Verification of truth — geographical necessity: A river must have which of the following?

Introduction / Context:
We are to identify the feature without which a river cannot be defined as such. Focus on geomorphological essentials rather than typical but optional attributes.

Given Data / Assumptions:

• A river is a natural flowing watercourse confined within lateral margins.
• Biota (fish, weeds) and human artifacts (boats) vary widely.

Concept / Approach:
The defining structure of a river is flowing water within banks (lateral boundaries). Therefore, “banks” are necessary for the existence/identity of a river channel.

Step-by-Step Solution:
Check each option: fish/weed/boats are nonessential and may be absent.Banks provide the confining sides; without banks, the feature would be an unconfined water body (e.g., lake/floodplain), not a river channel.

Verification / Alternative check:
Even braided rivers have multiple channels; each thread has banks. Seasonal or ephemeral rivers still have banks when defined as channels.

Why Other Options Are Wrong:
They are contingent on ecology or human use.

Common Pitfalls:
Assuming fish are universal to rivers; many stretches are fish-poor or temporarily deoxygenated.

Final Answer:
Banks