Laboratory Observations of Ice Jams in Channel Confluences

Abstract

Presented here are detailed descriptions of ice-jam processes at river-channel confluences. The descriptions are based on observations obtained from a small-scale hydraulic model of variable geometry and on field observations interpreted from published and unpublished accounts of ice jams in the vicinity of confluences. It was found that, in addition to three straightforward jamming processes, several relatively complex processes may lead to ice jams in channel confluences: (1) the merging of ice runs; (2) hydrodynamic pressure from a confluent flow impacting an ice run from the second confluent channel; (3) ice congestion at a confluence bar; and (4) ice grounding on confluence deltaic bars. The presence of a confluence bar, for example, is a significant factor triggering ice jams at the confluence of the Mississippi and Missouri Rivers. Tentative formulations of the three jam processes are proposed. The following three straightforward processes cause ice jams at river confluences: (1) a stationary ice cover exists immediately downstream of a confluence; (2) large ice pieces arch in the confluence; and (3) channel flow below the confluence is sluggish (e.g., the special case of flow entering a reservoir or lake).