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Arkansas Post National Memorial is a landscape built almost entirely by a river. This summary comes from the National Park Service's Geodiversity Atlas, drawn from the park's Geologic Resources Inventory.
The park's Geologic Resources Inventory report. Credit: National Park Service.
Young sediments, old foundation
Apart from the deeply buried Jackson Group, from the Eocene (56 million to 34 million years ago), the rocks here are hardly rocks at all: young, loose clays, silts, sands and gravels laid down by the Arkansas River, on top of glacial outwash from the Pleistocene ice ages (2.6 million to 11,700 years ago). Over millennia the river has swung back and forth across its floodplain, leaving abandoned channels, natural levees, point bars and backswamps. People left their mark too — the geologic map records American Indian mounds, flood-control structures and agricultural fill as artificial deposits.
Why people settled here
This is the first high ground on the banks of the Arkansas River upstream from its junction with the Mississippi. Coarser Pleistocene terrace deposits lie beneath the high ground in both of the memorial's units. With torrential floods drowning low ground along the river almost every year, high ground was essential for settlement.
The Memorial Unit sits on that terrace northwest of the river's main channel, surrounded on three sides by Post Bend, Post Bayou and Little Post Bayou — all abandoned channels of the Arkansas. The memorial's boundary follows the 49-meter (162-foot) elevation contour.
The French Cross exhibit near the visitor center. A cabin and cross stood at the post Henri de Tonti founded as "Poste de Arkansea" in 1686. Credit: National Park Service / Ed Wood.
Floods and flood control
Nearly 4,000,000 acres (1,600,000 hectares) of the lower Arkansas River basin can be flooded by the Arkansas, White or Mississippi rivers, alone or together.
| Period | Floods |
|---|---|
| 1833–1872 | At least five large floods |
| 1872–1950 | 13 major floods |
| 1927 | Especially devastating: the whole Arkansas Post site went under, and much of the post–Civil War community was destroyed or damaged |
In the mid-20th century the Army Corps of Engineers built levees, canals, locks and dams across the Arkansas and Mississippi valleys for navigation and flood control. The Arkansas Post Canal, part of the McClellan-Kerr Arkansas River Navigation System linking Tulsa to the Mississippi, passes the memorial. The Wilbur D. Mills Dam, visible from the nature trail, stands on the river between the memorial's two units, and Lock #2 lies on the canal between the Arkansas and White rivers. The Corps also armored the peninsula's shoreline in the memorial with stone riprap. A levee was proposed too, but rejected because it would have harmed the site's historical integrity.
Landforms of a meandering river
Below Little Rock, the Arkansas River enters the Mississippi's alluvial valley and flows 178 miles (286 km) with an average drop of just 0.6 feet per mile (0.1 m/km). That gentle slope slows the river, so it drops sediment fast — the lower Arkansas carries many million tons of suspended material a year — and forms classic S-shaped meanders. Sediment piles up fastest near the channel, building a ridge that pushes the flow sideways, so the channel widens, shallows and meanders. The river cuts into the outside of each bend, where flow is fastest, and builds deposits on the inside.
| Landform | What it is | Where to see it |
|---|---|---|
| Oxbow lakes | Meanders cut off when the neck of land between two bends is breached. They hold water, then slowly fill with fine clay "plugs" that can resist erosion | Lake Dumond, in the Osotouy Unit |
| Point bars | Crescent ridges of sand, silt and clay on the inside of bends, up to 10 feet (3 m) above low water, separated by clay-rich swales — "scroll-bar" topography | Well developed near the Memorial Unit and easy to see in aerial imagery |
| Natural levees | Sand and silt dropped along the banks in floods; the highest, most fertile ground, and the key to where people built, farmed and traveled. The thickest on the lower Arkansas are about 15 feet (5 m) high | The modern channel lacks large ones — it may not have occupied its position long enough before artificial levees were built |
| Backswamps | Low, densely vegetated areas behind the levees where slack floodwater lays down clay and organic matter | Across the floodplain |
The memorial's two lakes tell both stories: Lake Dumond is a natural oxbow filling with silt, while Park Lake, a 9-acre (4-hectare) lake in the Memorial Unit, was made by damming ravines in the 1930s, when the site was a state park.
The park's geologic map from the Geologic Resources Inventory. Credit: National Park Service.
Regional setting
Arkansas Post lies in the Coastal Plain physiographic province, sharing its geologic history with a region far larger than the park. Digital geologic maps, a report and a soil resources inventory for the memorial are available through the NPS DataStore.
Sources
Based on "NPS Geodiversity Atlas—Arkansas Post National Memorial, Arkansas," National Park Service; a work of the United States government in the public domain.
In queste pubblicazioniArkansas Post National Memorial
Licenza: CC0 1.0 (pubblico dominio) · Tratto da www.nps.gov
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