Recognition of Recent Meander Cutoff in the Arkansas River Alluvial Plain in West Central Arkansas and Implications for Land UseCathy Baker
Arkansas Tech University
Continent: North America
Country: United States
UTM coordinates and datum: 15H VB 94812 11442
Climate Setting: Humid
Tectonic setting: Passive Margin
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River floodplains have traditionally served many purposes. Settlements have built up worldwide along rivers because avenues of transportation and abundant water were available and because rich fertile lands could be cultivated. Large population centers today are concentrated on many floodplain areas of the United States, and land use issues in addition the threat of flooding exist.
Floodplains are comprised of materials deposited by both channel and "overbank" processes. The deposits range from highly porous and permeable sands and gravels to low permeability muds and clays. Highly permeable zones within floodplains can provide vast groundwater reserves in addition to the surface water resources; however, the very permeability of those materials can aid in the spread of contaminants. Very slowly permeable zones act to restrict the migration of pollutants and can serve in the limiting of contamination. The distribution of gravels, sands, silts, and clays in floodplain deposits is a consequence of river processes. Predicting the nature and distribution of materials ina floodplain, therefore, involves recognition of the landforms created by the action of the river.
Rivers which create well-developed floodplains (also known as "alluvial plains") most commonly possess a sinuous meandering pattern. The winding pattern develops in response to the interplay of erosion, transportation, and deposition by the river. Lateral erosion by swift currents on stream "outside" banks results in steep cut-banks while deposition by corresponding slower currents on "inside" banks results in the formation of sandbars known as "point bars". The thickest and aerially most extensive deposits of sands on floodplains are in the areas of most recently migrating point bars. Lateral erosion can eventually cut through meanders creating cutoffs. Cutoff meanders are eventually abandoned completely forming oxbow lakes and depressions. During flood stage rivers surge onto floodplains leaving deposits of muds comprised primarily of silt and clay. Thick overbank muds can accumulate on the most distant sections of floodplains. Areas of most recently migrating point bars, therefore, are typically the most vulnerable areas for groundwater contamination, whereas, more distant areas from the active channel usually are less vulnerable to groundwater contamination.
The Arkansas River is a generally southeasterly trending waterway that originates in the Rocky Mountains of western Colorado and joins the Mississippi River in southeastern Arkansas. The Arkansas River in the area of the Holla Bend National Wildlife Refuge exhibits well developed landforms commonly created by a meandering river (Figure 1). Landforms are clearly discernable using a number of sources: surface reconnaissance, aerial and satellite imagery, and topographic quadrangle maps; therefore, the Holla Bend area serves as an excellent model for recognition of meandering river landforms used in prediction of floodplain porosity and permeability characteristics. Landforms present within the area of Holla Bend National Wildlife Refuge include: floodplain, cutbanks, point bars, meanders, cutoffs, oxbow lakes (Figure 1 and Figure 2).
Because recent point bar deposits are most likely to expose permeable materials at the surface of the floodplain, recognition of the most recent meanders, point bars, , shallow, long, water-filled troughs, wetlands, or depressions (Figure 2). In field-scale reconnaissance, ridges and swells appear as a gently rolling landscape (Figure 3). Swales in field-scale view are commonly water-filled after flood events or during wet cycles (Figure 4). Ridges at the Holla Bend National Wildlife Refuge have been utilized to plant windbreaks for soil erosion reduction. Curved lines of southern pine trees mark the crests of many ridges throughout the refuge (Figure 4). and cutoffs is imperative when evaluating floodplains for land use practices that subject the area to potentially harmful contaminants. Although most states regulate the location of landfills, sewage waste treatment facilities, and environmentally harmful manufacturing facilities on floodplains, many potentially environmentally harmful facilities including animal confinement operations may still be sited on vulnerable materials.
Point bars of the Arkansas alluvial plain at Holla Bend are readily discernable in satellite and aerial views due to differences in coloration patterns related to different forage crops and woodlands in the area (Figure 1). As point bars migrate, slight depressions develop between sand bar crests creating alternating crests and troughs known as "ridges and swales" or a "ridge-and-swale topography". In topographic maps, the ridges are discernable as long low narrow ridges and swales are distinguishable as narrow
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