Boundaries that use natural features can change from time to time. If the change is gradual, the boundary moves with the change. If the change is sudden, the boundary freezes. There have been cases in which this has been at issue, including the many changes in the course of the Willamette River in Oregon. There was even a case in which it was an important question whether an area bordered by the Missouri River was in Nebraska or Iowa after the river bypassed one of its former curves and formed an oxbow.