When the jet reached ten thousand feet, it became clear that all of the above options were true: the reason why cities ran along the river, the reason why valleys are populated, and the logic of geography.

Why cities run along the river:
- Cities tend to develop near rivers because water is a vital resource for human settlements.
- Rivers provide a source of freshwater for drinking, cooking, and agriculture, which are essential for human survival.
- Additionally, rivers have historically served as transportation routes, facilitating trade and commerce.
- The presence of a river also supports various economic activities such as fishing and industries that rely on water.

Why valleys are populated:
- Valleys are usually fertile and provide suitable conditions for agriculture.
- The soil in valleys is often rich in nutrients due to the accumulation of sediment carried by rivers over time.
- Valleys typically have a moderate climate, making them suitable for human habitation and agricultural activities.
- The presence of rivers in valleys provides a reliable source of water for irrigation, enhancing agricultural productivity.
- Valleys also offer natural protection against harsh weather conditions, as they are often surrounded by mountains or hills.

Logic of geography:
- The geography of an area, including its natural features such as rivers and valleys, plays a significant role in determining human settlement patterns.
- Human civilizations have historically developed in areas that offer favorable geographical conditions, such as access to water, fertile land, and natural resources.
- The logic of geography suggests that human settlements tend to cluster in areas that provide essential resources and favorable living conditions.
- Understanding the logic of geography helps explain why cities run along rivers and why valleys are populated.

In conclusion, when the jet reached ten thousand feet, it became clear that the reasons why cities run along rivers, why valleys are populated, and the logic of geography are all interconnected. The presence of rivers in valleys provides essential resources, such as water and fertile land, which attract human settlements and influence the development of cities. Additionally, understanding the logic of geography helps explain the patterns of human settlement and the factors that contribute to the growth and development of cities.