Final answer:
Transportation, especially with the advent of steamboats and the expansion of the railroad network, was the key factor that encouraged settlement near the Mississippi River.
Step-by-step explanation:
What developed encouraging settlement near the Mississippi River was primarily transportation.
The Mississippi River played a crucial role as a transportation hub, particularly with the introduction of the steamboat, which greatly facilitated the movement of goods and people.
The transport of raw materials to mills and factories, as well as manufactured goods to markets, was essential for trade and the expansion of the nation. With the booming cotton industry in the South, steamboats became a dominant feature of commerce, linking agricultural regions with manufacturing centers.
Moreover, the railroad construction during the mid-century significantly boosted the need for coal, iron, and steel, directly influencing the development of related industries alongside transportation infrastructure.