Fluvial Features of the Arkansas River and Floodplain near Dardanelle, Arkansas

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1 Fluvial Features of the Arkansas River and Floodplain near Dardanelle, Arkansas Master Naturalist Field Trip: April 16, 2010 Trip Leader: Cathy Baker, Ph. D. Arkansas Tech University A. Objectives of Field Trip: 1. examine meandering river features of the Arkansas River and floodplain near Dardanelle, Arkansas, and at Holla Bend National Wildlife Refuge southeast of Dardanelle 2. consider environmental implications of different uses of the river and its floodplain B. Sites Visited: 1. Dardanelle: Dardanelle Dam 2. Russellville: River terrace at Okie Doke at the intersection of Ark. Hwy. 247 & Ark. Hwy. 7 north of the Arkansas River Bridge at Dardanelle 3. Dardanelle: River floodplain along Ark. Hwy. 27 southwest of Dardanelle 4. Dardanelle: River terrace at Brearley Cemetery west of Dardanelle on Ark. Hwy Dardanelle: River floodplain southeast of Dardanelle along Ark. Hwy Dardanelle: River & floodplain at Holla Bend National Wildlife Refuge southeast of Dardanelle off Ark. Hwy. 155 C. Overview: The Arkansas River in the region of Dardanelle and southeastward toward the towns of Atkins and Morrilton is a classic example of a meandering river developed in a region of low relief. Meandering rivers such as the Arkansas River in the Dardanelle area transport primarily sands, silts, and clays. Clays are carried in suspension giving the Arkansas River is muddy appearance during most of the year. Sands and silts are rolled and bounced along the river bottom. During periods of low flow the sands and silts stop moving and are deposited, creating sandbars known as point bars in the channel. In areas of low relief rivers are easily obstructed, flow is diverted, and lateral cutting dominates erosion. Meanders become much more winding through time as outside banks are cut away and the slower flow along an inside bank causes the deposition 1

2 of point bars. Literally, meanders grow toward outside banks and fill along inside banks. Through time the migration of meanders creates features such as meander scars and abandoned meanders known as oxbow lakes. Because of the low relief, after flooding events large amounts of sediment accumulate on either side of the stream channel forming a floodplain. During flood events, surge over the channel bank deposits fine (small) sized sand along the lip of the channel. A lip can build a low mound along the edge of the channel known as a natural levee. Natural levees block drainage of flood waters back into the meandering channel; as a consequence marshes and swamps can develop in floodplains. The marshes develop in the lowest parts of the floodplains when the floodplain is adjacent to the valley side slopes. Waters drain from adjacent uplands and from the floodplain toward the edge of the floodplain. Commonly streams develop along the edges of floodplains. Such streams are called yazoo streams, or in much of Arkansas and the rest of the southern states, bayous. During especially high floods natural levees can be breached, and thick sand sheets can be washed across floodplains. Normally floodplains have finer (smaller) grained materials forming the surface of the floodplain, but breaks can create sand splays and thick sand accumulations across the surface of a floodplain. The Arkansas River in the past has experienced periods of higher flow than the levels that exist today. During the Pleistocene (better known as the Ice Age ), snow melt from the Arkansas River headwaters in Colorado produced much higher flows than today. The Pleistocene floodplain was higher than today. As the Arkansas River flow diminished and the river shrank the floodplain shrank. The abandoned floodplain is marked by a flat surface and by a stair-stepped effect. The stairstepped floodplains are known as river terraces. The Pleistocene floodplain which comprises much of the farm land in the Dardanelle and the Holla Bend areas is much larger than the modern floodplain. D. Roadlog : 1. Dardanelle: Dardanelle Dam Take Ark. Hwy. 7 to Dardanelle. Immediately after crossing the bridge, take Front Street north to Dam Road. Take Dam Road north to Dardanelle Dam. At the dam there is a good view of the Arkansas River. The Dardanelle Dam site is a good location to recognize river features such as meanders, point bars, cut banks, and floodplain. The Pleistocene floodplain can be seen across the river encompassing most of an Army Corps of Engineers park, Old Post Road Park, Russellville. The top of the modern floodplain can be identified by a step down from the broad Pleistocene floodplain that comprises almost all of Old Post Road Park. Much of the modern floodplain has been swept of vegetation along both sides of the 2

3 channel at the dam. On the east side of the channel (the inside bank), the modern floodplain is covered with rock boulders. On the west side of the channel (the outside bank) the modern floodplain is primarily sand with stranded driftwood from the most recent flood. The U.S. Army Corps of Engineers engineered the dam with river dynamics in mind. The deepest part of any meandering river channel is along the eroding outside bank. The shallowest part of any meandering river channel is along the depositing inside bank. The dam was constructed with the navigation channel along the outside bank where flow naturally cuts and maintains a deeper channel. However, because the river was equipped with hydroelectric capability, a power house was constructed. The power house was constructed on the west side of the channel. Release of water through the power house creates unnaturally high flow along the west side of the channel. High flow has caused erosion of the point bar along the inside bank downstream from the power house. Wooden groins and rock jetties have been constructed to preserve the point bar along the Dardanelle side of the river channel 2. Russellville: River terrace at Okie Doke at the intersection of Ark. Hwy. 247 & Ark. Hwy. 7 north of the Arkansas River Bridge at Dardanelle Take Front Street south back toward the business area of Dardanelle. Turn left onto Ark. Hwy. 7 and cross the Arkansas River Bridge. Roughly two tenths of a mile from the bridge is a stop light and intersection with Ark. Hwy The soil exposure is at the northeast corner of this intersection. The eroded soil profile at this site represents an older floodplain level of the Arkansas River, a terrace. The terrace has been interpreted to be overbank deposits of a natural levee ranging into a floodplain. Certainly the soils are rich in fine sands, silts, clays and a few larger grains and gravels of chert (flint). A well developed soil profile is present showing an O horizon with a light tan A horizon and a redder more clay rich B horizon. Sediments washed from the soil are fine sandy to silty to the touch. Cracks in the soil are indicative of clays, some of which are swelling varieties of clay. Deep soil fractures can be observed. The fractures originate from shrink-swell from clays and from freeze-thaw of soil moisture during winter months. Water seeping deep into the soil has reduced the amount of oxygen along the fractures. The soil on either side of the fractures are gray (reduced/nonoxygenated). The gray appearance along fractures is known as gleying and is indicative high amounts of clay and percolation along fractures. 3. Dardanelle: Floodplain southwest of Dardanelle along Ark. Hwy. 27 The Pleistocene floodplain can be viewed southwest of Dardanelle. 3

4 Take Ark. Hwy.7 west from the Arkansas River Bridge at Dardanelle to the intersection with Ark. Hwy. 22. Take Ark. Hwy. 7 south of Dardanelle to Ark. Hwy. 27. next stop light. Take Ark. Hwy. 27 west. You are driving along the Pleistocene floodplain. Note how flat and smooth the land surface appears. Ahead (on your right) you will see an abrupt change in slope. That abrupt change in slope is the result of erosion of a terrace. As in most cases, the floodplain slopes very slightly toward the valley side slope, and streams develop right along the valley side. You will cross Smiley Bayou, a yazoo stream developed along the valley side. Most unaltered bayous meander along the edge of the floodplain. Smiley Bayou does not meander at the Ark. Hwy. 27 bridge. The bayou has been straightened to reduce the amount of land occupied by the river and increase amount of land that can be brought into production. 4. Dardanelle: River terrace at Brearley Cemetery west of Dardanelle on Ark. Hwy. 27 Cross Smiley Bayou on foot and you will arrive at the site. Terrace deposits from past higher flow periods of the Arkansas River during the Pleistocene are also present at Brearley Cemetery (on the right-hand side of the road). The nature of the terrace deposits at Brearley Cemetery are quite different from the terrace deposits at Okie Doke. The deposits contain a wider range in sizes of grains, notably some large cobbles. The deposits are channel deposits. The cobbles are of chert (flint) that is not known locally and must have been transported a good ways along the river during periods when the river was much younger. The original, Old Brearley Cemetery is located on a small lot in northern Dardanelle. As the town grew a larger site was needed. The current Brearley Cemetery is located on the terrace. Much of the surrounding upland is shale and shales interlayered with sandstones much harder than the unconsolidated materials of the terrace. The site was obviously chosen for easier digging and to be above potential flooding. 5. Dardanelle: River floodplain southeast of Dardanelle along Ark. Hwy. 155 The Arkansas River floodplain can also be viewed south of Dardanelle. Return to Ark. Hwy. 7 and proceed south to Ark. Hwy. 155 (the road to Holla Bend National Wildlife Refuge). The road initially heads east for about 1 ½ miles before bending back to the south. All of this flat area is Pleistocene floodplain. The road proceeds south for about another mile before bending back to head directly east. Approximately ½ mile of roadway proceeding east again, there is an almost imperceptible slope in the road. Looking to your 4

5 right, you will see a step extending off to the south. This step is the top of the Pleistocene floodplain. As you proceed east from this point for the rest of the trip, you are driving on the modern/current floodplain. Flooding in 1927 breached the levees protecting this section of the floodplain. Sands scoured and washed from the channel during the flood (an event known as evulsion) and buried the fertile silts and clays of the floodplain under a sheet of sand. Farmers had to abandon fields because of the infertility of the sands and lack of heavy equipment to remove such a massive volume of sand. Today, the land is still not used for row-crop farming, but is in pastureland. Stop and sample the soils they are sandy. In this area there is a hog confinement operation with confinement buildings and effluent pond. What is the potential for contamination in this area if this pond were to leak or to overflow? 6. Dardanelle: River & floodplain at Holla Bend Wildlife Refuge southeast of Dardanelle off Ark. Hwy. 155 Holla Bend National Wildlife Refuge preserves many meandering stream features. Proceed further on Ark. Hwy. 155 to the entrance of the refuge. At the visitor s center stop, pay your fees, and pick up a map of the refuge. We will use it as a map and road log for this trip. Holla Bend is an old meander loop in the Arkansas River. However, it is an engineered rather than natural cut off feature. During the middle 1950 s the U.S. Army Corps of Engineers created a cut-off in the project to prepare the river for navigation and in flood control. The Holla Bend National Wildlife Refuge incorporates all of the old meander cut-off and some of the surrounding floodplain and cut channel. a. Proceed into the refuge. As you drive into the refuge you will pass over a long earthen dike this is the dike that was built to cut-off the old Holla Bend loop and keep the river from reoccupying its channel. b. After crossing the dike take the first left-hand turn. This loop will take you along the Arkansas River channel. Across the channel, point bars can be clearly seen. The Holla Bend National Wildlife Refuge management plan has included planting of trees; however, the trees chosen include willow and pin oaks and none of the native floodplain varieties of poplars such as the cottonwood tree. c. If you wish, you can continue on the east end of the refuge where a boat ramp is shown in the refuge brochure. At the east end of the refuge is another dike that prevents the river from re-occupying the Holla Bend cutoff. In this case the dike also serves as a dam and creates an oxbow lake. 5

6 d. Running roughly through the center of the refuge is Center Road. Take Center Road through the center of the refuge. The road bisects an area rich in meandering scars of old point bar deposits. Meanders scars are marked by ridge-and-swale features. As you look southward along Center Road you can see subtle dips and rises in the road. These are the ridges and swales. As you drive south, you can see pine-tree wind breaks that top the ridges and bend to the east and west following the scars of the meanders. As you drive south, you can see depressions that bend to the east and west following the swale scars of the meanders. The ridge-and-swale landscape is indicative of old sandy point bars. The whole area is underlain by sands. Sands drain quickly. Even in rainy years little water stands on much of the refuge except in the old channel and oxbow sections. Vast groundwater reserves are located in the sands underlying the refuge, and water is used to irrigate forage crops planted for local wildlife and migratory birds. e. Proceeding south on Center Road to Holla Bend Road, you can visit the wildlife viewing stand in the southern part of the refigure. The viewing stand allows good views of the Old River Channel and modern floodplain. f. As you leave the refuge following the roads toward Lodge Lake and Long Lake, you can look back to your right and see the ridge-andswale landscape of the meander scars. 6

1 2 4 3 5 6 Figure 1: map showing locations

7 Figure 1: map showing locations for field trip stops (Map courtesy DeLorme) 7

8 Figure 2: Aerial photograph view of the field trip area (photograph courtesy Google Earth) 8

9 Figure 3: Arkansas River fluvial features at Dardanelle: channel, point bars, floodplain, meander scars, groins, and jetties (photograph courtesy Google Earth) 9

10 Figure 4: Arkansas River fluvial features south of Dardanelle: meander scars, cutoffs, point bars, river terraces and splay features (photograph courtesy Google Earth) 10

11 Figure 5: Arkansas River fluvial features at Holla Bend National Wildlife Refuge: point bars, oxbow lake, meander scars, ridge and swell topography, river cutoffs, and terraces (photograph United States Department of Agriculture) 11

Erosion Surface Water. moving, transporting, and depositing sediment.

Erosion Surface Water. moving, transporting, and depositing sediment. + Erosion Surface Water moving, transporting, and depositing sediment. + Surface Water 2 Water from rainfall can hit Earth s surface and do a number of things: Slowly soak into the ground: Infiltration