SIGNATURE OF 2008 SPRING FLOOD IN THE LOWER WHITE RIVER FLOODPLAIN, ARKANSAS

The lower White River in SE Arkansas is a tributary to the Mississippi River that typically floods each spring. The upstream section of the lower White River floodplain (LWRFP) has undergone major land use change from bottom land forest to agriculture. In contrast, the downstream section of the floodplain is designated as a RAMSAR wetland and is protected as the forested National Wildlife Refuge. LWRFP has many oxbow lakes located within disturbed and protected areas which receive an influx of river water, sediment and biota during major floods. The flood signatures archived in these oxbow lakes can be used to identify historic trends in flooding caused by human and natural causes.

In 2008 spring the White River experienced 50-100 year flooding. Water and sediment deposited in the oxbow lakes were collected to analyze the flood signature. We also aimed to test if the location of the oxbow lake relative to the river and its location within agricultural or protected area would affect flood signature. Lakes were sampled during the flood (n=12) in March, 2008 and after the flood (n=18) in August, 2008. During the flood the lakes were turbid, but not influenced by their distance from the river or duration of inundation. The lakes were abundant in planktonic diatoms- Aulacoseira granulata, A. pusilla, and Cyclotella pseudostelligera. After the flood the lakes recorded higher alkalinity and conductivity values. These lakes were rich in benthic diatoms- Gomphonema parvulum and Navicula radiosa, however the planktonic diatoms were still high in abundance.

No significant statistical difference was observed in other limnological parameters such as nutrient and metal concentrations in lakes sampled during and after the flood and amongst lakes located in agricultural or protected areas in the floodplain, except for few outlier lakes. The absence of significant trends in the limnology across the LWRFP points to the homogenization effect of flooding. Low heterogeneity in the lakes after the flood indicates that the floodplain still had certain levels of connectivity with the river as was also observed in field. This study provides useful insight about the prevailing limnological conditions during a major flood event; and can serve as an analogue for identifying past flood events from sedimentary records in the study area.