Current, erosional and depositional patterns at a river confluence and bend in the Nottawasaga River, Angus, Ontario

At a bend and confluence of the Nottawasaga River, sixty-four measurements of flow velocities were recorded at points along vertical profiles regularly spaced across a series of cross-sections. From the summarization of the data into a ‘three dimensional’ diagram, the current velocity patterns in the river reach were evident. As observed at low discharge levels, the low speeds and directional uniformity of the flow in the Nottawasaga River entering the study reach suggests that the observed asymmetrical channel the study reach suggests that the observed asymmetrical channel was a remnant of higher discharge levels. On the other hand, in the Pine River prior to the entrance into the confluence, channel morphology was being influenced by much higher flow speeds in combination with two cells of contra-rotating helicoidal flow. Active dunes and ripples were observed migrating along the asymmetrically shaped channel bottom. The erosional and depositional pattern of the tributary suggests that the Pine River mouth was slowly migrating upstream. The fusion of the waters at the confluence resulted in very complex flow patterns. In the waters entering from the tributary, the presence of a helicoidal flow cell appeared to be causing a large bar to build along the bank immediately downstream from the Pine River mouth. Across the channel, the deflection of the Nottawasaga River flow was causing erosion of the opposite bank. Downstream at the riffle area, flow in the Nottawasaga River accelerated as channel morphology became almost symmetrical. Around the bend, the development of a high speed filament and helicoidal flow cell were reflected by the asymmetrical channel and the erosional and depositional pattern along the banks. A large eroded area in the bank suggested that a proximal reverse circulation observed in the flow pattern along the outside of the channel was capable of causing erosion during high discharge levels. The overall velocity pattern observed at the bend became much less intense as flow entered a deep pool that was situated in the downstream section of the study reach. Paleocurrent measurements of ripple marks on the point bar at the bend and small ‘side’ bars immediately downstream gave an indication of flow patterns occurring along the inner bank of the at high discharge levels. These measurements showed that a large reverse circulation exists along the inner bank causing scouring of the downstream end of the point bar. Excavations into the slumped area opposite the point bar revealed numerous sedimentary structures. Amongst these structures was evidence of point bar accretion, upstream migration of the tributary, a channel cut-off, and vertical accretion deposits on the floodplain