IN-CHANNEL SURFICIAL FINE-GRAINED SEDIMENT LAMINAE .1. PHYSICAL CHARACTERISTICS AND FORMATIONAL PROCESSES

An in-channel surficial depositional feature (surficial fine-grained l aminae) composed of loosely bound fine sediment deposited during low f low conditions has often been observed in river systems in south-weste rn Ontario. The physical characteristics of this feature have been det ermined by a direct observation image analysis system. This sediment c onsists primarily of flocculated fine-grained material. The size distr ibutions of surficial fine-grained laminae and suspended sediment were not significantly different. Each distribution is bimodal in nature a nd shows a characteristic grain size deficiency in the 4-5 mum size ra nge. This observation suggests that flocculation and not low discharge or low competence is the dominant mechanism for the formation of surf icial fine-grained laminae under conditions of low flow in fluvial sys tems of south-western Ontario. A quantitative assessment of this featu re shows its potential importance as a source of fine-grained sediment and associated contaminants for downstream transport.