Final answer:
Merchants were the group that most often accumulated riches during the gold rush, as they provided goods and services to miners.
Step-by-step explanation:
The men who mostly accumulated riches during the gold rush were primarily merchants. The gold rush era was marked by a rush of prospectors, but the easily accessible placer gold was quickly exhausted. As the California Gold Rush progressed, the individual prospector with his pan was replaced by larger mining operations. It required significant investments and resources to continue mining, which were beyond the reach of most miners. Consequently, those who provided goods and services to the miners, the merchants, often ended up making more consistent profits and thus could amass wealth more effectively compared to the prospectors.