All Logjams Are Not Created Equal. Issue 8 (16th August 2021)

• Record Type: Journal Article
• Authors: Livers, Bridget; Wohl, Ellen
• Is Part Of: Journal of geophysical research. Volume 126:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-16
• Subjects: logjams -- instream wood -- rivers -- large wood; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006076
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Logjams create diverse physical and ecological effects in stream channels, including at least temporary storage of water, sediment, and particulate organic matter. We hypothesize that logjams that span the entire bankfull channel width in channels ≤25 m wide are more effective in storing these materials than non‐channel spanning logjams. We test this hypothesis by systematically comparing characteristics of 183 logjams from 17 stream reaches in the Southern Rocky Mountains. Our data set is novel in that it evaluates naturally occurring logjams in unaltered streams in a single study area specifically based on whether they span the stream channel. We find that channel‐spanning logjams have a significantly larger number of wood pieces, longer & wider pieces, more ramp and bridge pieces, and greater logjam height and volume, both as raw data and when standardized by average channel width. Channel‐spanning logjams also have significantly greater backwater pool volume and volume of particulate organic matter stored in backwater pools and in logjams. Restoration employing engineered logjams in relatively small channels currently focuses on non‐spanning logjams, but could be expanded to include spanning logjams. Plain Language Summary: Logjams are accumulations of three or more large wood pieces in streams and stream environments. Logjams can obstruct flow and create frictional resistance in small stream channels, creating many physical and beneficial ecological effects in stream environments. This includes, but is not limited to, temporary storage of water, sediment, and organic matter, which translates to habitat and nutrient availability for aquatic organisms. Despite these benefits, deforestation and active wood removal from stream environments have significantly reduced the size and abundance of logjams throughout the temperate latitudes, contributing to the loss of ecological integrity and the simplification of stream channels. Stream restoration projects are increasingly using engineered logjams (ELJs), which are less likely to span a stream channel than naturally occurring logjams. Limited understanding of how logjam characteristics relate to specific effects constrains our ability to evaluate whether ELJs have comparable effects to natural logjams. We systematically evaluate characteristics and effects of 183 logjams in unaltered headwater Southern Rocky Mountain streams. We find that channel‐spanning logjams have significantly greater effects, such as pool and organic matter volume storage, than non‐channel‐spanning logjams, and recommend considering channel‐spanning ELJs in restoration projects in small streams to maximize retention. Key Points: Logjams that span the bankfull channel create significantly greater storage of water and organic matter than non‐spanning logjams Channel‐spanning logjams also have significantly more wood, larger pieces of wood, and more ramp and bridge wood pieces Stream rehabilitation projects employing engineered logjams should consider channel‐spanning logjams to achieve greater retention