An ice dam occurs when water builds up behind a blockage of ice. Ice dams form either when glacier blocks a river and forms a lake or when ice chunks in a river are blocked by something and build up to form a dam, often called an ice jam. Glacial ice dams have historically resulted in massive outburst floods. River ice jams can cause flooding upstream during the jam, flooding downstream when the jam releases, and damage from the ice itself on structures and ships in or near the river. Ice jams on a lake or ocean occur during the spring break-up if wind driven ice piles up along a shoreline.