Final answer:
The physical cause of river floods that is most predictable is heavy rainfall. When there is a large amount of rainfall, the water level in rivers and streams can quickly rise, causing flooding.
Step-by-step explanation:
The physical cause of river floods that is most predictable is heavy rainfall. When there is a large amount of rainfall, the water level in rivers and streams can quickly rise, causing flooding. This is especially true in areas with steep slopes or with soil that cannot absorb water quickly.
For example, if there is a period of heavy rain for several days, the rainwater will flow into the rivers and streams, causing them to overflow their banks. As a result, the excess water spreads out over the surrounding land and causes flooding. This is why it is important to monitor weather forecasts and be prepared for potential flooding during periods of heavy rainfall.