Controls on river evolution in incising post-glacial bedrock rivers along the North Shore of Lake Superior, Minnesota, USA

Base-level fall can trigger river incision, often creating discrete knickpoints. As these knickpoints migrate upstream through heterogeneous bedrock, they can be modified and transformed.  We investigated the role of stepped base-level fall vs. bedrock heterogeneity on knickpoint evolution and river incision in a series of rivers on the North Shore of Lake Superior.  The most recent period of base level fall is thought to have initiated around 10.8 to 10.6 ka (Breckenridge, 2013) as low eastward-draining outlets opened on what is now Lake Superior, initiating incision on local rivers.  As incision migrated upstream and rivers downcut through glacial sediments and underlying bedrock, they created river terraces and discrete knickpoints. 

Knickpoints were identified as reaches with high stream power along river long profiles. Terraces were mapped in the field and on aerial lidar data, with several dated using optically-stimulated luminescence in overlying alluvium. Base-level fall chronology relied upon detailed mapping of shorelines and outlets in published literature. Bedrock competency was quantified in situ through a combination of compressive strength (measured via Schmidt hammer), fracture intensity, and fracture density. Samples were run through an abrasion flume to measure attrition rates on different lithologies.

Results indicate that while the highest suite of terraces aligns with the elevation of the highest lake-level stand of glacial Lake Duluth, the predecessor to western Lake Superior, other terraces do not align with intermediate lake levels. Although lake-level fall initiated overall incision in the system, the stepped nature of the lake level fall was rapid enough that it did not form discrete sets of terraces and knickpoints. OSL ages indicate incision history is complicated, with some sites showing steady incision through the past 10.8 ka and others showing faster rates initially. The widest valleys and most abundant terraces are located in areas where rivers are primarily migrating and incising through surficial glacial deposits. Knickpoints are currently located on the most competent bedrock lithologies. 

The most common lithology present, basalt, has variable competency characteristics depending on location within the lava flow (resistant and massive flow bases vs. weaker often vesicular flow tops) as measured by compressive strength, fracture intensity and density, and experimental attrition rates. Interestingly, the weakest and most fractured surfaces in otherwise high competency basal basalt flows, were found in and around knickpoints, potentially indicating that physical weathering occurs in high stream power reaches prior to erosion.