BEDROCK KNICKPOINTS, RIVER TERRACES AND POST-GLACIAL INCISION IN SOUTHERN NEW ENGLAND

Longitudinal river profiles, knickpoints and terraces are crucial to understanding landscape response to changes in base level and sediment flux. In this study, we analyze longitudinal profiles and fill/cut-fill terraces to investigate river incision and transient response to deglaciation and baselevel changes along the Farmington and Housatonic Rivers in Massachusetts and Connecticut. We utilize LiDAR (Light Detection and Ranging) data and Geographic Information System (GIS) to extract longitudinal river profiles and compile datasets for stream terrace elevations. Fieldwork at select sites assisted in understanding the stratigraphy of terrace deposits. Results reveal river profiles marked by numerous knickpoints, and over 300 instances of terrace trends resting well above modern floodplains of the Farmington and Housatonic Rivers. Deposit heights above the modern Farmington River range from 2 to 40 m (2-18 m for mapped stream terraces), and 3-45 m (3-20 m for mapped stream terraces) above the Housatonic River. More terrace levels and greater amounts of incision are consistently found downstream of bedrock knickpoints. Conservative estimates are used to establish the initial, highest river level following glacial retreat. We propose three different knickpoint migration and/or emergence scenarios to explain the interaction of relatively rapid and easy incision into unconsolidated fill vs inhibited incision once the underling bedrock is exposed along the channel. These conceptual models assist in describing complex slope and incision patterns observed along these rivers, as well as lead to a robust prediction for terrace patterns and ages where dating is not possible. Overall, our results indicate that the Farmington and Housatonic Rivers are in transient adjustment influenced primarily by bedrock legacy rather than base level controls. This research is fundamental to understanding post-glacial landscape evolution in southern New England, and the methods used here can be easily transferred for use in other deglaciated regions around the world.