From the Overview: "An ice jam may be defined as an accumulation of ice in a river, stream or other flooding source that reduces the cross-sectional area available to carry the flow and increases the water-surface elevation. [...] In the northern United States, where rivers can develop relatively thick ice covers during the winter, ice jams can contribute significantly to flood hazards. Although flow discharges may be low relative to free flow flood, the stages of ice jam flooding may be among the highest on record. Ice jams typically occur repeatedly in the same locations, and ice jam flooding tends to be local and highly site specific. In areas likely to be selected for an enhanced study by FEMA or one of FEMA's Mapping Partners, historical documentation will usually indicate if ice-jam flooding is a significant factor warranting consideration. In regions of the United States where ice jams are typical, the Mapping Partner that performs an enhanced study for a FEMA-contracted Flood Risk Project or community-initiated map revision shall investigate historical floods for evidence of ice-jam contribution as part of the reconnaissance effort."
Guidance Document 94
U.S. Federal Emergency Management Agency: https://www.fema.gov/