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Title: Measuring connectivity of floodplain waterbodies to the lower Mississippi River
Authors: Oliver, Amanda J. M.
Murphy, Catherine E.
Little, Charles D., 1946-
Killgore, K. Jack
Keywords: Mississippi River--Alluvial plain
Floodplain ecology
Optical radar
Digital elevation models
Publisher: Coastal and Hydraulics Laboratory (U.S.)
Series/Report no.: MRG&P Tech Note;MRG&P Tech Note No. 1
Abstract: Introduction: Prior to levee construction, Mississippi River floodwater spread across a 30- to 124-mile-wide floodplain, exchanging nutrients, sediment, and organisms between the main channel and floodplain waterbodies. Levees now constrict the floodplain to an average 5-mile width and thus also reduce the number of waterbodies that the river can connect to (Biedenharn et al. 2000). Additionally, Lower Mississippi River (LMR) bendway cutoffs have increased the river’s slope and stream power (Biedenharn et al. 2000), potentially changing connectivity. Maintaining a gradient of connectivity—from waterbodies that are always connected to those that are rarely connected—is essential for ecosystem health (Ward and Tockner 2001). Rarely connected waterbodies support unique species assemblages (Tockner and Stanford 2002) and ecosystem diversity (Lubinski et al. 2008; Thomaz et al. 2007) while more frequently connected waterbodies can sequester and exchange nutrients from the main channel and provide areas for spawning, rearing, and refuge for riverine organisms (Baker et al. 1991; Stoffels et al. 2014). The U.S. Army Corps of Engineers has the ability to change connectivity by notching dikes, dredging to remove sand plugs and to maintain tie channels, and strategically placing revetment and dikes. To quantify connectivity and evaluate habitat restoration potential, a study was initiated encompassing the river and floodplain downstream of Helena, Arkansas, extending from approximately River Mile (RM) 620 to 642. This area was selected because of available long-term gage data, elevation data, accessibility, and the presence of numerous waterbodies that ideally were discrete and arrayed along a gradient of connectivity. The study attempted to answer several questions, one of which was how the biological community and biogeochemical processes differ between waterbodies with different degrees of connection. Elevation data and gage data were used to locate connecting channels, to identify connection thresholds and locations, and to determine connection frequency. Ward et al. (1999) define connectivity as the transfer of water and material between the river’s main channel and the floodplain. A connection threshold is “the elevation river water must reach to enter” or, conversely, the elevation below which “water cannot gravity drain from” a discrete waterbody (Cobb et al. 1984) (Figure 1). This technical note documents a portion of the study: the data and methods to develop an elevation model and to determine connection thresholds and connection frequency.
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