 locations in Tennessee was related to the corresponding flood frequency. Drainage areas for the reference sites ranged from 0.02 to 107-sq. mi., while field measured bankfull discharges for the locations ranged from 2.7 to 4,450 cfs. Before performing an analysis by Ecoregions, an average return period of 1.49-yr appeared to support the assumption that the 1.5-yr flow event approximates bankfull discharge for the streams in the state. Distributed magnitudes of the recurrence interval that represents bankfull discharge increased from west to east as Ecoregions and climate divisions varied in the state. A 1.5-yr return period could be used to determine the magnitude of the bankfull discharge observed in streams within the Ecoregions 68, 69 and 71, while a 1.7-yr recurrence interval should be used for streams within the Blue Ridge region (Ecoregion 66). For streams within the Southeastern Plains (65) and the Mississippi Valley Loess Plain, the return period used to represent bankfull discharge is 1.2-yr. Findings could bring important insights toward a better understanding of the relationship between recurrence intervals and bankfull characteristics, and their importance.
