Inner structure of anchor ice and ice dams in steep channels

Two main types of ice crystals were observed: columnar ice crystals and fixed-frazil ice crystals. Relatively large columnar ice crystals grew upwards (away from the bed) perpendicularly to the local flow surface and were only observed in ice dam samples. Fixed-frazil crystals originated from the deposition and/or interception of drifting frazil particles that became “fixed” to the bed and to existing ice accumulations. The data suggest that in-situ growth of fixed-frazil ice crystals was the dominant process (accounted for the most ice development) for both anchor ice and ice dams. Fixed-frazil ice consists of a bonded mass of interlocking plate shaped crystals demonstrating a preference to be perpendicular to the bed and water surfaces. The mass displays some variability in porosity (mean value 41%). When analyzed using CAT scan slices, the interlocking plates appear as a tree-like or dendritic structure. The average size of resulting crystal segments and the maximum size of individual whole crystals within fixed-frazil ice structures were significantly more important for ice dam samples than for anchor ice samples.