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Abstract Ice formation is generally considered to exclude many particles and most solutes and thus be relatively pure compared to ambient waters. Because river ice forms by a combination of thermal and mechanical processes, some level of sediment entrainment in the ice column is likely, though reports of sediment in river ice are limited. We observed high and sporadic levels of silt and sand in ice of the Kuskokwim and Tanana rivers (Alaska, the United States) during routine field studies. These observations led us to make a more comprehensive survey of sediment entrainment in river ice of the Kuskokwim and Yukon rivers and several of their tributaries. We collected and subsampled 48 ice cores from 19 different river locations in March 2023, which included concurrent measurements of water turbidity, velocity, and depth. Approximately 60% of cores contained detectable levels of sediment, averaging 438 mg/L with median concentrations exceeding 1000 mg/L in three cores from the Yukon and Kuskokwim main stems. Many cores had even higher concentrations at certain intervals, with seven cores having subsamples exceeding 2000 mg/L; these were often located in the middle or lower portion of the ice column. Jumble ice, formed mechanically by frazil‐pan jamming during freeze‐up, was generally the best predictor of higher sediment entrainment, and these locations often had higher under‐ice velocities and depths. Our observation of high and widespread sediment entrainment in northern river ice, particularly in jumble‐ice fields, may have implications for sediment transport regimes, ice strength and transportation safety, and how rivers break up in the springtime.
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Freeze‐Up Ice Jams and Channel Hydraulics Cause Hazardous Open Water Zones Within Winter Ice Cover on the Kuskokwim and Yukon Rivers and Their Tributaries
Abstract Timing and completeness of freeze‐up on northern rivers impact winter travel and indicate responses to climate change. Open‐water zones (OWZs) within ice‐covered rivers are hazardous and may be increasing in extent and persistence. To better understand the distribution, variability, and mechanisms of OWZs, we selected nine reaches totaling 380 river‐km for remote sensing analysis and field studies in western Alaska. We initially identified 48 OWZs from November 2022 optical imagery, inventoried their persistence into late winter and interannual consistency over previous years, and at a subset measured ice thickness, water depth and velocity, and physicochemistry. The most consistent locations of OWZ formation occurred below sharp bends and channel constrictions, whereas locations associated with river bars and eroding banks were more transient. Of 359 OWZs identified in early winter over 6 years, 8% persisted into late winter―all on the Yukon River mainstem. Although several OWZs were in locations where we anticipated groundwater influence, we found no field data indication of groundwater upwelling. Observations of jumble ice upstream of many OWZs led us to examine freeze‐up ice jam locations in optical imagery, which showed strong correspondence to downstream OWZs. We hypothesize that reaches downstream of ice jams are much slower to freeze‐over due to restricted ice transport and high turbulence caused by channel form and ice‐affected hydraulics. Future work should focus on evaluation of this and other competing hypothesis at both reach and river network scales to predict OWZ locations and occurrence relative to other processes affecting river freeze‐up in northern climates.
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- Award ID(s):
- 1836523
- PAR ID:
- 10590553
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 61
- Issue:
- 4
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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