Geodiversity refers to the full variety of natural geologic (rocks, minerals, sediments, fossils, landforms, and physical processes) and soil resources and processes that occur in the park. A product of the Geologic Resources Inventory, the NPS Geodiversity Atlas delivers information in support of education, Geoconservation, and integrated management of living (biotic) and non-living (abiotic) components of the ecosystem.
Geologic Features and Processes
With the exception of the deeply buried Eocene (56 million–34 million years ago) Jackson Group (geologic map unit Tj), the geologic units in the memorial and surrounding area are young, unconsolidated, clays, silts, sands, and gravels associated with the Arkansas River fluvial system deposited atop a foundation of glacial outwash deposits from the Pleistocene (2.6 million to 11,700 years ago) ice ages. The Arkansas River has meandered back and forth across its floodplain for millennia, leaving a complex pattern of abandoned channels, natural levees, point bars, and backswamp deposits. Human activities left their mark on the landscape and are recorded in the geologic record (and on the geologic map; scale 1:24,000) as artificial fill in the form of American Indian mounds, flood control structures, and agricultural fill.
This location factored heavily in the region’s human history because it is the first high ground encountered on banks of the Arkansas River upstream from its confluence with the Mississippi River. Coarser-grained, Pleistocene terrace deposits (Qt) underlie the high ground at both units of the memorial. Almost annually, torrential floods of the Arkansas River would inundate any low ground along its banks, making higher ground a necessity for successful settlement.
The Arkansas River and its tributaries dominate the landscape within and surrounding Arkansas Post National Memorial. Flooding, erosion, dredging, and sedimentation are important resource management issues. The Memorial Unit is located on terrace deposits (geologic map unit Qt) that provide high ground northwest of the main channel of the Arkansas River. Post Bend, Post Bayou, and Little Post Bayou surround the Memorial Unit on three sides. The bayous are abandoned channels of the Arkansas River. The 49 m (162 ft) elevation (above sea level) contour line is designated as the memorial boundary.
Flooding and Flood Control Structures
Nearly 1,600,000 ha (4,000,000 ac) of the lower Arkansas River basin is subject to flooding by the Arkansas, White, and/or Mississippi rivers. Because three major rivers could inundate the area singly or in combination, the memorial area experienced frequent major floods prior to the construction of Army Corps of Engineers (ACOE) river control structures (Waterways Experiment Station 1951). At least five large floods occurred between 1833 and 1872. From 1872 to 1950, 13 major floods took place (Waterways Experiment Station 1951). The 1927 flood was particularly devastating, inundating the entire Arkansas Post site, and destroying or damaging much of the post- Civil War community (Coleman 1987; Evans 2005).
To promote river navigation, mitigate flooding, and maintain the current channel location, the ACOE constructed extensive levees, canals, locks, and dams throughout the Arkansas and Mississippi river valleys during the middle 20th century. The Arkansas Post Canal passes Arkansas Post and is part of the ACOE McClellan-Kerr Arkansas River Navigation System connecting Tulsa to the Mississippi River along the Arkansas River. The Wilbur D. Mills Dam—visible from an overlook on the memorial’s nature trail—is located on the Arkansas River between the two units of the memorial. Lock #2 is located on the Arkansas Post Canal between the Arkansas and White rivers. As part of the Mills dam and lock project, the ACOE constructed a stone revetment (riprap) between elevations 49 and 50 m (160 and 165 ft) within Arkansas Post National Memorial, along the peninsula’s shoreline (Carrera 1976; Evans 2005; E. Wood and K. McCallie, superintendent and resource manager, Arkansas Post National Memorial, conference call, 10 April 2013). A levee system was proposed but rejected due to its impact on the historical integrity of the site (Evans 2005).
Virtually all landforms in the Arkansas Post area result from variations in the energy of the Arkansas River’s flow and the quantity and physical characteristics of sediments derived from surrounding areas (Saucier 1994). Fluvial processes both construct (deposit) and erode landforms. Meandering river channels, oxbow lakes, point bars, natural levees, and backswamp deposits are the primary fluvial features within and near the memorial. Today, the river banks near Arkansas Post are heavily modified by ACOE river control structures.
Meandering River Channels
The characteristic meandering channels of the Arkansas River are a result of an exceptionally low gradient (slow flow) and high sediment load. Below Little Rock, the Arkansas River enters the alluvial valley of the Mississippi River and flows for 286 km (178 mi) with an average gradient of just 0.1 m/km (0.6 ft/mi). The river’s low channel gradient reduces its sediment-carrying capacity, resulting in rapid sedimentation. The lower Arkansas transports many million tons of suspended material each year (Waterways Experiment Station 1951).
The Arkansas River forms a series of fairly uniform S- shaped curves that define a classic alluvial meander pattern. Nearly all stages of the pattern’s development are visible in the lower Arkansas River valley (Waterways Experiment Station 1951). Because sedimentation rates are highest near an active channel, an alluvial ridge develops in the meander belt at a higher elevation than the adjacent floodplain (Saucier 1994). This elevated topography diverts the river’s flow laterally, causing the channel to broaden, shallow, and meander.
As a river flows through its channel, around curves, the flow velocity (and thus erosive energy) is greatest on the outside of the bend (see “Point Bars” section). The river erodes into its bank on the outside of a curve and leaves point bar deposits on the inside of the bend. As the process continues, the outside bend retreats farther, while the inside bend migrates laterally, thus creating migrating meanders.
As meander bends migrate, the “neck” of land between two bends narrows. Eventually the neck may be cut through and the meander is abandoned by the stream. Abandoned meanders (Qcmac), commonly known as “oxbows,” are prominent features on the regional landscape (Ausbrooks and Prior 2009a, 2009b). Water may periodically flow through the abandoned meander during floods or particularly high flow events. The oxbows hold water as ponds and lakes (e.g., Lake Dumond at the Osotouy Unit), but over time will fill in with very fine-grained sediment, often clay. Such “clay plugs” may be slightly more resistant to erosion than surrounding sediments, slowing channel migration (Waterways Experiment Station 1951).
Point bars (Qcmpb) are crescent-shaped ridges of sand, silt, and clay deposited on the inside of meander loops where the water’s velocity is slowest (Waterways Experiment Station 1951; Ausbrooks and Prior 2009a, 2009b). A point bar’s shape conforms to the curvature of the source channel, but bars may truncate each other forming a complex pattern as meandering continues (Waterways Experiment Station 1951). As a meander migrates, successive point bars build up laterally and may reach heights as much as 3 m (10 ft) above mean low water levels. The curved, sand- and silt-cored ridges are separated by low, clay- and silt-rich swales. The alternating ridges and swales, sometimes referred to as scroll-bar sequences, define classic point–bar accretionary topography (Waterways Experiment Station 1951; Saucier 1994). This topography is particularly well developed near the Memorial Unit and is easily visible in aerial imagery.
During high flows or floods, a river deposits natural levees (Qsonl) of sand and silt along its banks. These deposits represent the relatively coarse-grained component of a river’s suspended sediment load and often form the highest areas of an alluvial region’s land surface. Natural levees also create fertile farmland. For these reasons, they have been the most significant landforms in human-settlement patterns, influencing building locations, transportation routes, agriculture, and industry development (Saucier 1994).
The width of a natural levee is proportional to the size (volume) of the river responsible for its formation; the height of a natural levee is an indicator of the difference in water levels between normal flow and flood stage. Both dimensions increase as a function of age (Saucier 1994). In the lower Arkansas River, the thickest natural levees are approximately 5 m (15 ft) high with the coarsest sedimentary components at their crests grading landward into finer grained, backwater deposits (Waterways Experiment Station 1951; Ausbrooks and Prior 2009a, 2009b). They typically are largest on the outside of bends (Waterways Experiment Station 1951).
The modern Arkansas River channel lacks significant, flanking natural levees (Qsonl). This may be because it had not occupied its current position long enough to build up large levees prior to the installation of artificial levees (Qafe) (Waterways Experiment Station 1951).
Backswamps are low-lying areas that retain water during floods or high flow. They are separated from the river channel by natural levees (Qsonl) (Waterways Experiment Station 1951; Ausbrooks and Prior 2009a, 2009b). Low relief and dense vegetation characterize a backswamp. The relatively slack floodwaters lay down layers of (fine-grained) clays and organic material (Waterways Experiment Station 1951).
Park Lake in the Memorial Unit, and Lake Dumond in the Osotouy Unit are the two most prominent lacustrine features within the memorial. Park Lake is an artificial lake dating to the memorial’s history as a state park. Lake Dumond is an abandoned meander or oxbow lake that is currently filling with silt.
Post Bayou, Post Bend, and Little Post Bayou surround the Memorial Unit. These are abandoned Arkansas River channels. The Memorial Unit also contains a 4-ha (9-ac) artificial lake, Park Lake (see cover). The lake formed by damming ravines in the 1930s (Evans 2005). Lake Dumond is an oxbow lake within the Osotouy Unit.
Geology Field Notes
Students and teachers of college-level (or AP) introductory geology or earth science teaching courses will find that each park's Geologic Resource Inventory report includes the Geologic History, Geologic Setting, and Geologic Features & Processes for the park which provides a useful summary of their overall geologic story. See Maps and Reports, below.
Arkansas Post National Memorial is a part of the Coastal Plain Physiographic Province and shares its geologic history and some characteristic geologic formations with a region that extends well beyond park boundaries.
Geologic Resources Inventory
- Scoping summaries are records of scoping meetings where NPS staff and local geologists determined the park’s geologic mapping plan and what content should be included in the report.
- Digital geologic maps include files for viewing in GIS software, a guide to using the data, and a document with ancillary map information. Newer products also include data viewable in Google Earth and online map services.
- Reports use the maps to discuss the park’s setting and significance, notable geologic features and processes, geologic resource management issues, and geologic history.
- Posters are a static view of the GIS data in PDF format. Newer posters include aerial imagery or shaded relief and other park information. They are also included with the reports.
- Projects list basic information about the program and all products available for a park.
Related ArticlesArkansas Post National Memorial
National Park Service Geodiversity AtlasThe servicewide Geodiversity Atlas provides information on geoheritage and geodiversity resources and values within the National Park System. This information supports science-based geoconservation and interpretation in the NPS, as well as STEM education in schools, museums, and field camps. The NPS Geologic Resources Division and many parks work with National and International geoconservation communities to ensure that NPS abiotic resources are managed using the highest standards and best practices available.
For more information on the NPS Geodiversity Atlas, contact us.