Natural boundaries between states or countries are found along rivers, lakes, deserts, and mountain ranges. Among them, river boundaries would seem to be ideal: they provide clear separation, and they are established and recognized physical features. In reality, however, river boundaries can change as rivers change course. Following flooding, a river’s course may shift, altering the boundary between states or countries. For example, the Rio Grande, separating the United States and Mexico, has frequently shifted its course, causing problems in determining the exact location of the international boundary.
*boundary: 경계
A river seems to be ideal in ___(A)___ boundaries, but in fact it isn’t, because its course is ___(B)___.