Meandering river evolution in an unvegetated permafrost environment

Abstract

Meandering river sedimentary facies are associated with the rise of terrestrial vegetation in the rock record, but modern examples of highly sinuous, single-threaded streams are also found in many modern non-vegetated environments. Prior workers have speculated that permafrost or ice-cemented sediment could provide the cohesion needed to generate single-threaded, meandering channels, however, the presence of bank vegetation in the overwhelming majority of terrestrial permafrost sites in which meandering rivers are present has made it impossible to disentangle the relative impact of bank cohesion provided by frozen sediments from that provided by roots. Here, we examine topographic change over 13 years at the Onyx River, an unvegetated, permafrost-affected, sand-and-gravel-bottomed river in the McMurdo Dry Valleys (MDV) of Antarctica, supported by ∼40 years of image data. On the basis of lidar change detection and satellite/airborne image analysis, we find that meandering river processes (cut bank erosion, point bar growth, and meander migration) can occur in this ice-cemented, unvegetated permafrost river system. We find that the Onyx River's upper, single-threaded reach is meandering, and that bank cohesion is provided by ice-cemented permafrost. Meander migration rates within the Onyx's meandering reach are extremely close to values predicted based on observations of meander migration rates in other unvegetated rivers, however, the short (∼70 day) duration of Onyx River discharge suggests these migration rates should be considered low estimates, which could be up to ∼5 times greater during the short, summer erosion season. Together, these observations suggest that ice-cemented permafrost is a weak form of bank cohesion that can result in rapid landscape change in response to thermokarstic fluvial erosion, producing dynamic river channels, especially where warm meltwater encounters ice-rich permafrost deposits.