The Migration Diary of a Knickpoint in a Channelized Stream With a Cohesive Bed

Abstract

This paper examines the coevolutionary roles of variableflow features and heterogeneoussubstrates on knickpoint migration. A conceptual model is used to depict distinct migration mechanismsin a channelized headwater system of a representative intensively managed watershed in the Deep LoessRegion of the U.S. Midwest. Throughout this region, entrenched streams have eroded into alluvial‐basedstrata that are horizontally consistent but vertically heterogeneous. Variableflow in these streams switchesbetween vented conditions, where water shoots from the knickpoint face as an aerated plunging jet, andunvented conditions, where water clings close to its face through suction. These switches affecthydrodynamic mechanisms controlling migration but cannot explain fully observed alternating slow‐fastmigration rates. It is hypothesized that vertically heterogeneous strata in entrenched streams, specificallystrong‐over‐weak layers, facilitate observed changes in knickpoint migration rates. Preferential erosion ofweaker sublayers is thought to trigger other erosional mechanisms to accelerate migration even at lowflows.Herein, a migration“diary”documents a representative knickpoint's displacement over time withflowmeasurements and site stratigraphy. Slower migration occurred from 2006 to 2009 throughfluvial erosion ofa resistant silt loam cap. From 2009 to 2012, migration accelerated as the knickpoint face reached a criticalheight through scour. Aflow back roller formed, which preferentially eroded a weaker sand sublayer.Undercutting/cantilever failure cycles persisted, causing 30 m of retreat within 2 months (i.e., 62.5% of thetotal migration over 78 months). Comparisons with establishedfluvial hydraulics‐based andundercutting/cantilever‐based migration models confirm the hypothesis and importance of both stratifiedsubstrates and hydrodynamic controls on knickpoint migration