Rapid River Meandering in the Lowermost Arkansas River, Arkansas, USA

Abstract:

The Arkansas River below Pendleton (40 km above the confluence with the Mississippi River) is among the most rapidly meandering rivers in North America. This is remarkable considering it is downstream of the McClellan-Kerr Arkansas River Navigation System (MKARNS), a significantly engineered and heavily sand-mined section of river. We place this reach within the context of other meandering rivers and determine the causes for rapid meandering. We use Google Earth Engine to quantify the migration rate of meander bends using Landsat imagery between 1984 and 2014 and historical maps to confirm similar behavior before the implementation of MKARNS. There is minimal migration upstream and immediately downstream of the lock and dam at Pendleton, AR, followed by significant migration further downstream. Preliminary measurements indicate the active reach, the final 30 km, migrates about 60 m/yr, with some sections migrating at 80 m/yr in a river ranging from 200-500 m wide. This is about 0.2 channel widths per year, far higher than previously measured pristine Amazon tributaries (0.003-0.07 channel widths per year). Engelund-Hansen and USACE models indicate bed material sediment transport on the order of 1 MT/yr in the entire river below Pendleton. Using bar migration rates and estimates for channel depth, sediment density and porosity, the estimated sediment involved in point bar deposition in the active reach is also on the order of 1 MT/yr. Therefore, the proportion of bed material involved in bar migration transitions from near zero to almost 100%. Furthermore, in the meander migration previously described, meander bends migrate downstream as opposed to the more common lateral expansion. Finally, sinuosity has notably decreased in this region, lowering from 2.0 to 1.5 from 1984-2014 – much of this straightening occurred in the last 5 years. Remote sensing, sedimentology, and hydrodynamic models will be presented to attribute this remarkable meandering behavior to river engineering, sediment transport dynamics, or the arrival of the Arkansas River on the Mississippi River floodplain.