Seward Highway turnouts were filled with people Tuesday afternoon, April 19, 2011, to watch the bore tide roll into Turnagain Arm. A bore tide is a rumbling, churning wave that forms on the leading edge of an incoming tide as it rises into a relatively narrow place. Turnagain Arm and Knik Arm are the only places in the United States in which the phenomenon regularly occurs, according to Alaska State Parks. Extreme tide differentials this week could make for visible bore tides on Wednesday and Thursday.