流域

1. Origins, structure, and basic meaning

• Structure: The hydrological noun 流域 (りゅういき) is the basin or watershed whose water drains into a river.
• Basic usage: Towns may lie 川の流域に, and disaster planning can be coordinated 流域全体で.

2. Common pitfalls and similar expressions

• Land draining into a river
  • It is the broad land area whose water drains into the same river system, not just the river surface or banks.