Boundaries between cities, states, and countries can be physical and/or political. Some last for decades, others change more frequently. Water boundaries present unique challenges. Whereas natural obstructions to transportation by land can create physical boundaries, rivers can change course or dry up, then change course again. Sometimes flooding took its toll on altering the river. Other than environmental change or natural occurrences, disputes arose over badly drawn maps, political friction or rivalries, and a variety of legal claims. The Rio Grande Basin drains a more than 330,000-square-mile area in the southwestern United States and northern Mexico, which includes numerous vegetation types, desert and woodland ecotones, and a variety of urban and rural peoples. The United States had bought the Louisiana territory with undefined boundaries, which also was true when France acquired it from Spain.