LITHOLOGIC CONTROLS ON LANDSCAPE EVOLUTION IN POST-GLACIAL BEDROCK RIVERS, LAKE SUPERIOR BASIN

River incision in bedrock landscapes is controlled by flow, sediment flux, and properties of the underlying lithology such as fractures, cleavage, and bedding planes. The objective of this study is to identify the role of substrate erodibility (crystalline and sedimentary bedrock, Quaternary sediments) as a control for vertical incision and lateral planation in rivers responding to base level fall following deglaciation in the Lake Superior basin. These controls were explored in three Lake Superior bedrock river watersheds with varying lithologic complexity and competency to investigate the formation, position, and migration rate of knickpoints, and the preservation of strath terraces resulting from valley widening and incision. Strath terraces were mapped and dated as a means of spatially and temporally constraining longitudinal profile evolution following base level fall. Bedrock lithologies outcropping in the channel were characterized through three erodibility metrics (uniaxial compressive strength, fracture intensity, fracture density) to compare with the hydraulic geometry and unit stream power of the channel. Preliminary results indicate that higher strength bedrock creates steeper sloping, narrower channels with higher unit stream power, and that knickpoints in the present profile are found only in these high strength units. Extensive floodplains and strath terraces are only found overlying Quaternary sediments, suggesting that the differences in relative erodibility between Quaternary deposits and crystalline bedrock limits lateral planation and valley widening in bedrock. We propose a conceptual model of landscape evolution in Lake Superior streams where initial lake level fall triggers incision through Quaternary deposits and the underlying bedrock. Bedrock knickpoints forming in the newly exposed knickzone migrate upstream at a rate that is controlled by flow (upstream area) and erodibility of lithology (strength and the degree of fracturing). The signal of base level fall for upstream Quaternary reaches is pinned by the downstream exposed bedrock, setting a "local base level", allowing for lateral planation and valley widening in the weaker parent material.