River terrace development of the lower Saugeen River Valley, Southern Ontario

grantor: University of TorontoFollowing the retreat of glacial ice from southern Ontario $\sim$12.5 ka BP, a river channel has continually occupied the Saugeen Valley. The Saugeen River, initially a laterally unstable glaciofluvial channel, has repeatedly adjusted to fluctuations in relative baselevel generated by glacioisostatic rebound and lake level changes. These adjustments included channel incision and, in consequence, the formation of river terraces. The Saugeen River Valley has four levels of river terraces preserved in its lower 40 kilometres. Each terrace, T1 through T4, reflects adjustments to former lake levels in the Huron Basin--Algonquin, Nipissing, Nipissing II, and Algoma respectively. However, glacioisostatic rebound is of greater importance in their formation than lake level change. Created when the rate of glacioisostatic rebound was large, the highest and oldest terrace, T1, is the only continuous one throughout the study area. Large rates of rebound required the T1 paleochannel to adjust its gradient significantly and did not allow the channel to reestablish grade. After $\sim$6 ka BP, the duration of each post-glacial lake became successively greater and the rate of rebound decreased. The combination of these two factors resulted in T2, T3, and T4 terrace surfaces that do not have gradients significantly different from that of the modern channel. Their respective channels were able to reestablish graded conditions. Unlike the other three terraces, the T4 is present only in the lower half of the valley because the rate of rebound during its creation was relatively small. Lake level changes only affected channel reaches near the river mouth. The existence of the three terrace surfaces younger than $\sim$6 ka (T2, T3, and T4) suggests three post-glacial lakes occupied the Huron Basin since 6 ka BP.M.Sc