Northward-flowing rivers of Alaska inundate extensive areas of sea ice during spring breakup. Drainage of large volumes of fresh water through the ice at holes and cracks (strudel) causes scour depressions more than 4 m deep and as much as 20 m or more across in the sea floor below. These modern strudel scours and their filled counterparts were studied by side-scan sonar, echosounder, high-resolution seismic gear, diving observations, and sediment sampling. Strudel scours occur within 30 km of river mouths. Their shapes and distribution patterns correspond to those of strudel seen in the ice canopy. The scours are most numerous in the inner areas of overflow, as many as 25 per kilometer of ship's track. Strudel scours also occur outside of the 1970-72 overflow areas; these must be relatively old. One strudel scour investigated by diving was surrounded by a ridge, had vertical walls exposing a tundra horizon, and terminated at a gravel layer 4 m below a lagoon floor. Another one terminated at a semiconsolidated layer of silty clay. Both the gravel and silty clay are pre-Holocene deposits. From highly irregular bedding in seismic records it is apparent that virtually all of the Holocene deposits and significant parts of underlying older sediments around river mouths have been reworked by strudel scour. This mixing of sediments by vertical strudel flow causes sediment types to vary greatly over small areas. These new observations add further complicating factors to be considered in the interpretation of shallow-water deposits of cold climates.