VARIABILITY OF BED MORPHOLOGY AND SEDIMEN TARY STRUCTURES AT A DISCORDANT RIVER CONFLUENCE DURING LOW FLOWS

Variability of bed morphology and sedimentary structures at a discorda nt river confluence during low flows. Discordant river channel conflue nces are highly turbulent environments. Their morphology, which is car acterized by a tributary mouth bar, shows great temporal variability. In this study, we examine the relationships between hydraulics, bed mo rphology and sediment transport at a discordant river channel confluen ce at low flow stage. During two consecutive summers, we surveyed almo st weekly the confluence of the Bayonne and the Berthier rivers, Lanau diere, Quebec. Significant morphological changes occurred on almost th e entire channel bed even if flow stages were most of the time between 1/20 and 1/5 bankfull level. Sedimentary facies illustrate particular ly well the tributary mouth bar progradation due to each watershed cha racteristics. Low flow conditions for each river varied considerably a nd incongruently. These variations resulted in changes in the location of the mixing layer between the joining flows and the extent of progr adation of the tributary mouth bar into the main channel. We propose a three-phase model showing that the position of the tributary mouth ba r, and whether it is receding from, prograding or fully extending into the confluence, is closely related to the development and lateral exp ansion of the mixing layer-between the two streams. Zones of depositio n and erosion in the confluence are then associated with the position and dynamics of the mixing layer.