- Soil loss causes grooves in soil
- Soil erosion means soil loss through wind / water / animals
- Therefore, soil erosion caused by water will cause grooves in soil

- Soil erosion caused by water will cause grooves in soil
- Rainwater is a kind of water
- Therefore, rainwater can cause grooves in soil

- Rainwater can cause grooves in soil
- A riverbank is made of soil
- Hence, rainwater can cause grooves in a riverbank

Therefore, rainwater can cause grooves in a riverbank
Grooves running down the sides of a riverbank are most likely caused by
- Soil loss causes grooves in soil 
- Rainwater is a kind of water 
- A riverbank is made of soil 
- Soil erosion means soil loss through wind / water / animals