Final answer:
The exchange of trade between the Coastal and Plateau Indians occurred in the Pacific Northwest, a region that supported diverse cultural exchanges and trade networks.
Step-by-step explanation:
The thriving exchange of trade between the Coastal and Plateau Indians took place in the Pacific Northwest. This region was known for its rich environment that combined marine resources with those of the land, enabling diverse cultures to develop complex trade networks. These networks facilitated the exchange of goods such as food, tools, and raw materials, as well as cultural practices like the potlatch ceremony, which is specifically associated with the peoples of this region. The Pacific Northwest's geography, with its numerous rivers and coastal inlets, was ideal for long-distance trade.