Channel Form and Processes in a Formerly Glaciated Terrain

Despite that many places around the world in general, and North America in particular, were glaciated during the last ice age, relatively little is known about rivers that evolved over glaciated landscapes once they deglaciated. They are commonly categorized as alluvial with a glacial legacy, and often described as simple gravel-bed or sand-bed rivers. Alternatively, they are classified as bedrock rivers because the glacial deposits have been eroded and the underlying rock is exposed. However, the glacial history of these rivers is important and my research shows that classification for them is "semi-alluvial". This work shows that classification is important, not only for scientific accuracy but for river management that must be based on a sound understanding of river form and process. Poor understanding can be costly (i.e. restoration and management that do not achieve their goals; failed infrastructure), leading to wasted resources and inefficient functioning of the river. Two major geomorphologic phases can be discerned in formerly glaciated terrains: Deglaciation which exposed the landscape to erosion when ponded meltwater was abundant and led to outburst flooding. These catastrophic floods cannot occur under the modern climate of southern Ontario. Evidence for glacial lakes is found in lacustrine deposits upstream, and on top of moraines while spillways indicate where glacial lakes drained and carved deep valleys into the terrain. Spillways reveal outburst flooding with boulder lags, terraces, flow scars and possibly mounds and berms away from the modern misfit channel. Postglacial incision and deposition during a single flood event or a single hydrological year. Human disturbance over the past two centuries, includes agriculture, channelization, millponds and weirs. The dissertation addresses two parallel paradigms in fluvial geomorphology: Which processes are responsible for most of the geomorphic work that shapes the landscape catastrophic flood(s) or small-scale erosion and deposition that bit by bit give the channel and valley its present morphology? My work will show that the outburst flooding of Glacial Lake London (paper 1) sets the stage for the postglacial morphology and small-scale processes we see today (paper 2). The third paper attempts to explain these small-scale processes using a 1D hydraulic model that can answer dynamic questions like bankfull discharge, water surface slopes of discharges, and velocity reversal hypothesis. The findings show that the outlet of the Thames River near the neighborhood of Byron is a deeply-incised spillway channel that formed from the catastrophic drainage of Glacial Lake London during deglaciation. The dry lake bed serves today as the baselevel for the upper Thames River and its tributaries that incised and exposed the lacustrine and glacial sediments. The till exerts a strong control over channel form and resulting processes. Hydrological and sedimentary metrics often used to assess a river's condition produce contradictory results, doubting their validity without a-priory knowledge of what is the reference till river. With very little research done on semi-alluvial rivers in till, we propose that the scientific and engineering community focus on these rivers as they are quite abundant in N. America. Even so, evidence from tillbedded rivers and this study show they are quite different than alluvial and bedrock channels.