Abstracts

The Great Flood of 2011 and its Influence on the Mississippi River Valley Alluvial Aquifer: Did the River Recharge the Aquifer or What?

Author(s): Byrd, C.

The flooding along the Mississippi River during the months of April and May, 2011 was among the worst in the last 100 years. According to one source, this flooding event was comparable to the flooding that occurred in 1927 and 1993. Data from five gages that include stations at Memphis, Tennessee down to Vicksburg, Mississippi indicate that an all-time record was set at the gage at Vicksburg, Mississippi, and near-record stages were recorded at the other four stations. Seven states were impacted by the floodwaters, and for the first time in many years, the Morganza Spillway was opened to deliberately flood roughly 4,600 square miles of rural Louisiana so that New Orleans and Baton Rouge could be spared.

With all the record setting stages along the Mississippi River adjacent to Mississippi, one may ask, "What influence did the record-high river stages have on the Mississippi River valley alluvial aquifer (MRVA), which is the shallow aquifer in the alluvial plain (Delta) in the northwest portion of the state?" Unfortunately, the answer is not as straight forward as one may wish.

The Mississippi River is the western boundary for both the Delta and the alluvial aquifer. The depositional history of this river system is very complex, thus the geology of the alluvial aquifer is very complex. As the Mississippi River and its tributaries migrated throughout the alluvial plain, many stream channels were eventually cut off from the main river creating oxbow lakes. Through time, many of these oxbows were filled in with very fine-grained sediments, such as silt and clay. If the fine-grained sediments within an oxbow is of sufficient thickness, a "clay plug" is formed that serves to prevent any flow of water through it. Remnants of these old streams and oxbows are present all along length of the Mississippi River. However, if mostly fine- to coarse-grained sand and gravel was deposited, the river and the MRVA are most likely in very good hydrologic connection.

Water levels were collected during early May (the period of peak flooding) by the Office of Land and Water Resources staff. This data along with water level data collected by the Yazoo Mississippi Joint Water Management District staff during early April and early June were correlated with River stages. Then an analysis of geologic data was combined with the water levels and river stages to try to determine hydrogeologic connection.

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1 Best Management Practices #1
2 Delta Water Assessment
3 Flood Assessment & Mgmt.
4 Wetlands
5 Watershed Mgmt. #1
6 Non-Point Source Assessment
7 Modeling
8 Water Quality
9 Best Mgt. Practices #2
10 Delta Water Conservation
11 Sedimentation
12 Storm Water
13 Watershed Mgt. #2
14 Public Water Systems
15 Surface Water Assessment & Evaluation

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