Final answer:
Building a city at a confluence site offers advantages such as break-of-bulk points for trade, favorable conditions for industries like steel production, and the availability of water and fertile land. These factors combined support population growth and trade but can lead to agglomeration diseconomies if not managed properly.
Step-by-step explanation:
The advantage of building a city at a confluence site includes significant trade and industrial benefits. Confluence sites create break-of-bulk points where multiple forms of transportation such as rivers, roads, or rails intersect, facilitating easier and more cost-effective transfer of goods. This was instrumental in the growth of cities like Pittsburgh, Pennsylvania because these cities could capitalize on the efficiency of transporting heavy materials like iron and coal that were essential for industries such as steel production.
Historically, cities built at confluence sites or near waterways also had the advantage of access to fresh water and more fertile land for agriculture, supporting larger populations and enabling the growth of trade. Additionally, they often became cultural and administrative centers, with facilities for trade, administration, and defense, as well as infrastructures like warehouses which provided employment opportunities. However, such advantages can eventually lead to agglomeration diseconomies, including congestion, pollution, and overstretched facilities, which necessitate careful urban planning to maintain a city's livability and efficiency.