RIVER INCISION, INVERTED LONG PROFILES, DOWNSTREAM COARSENING, AND TERRACES: WHAT GLACIATION HAS WROUGHT ON INCISING MINNESOTA RIVERS AND HOW IT IMPACTS MANAGEMENT OF RIVERS TODAY

Many upper Midwest rivers were impacted by the last glaciation. Changes in base level associated with fluctuating lake levels, glacial lake drainage, and stream capture events led to distinct features in the post-glacial fluvial landscape. Here, the focus is on two post-glacial fluvial systems in Minnesota. In both cases, rivers were subjected to base level fall associated with glacial lake drainage. Response to base level fall has led to a series of distinct characteristics in the incising portion of the fluvial systems: inverted long profiles, stream terraces, narrow (or absent) floodplains, downstream coarsening, downstream increases in stream power, and a lack of normal hydraulic geometry relationships.

In the Le Sueur River in south-central Minnesota, rivers are incising through stacked glacial tills. Easily erodible substrate has allowed rapid upstream knickpoint propagation and laterally mobile channels, creating hundreds of terraces. Numerical modeling coupled with OSL and C¹⁴ dating of terrace deposits show that the Le Sueur is behaving like a detachment-limited channel rather than an alluvial one. Only models that incorporate downstream coarsening could successfully predict the modern long profile. Tributaries draining into Lake Superior along Minnesota’s North Shore are primarily incising through competent igneous bedrock overlain by thinner glacial deposits. Incision rates are slower, and long profiles remain inverted. Because of this, stream power increases rapidly downstream in the lower incised portions of mainstem channels, and hydraulic geometry relationships do not follow standard patterns. High fine sediment loads occur where mainstem channels incise or migrate into clay-rich glacial deposits.

Management of these post-glacial rivers must be approached with an understanding of the on-going incisional history and its ramifications. High bluffs can be a primary source of fine sediment. In many cases, increased flows from land use change are exacerbating erosion, made worse by the lack of a connected floodplain throughout the incised valley. Efforts by management agencies to reconnect channels to floodplains as a means to ameliorate changes in watershed hydrology and slow erosion may be more successful in channels with slower incisional rates than in rapidly-incising reaches.