Plum Bayou Mounds Archeological State Park (3 LN 42), formerly known as "Toltec Mounds Archeological State Park", also known as Knapp Mounds, Toltec Mounds or Toltec Mounds site, is an archaeological site from the Late Woodland period in Arkansas that protects an 18-mound complex with the tallest surviving prehistoric mounds in Arkansas. The site is on the banks of Mound Lake, an oxbow lake of the Arkansas River. It was occupied by its original inhabitants from the 7th to the 11th century. The site is designated as a National Historic Landmark.

The identification of the site with the Toltec of Mexico was a 19th century misinterpretation. It was thought that the Toltec people lived in North America and built the mounds. Mary Knapp was the co-owner of the land from 1849 to 1905. She and her first husband purchased the land in 1849 and the land passed to her after his death. She and her second husband made subsequent land purchases near the site. Mary was interested in archeology and had become acquainted with Dr. William Barry who was also interested in Native American artefacts and customs. Mary entered into correspondence with Barry discussing the "Toltec Mounds" site and Barry passed her letter on to Joseph Henry, the secretary of the Smithsonian Institution. Mary would go on to correspond directly with Henry as well as forward artefacts she had collected from the site to the Smithsonian.

This would lead to investigations at the site by archaeologist Edward Palmer from the Smithsonian Institutions Bureau of American Ethnology in 1883 as well as by others which proved that the indigenous ancestors of regional Native Americans built these mounds and all other mounds within the present-day United States. They were part of mound building cultures that flourished from the Late Archaic period into the Protohistoric period. They built earthwork mounds for religious, political and ceremonial purposes, connecting them to their cosmology.

Originally, the name Plum Bayou was borrowed from a nearby waterway and applied to the distinctive culture of the site, discussed below. The site was officially renamed in November 2022 following consultation with the Quapaw Nation and the Arkansas Archaeological Survey.

The people who built the mounds at Plum Bayou Mounds had a culture distinct from other contemporary Native American groups in the Mississippi Valley. Plum Bayou sites are found throughout the White River and Arkansas River floodplains of central and eastern Arkansas, but are also found as far west as the eastern Ozark Mountains. Plum Bayou Mounds is the largest site of the eponymous culture. Their relationships with neighboring cultures such as the Coles Creek culture to the south and Fourche Maline culture to the southwest are still under investigation. The people lived in permanent villages and hamlets throughout the countryside. They built sturdy houses, farmed, gathered wild plants, fished, and hunted.

Mound groups, such as this one, were religious and social centers for people living in the surrounding countryside. Plum Bayou Mounds itself had a small population, made up primarily of political and religious leaders of the community and their families. This center was occupied from the 7th to the 11th century.

Located on the banks of an oxbow lake, the archaeological site once had an 8–10-foot-high (2.4–3.0 m) and 5,298-foot-long (1,615 m) earthen embankment and ditch on three sides. The other side was the lake, now called Mound Pond. Eighteen mounds were built inside the high curving 1 mile embankment, and two were originally 38 and 49 feet (12 and 15 m) high. Mounds were placed along the edges of two open areas (plazas) which were used for political, religious, and social activities attended by people from the vicinity. At least two mounds were used for feasting, as indicated by discarded food remains. Deer were a favorite food. Mound locations seem to have been planned using principles based on the alignment with important solar positions and standardized units of measurement. Most of the mounds were flat-topped platform mounds with buildings on them. Other Native Americans lived on the site in the 15th century, but they did not build the mounds.

The site was declared a National Historic Landmark in 1978.,

Period	Lower Yazoo Phase	Dates	Tensas/Natchez Phase	Toltec Phase	Dates
Historic	Russell	1650–1750 CE	Tensas/Natchez	Quapaw ?	1673
Plaquemine/Mississippian culture Late Plaquemine/Mississippian Middle Plaquemine/Mississippian Early Plaquemine/Mississippian	Wasp Lake	1400–1650 CE	Transylvania/Emerald	Quapaw ?	1650
	Lake George	1300–1400 CE	Fitzhugh/Foster	–	–
	Winterville	1200–1300 CE	Routh/Anna	–	–
Transitional Coles Creek	Crippen Point	1050–1200 CE	Preston/Gordon	–	–
Coles Creek culture Late Coles Creek Middle Coles Creek Early Coles Creek	Kings Crossing	950–1050 CE	Balmoral	–	–
	Aden	800–950 CE	Ballina	Steele Bend	750–900 CE
	Bayland	600–800 CE	Sundown	Dortch Bend	600–750 CE
Baytown culture Baytown 2 Baytown 1	Deasonville	500–600 CE	Marsden	Dooley Bend	400–600 CE
	Little Sunflower	400–500 CE	Indian Bayou	–	–
Marksville culture Late Marksville Early Marksville	Issaquena	200–400 CE	Issaquena	–	–
	Anderson Landing	0–200 CE	Point Lake/Grand Gulf	–	–
Tchefuncte culture	Tuscola	400 BCE–0 CE	Panther Lake	–	–

Table taken from "Emerging Patterns of Plum Bayou Culture:Preliminary Investigations of the Toltec Mounds Research Project", by Martha Ann Rolingson, 1982.

• Poverty Point
• Watson Brake

• Official website
• Archeology at Plum Bayou Mounds Archeological State Park on YouTube
• "Plum Bayou Mounds Archeological State Park". Tripadvisor.