Which Native American group is known for having built huge, earthen

structures in the Mississippi River

Cahokia

Explanation:

Cahokia is the most sophisticated prehistoric Native civilization north of Mexico. Best known for large, man-made earthen structures, the city of Cahokia was inhabited from about A. D. 700 to 1400.

mound builders

Explanation:

The various cultures collectively termed "Mound Builders" were inhabitants of North America who, during a 5,000-year period, constructed various styles of earthen mounds for religious, ceremonial, burial, and elite residential purposes. These included the pre-Columbian cultures of the Archaic period, Woodland period (Calusa culture Adena and Hopewell cultures), and Mississippian period; dating from roughly 3500 BCE (the construction of Watson Brake) to the 16th century CE, and living in regions of the Great Lakes, the Ohio River Valley, and the Mississippi River valley and its tributary waters.[1]

Since the 19th century, the prevailing scholarly consensus has been that the mounds were constructed by indigenous peoples of the Americas. Sixteenth-century Spanish explorers met natives living in a number of later Mississippian cities, described their cultures, and left artifacts.[2] Research and study of these cultures and peoples has been based mostly on archaeology and anthropology.