PRESERVATION OF GLACIAL FEATURES IN RIVER VALLEYS: THE ROLE OF GRAIN SIZE AND ROCK UPLIFT

Glaciation affects rates of fluvial erosion and deposition, as evidenced by the dependence of river terrace formation on glacial cycles and hillslope-channel connectivity on glacial valley widening. However, many prior observations of rivers draining glaciated landscapes rely on a snapshot in time and on the preservation of relict fluvial and glacial surfaces. Here, we use a 1-D coupled glacial and fluvial model to assess how glaciation affects fluvial erosion and deposition downstream throughout several glacial cycles and consider the role that sediment loads play in inhibiting erosion downstream of glaciers. We run a suite of models with different rock uplift and bedload size conditions over eight 100 ky glacial cycles. In all model runs, glacial erosion forms a zone of low steepness in the headwaters that produces a convex profile at the transition between glacial and fluvial erosion. Comparisons between model runs with uplift rates of 0.05, 0.5, and 1.0 mm/yr shows that the length of the profile convexity decreases with increasing uplift rate. Varying the initial grain size in our model results in either erosion or preservation of the profile convexity during interglacials, depending on the sediment transport capacity. In model runs with an initial mean grain size of 20 mm, bedload is transported across the convexity without deposition. This allows bedrock erosion to adjust the channel profile to a new fluvial equilibrium. In contrast, runs with an initial mean grain size of 200 mm show deposition within the convex zone, which persists throughout the glacial cycle. Bedrock erosion by fluvial processes is inhibited and the glacial convexity remains in the valley profile, even for simulations in which glacial erosion occurs for only ~20% of the run time. These results indicate that the signal of glacial erosion in glaciated valleys is strongly controlled by the sediment size and transport more so than uplift rates. Our findings emphasize the importance of coupling sediment transport models with erosion models to accurately predict landscape development.