Parker et al. (2007) derived regime equations from natural British and US-American Rivers. The regime equations allow estmating the width, depth and slope of single-thread rivers by knowing the bankfull discharge and median grain size of the bed surface only. These equations may be used to estimate the width of a river needed after restoration of the riverbanks.
Parker, G., Wilcock, P. R., Paola, C., Dietrich, W. E., & Pitlick, J. (2007). Physical basis for quasi‐universal relations describing bankfull hydraulic geometry of single‐thread gravel bed rivers. J Geophys Res, 112, F04005, doi:10.1029/2006JF000549.