in 1892, after a lot of litigation between iowa and nebraska, the supreme court of the united states said that carter lake is part of iowa in "nebraska v. iowa", 143 u.s. 359 (1892). the general rule is that state borders follow small changes in the course of a river. however, the court said that an there's an exception when a river avulses one of its bends. in 1972, the supreme court made another ruling about carter lake when it ruled on a border dispute between the two states in "nebraska v. iowa", 406 u.s. 117 (1972).