Flood producing mechanisms identification in southern British Columbia, Canada A. Loukas a, * , L. Vasiliades b , N.R. Dalezios c a Department of Civil Engineering, University of Thessaly, Pedion Areos, 383 34 Volos, Greece b Department of Management of Environment and Natural Resources, University of Thessaly, Pedion Areos, 383 34 Volos, Greece c Department of Agriculture, University of Thessaly, Pedion Areos, 383 34 Volos, Greece Received 17 May 1999; received in revised form 25 September 1999; accepted 22 October 1999 Abstract The causes of peak flows in two climatically different mountainous-forested basins of British Columbia have been identified. The U.B.C. watershed model was used to identify the causes of peak flows, since this model separately calculates the runoff components, i.e. rainfall, snowmelt and glacier runoff. The results showed that the flood flows in the maritime basin of Upper Campbell are mainly generated by rainfall during the fall months and winter rain-on-snow events. Rainfall runoff constitutes the largest percentage of peak flow for all types of events. On the other hand, the flood flows in the inland basin of Illecillewaet are mainly produced by spring rain and snowmelt events, snowmelt events alone and summer events when runoff from the glacier melt contributes to peak discharge. However, snowmelt runoff is the dominant component of peak flows. Based on these findings, flood frequency analysis showed that considering the flow component frequency distributions marginally improves the probability distribution flows in the two examined watersheds.  2000 Elsevier Science B.V. All rights reserved. Keywords: Peak flows; Hydrologic simulation; Flood causes; Flood frequency 1. Introduction The annual series of peak flows at a station is normally treated as a sample from an implied single population. In fact, most annual flood series in Canada contain floods of two or more physical types; in particular, floods because of snowmelt and rainfall may occur at different times of year and have quite different characteristics. In many cases, it is necessary to categorize the peak flows according to the causes that produce them. Such a case is when, in flood frequency studies, the presence of floods from two or more populations produces a peculiar shape of frequency curve (Watt et al., 1989). The categorization of flood flows into various physical types should provide a better and reliable estimate of the magnitude of design floods which, in turn, is necessary for the design of hydro- technical projects. Another reason for the classification of flood flows is the case of forest management practices. Forest management can increase the magnitude of peak flows by altering a variety of hydrologic processes. For example, removal of vegetative canopy decreases interception losses, increases net precipitation, reduces evaporation and thereby increases soil moisture and, therefore, increases the magnitude of subsequent runoff events (Harr et al., 1975; Troendle Journal of Hydrology 227 (2000) 218–235 0022-1694/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-1694(99)00182-1 www.elsevier.com/locate/jhydrol * Corresponding author. Fax: +30-42174239. E-mail addresses: aloukas@uth.gr (A. Loukas), lvassil@uth.gr (L. Vasiliades), dalezios@uth.gr (N.R. Dalezios).