Laboratory observations of ice jams in channel confluences

Presented here are detailed descriptions of ice-jam processes at river-chan nel 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from a small-scale hydraulic model of variable geometry and on field observations interpreted from published and unpublished accounts of ice jams in the vic inity of confluences. It was found that. in addition to three straightforwa rd 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 conflue nt channel: (3) ice congestion at a confluence bar: and (4) ice grounding o n confluence deltaic bars. The presence of a confluence bar, for example, i s a significant factor triggering ice jams at the confluence of the Mississ ippi and Missouri Rivers. Tentative formulations of the three jam processes art: proposed. The following three straightforward processes cause ice jam s at river confluences: (1) a stationary ice cover exists immediately downs tream of a confluence: (2) large ice pieces arch in the confluence; and (3) channel now below the confluence is sluggish (e.g., the special case of fl ow entering a reservoir or lake).