Mississippi River Ecohydrology: Past, present and future

Ecohydrology & Hydrobiology 13 (2013) 73–83

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Original Research Article

Mississippi River Ecohydrology: Past, present and future Paul J. DuBowy * Mississippi Valley Division, U.S. Army Corps of Engineers, Vicksburg, MS 39181-0080, USA

A R T I C L E I N F O

A B S T R A C T

Article history: Received 28 November 2012 Accepted 27 February 2013 Available online 26 March 2013

For well over 100 years the twin objectives of navigation/transportation and flood-risk management have led to an intensively managed Mississippi River system which, hydrologically and hydraulically, has been radically altered. Additionally, human disturbances have led to more recent environmental impacts, including increased agricultural chemicals and industrial toxins, altered salinity and sediment loads, and introduction of non-native species, resulting in a riverine and riparian ecosystem far different from its historical condition. These anthropogenic impacts, combined, have led to reduced biodiversity, resilience and ecosystem services provided to society. The goals and objectives of ecosystem rehabilitation must include mechanisms to reverse the physical, chemical and biological alterations to the Mississippi River. Implementing Ecohydrology goals through the reestablishment of the historical floodplain is paramount to successful remediation. Likewise, the ability to measure project success is critical to evaluating the efficacy of the entire rehabilitation program. Published by Elsevier Urban & Partner Sp. z o.o. on behalf of European Regional Centre for Ecohydrology.

Keywords: Mississippi River Hydraulics and hydrology Navigation and flood control Nutrients and sediment Invasive species

1. Introduction Sustainability of large river systems requires an understanding of the historical conditions found in pre-settlement ecosystems and how anthropogenic changes have altered these systems. The restoration or rehabilitation of these large rivers can lead to increased carrying capacity and biological diversity with resulting resistance to stress. Ecohydrology is the analysis of integrated biological and hydraulic processes at the landscape level with resulting changes to hydrological, physical, chemical and ecological attributes of aquatic systems, river basins, and adjacent floodplains and riparian areas (Zalewski et al., 1997; Zalewski, 2000). By extension, the Ecohydrological process could be employed as a tool to guide rehabilitation protocols through a careful analysis of hydrologic drivers and other environmental stressors (Linke et al., 2012). These alterations to drivers and stressors result in modifications

* Tel.: +1 601 634 5930. E-mail address: [email protected].

to water quality, biodiversity, and other ecosystem goods and services. Consequently, Goals, Objectives, Targets and Metrics of restoration projects can emanate directly from these same Ecohydrological principles (DuBowy, 2010). The key to the rehabilitation of sustainable river or coastal systems is to incorporate human dimensions and values and find middle-ground between continued economic growth and the preservation and conservation of natural resources and human well-being (Weinstein, 2008; Chı´charo et al., 2009; Dufour and Pie´gay, 2009; Lockaby, 2009; Bunch et al., 2011). The Mississippi River catchment is the third largest river system in the world. The Mississippi River, itself, is over 3700 km long and, together with the Ohio and Missouri Rivers, drains all or parts of 31 U.S. states and two Canadian provinces. Because the Mississippi system varies widely in hydraulics and hydrology from source to the Gulf of Mexico, ecosystem sustainability likewise takes different forms in different regions along the river. The effects of river regulation, floodplain development, watershed modifications and inputs of agricultural chemicals present constant challenges to ecosystem rehabilitation along the

1642-3593/$ – see front matter . Published by Elsevier Urban & Partner Sp. z o.o. on behalf of European Regional Centre for Ecohydrology. http://dx.doi.org/10.1016/j.ecohyd.2013.02.003

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Mississippi River. Moreover, the continuing negative impacts of non-native species present additional challenges to river rehabilitation. Consequently, sustainable river and floodplain systems must be developed within the context of the potentially different directions that other societal uses have taken various reaches of the river (DuBowy, 2010). This Mississippi River case study can be examined as a primer for utilizing innovative Ecohydrological techniques and concepts in the potential rehabilitation of extensive and important aquatic ecosystems. The focus of this review is an examination of the anthropogenic causes which have resulted in the current altered/degraded river ecosystem, ongoing remediation efforts on the Mississippi River, and implementation of progressive planning and evaluation protocols to facilitate future rehabilitation efforts. 2. Historical conditions The Mississippi River is not a single homogeneous unit (Fig. 1). From its source in northern Minnesota to the Gulf

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of Mexico one can identify at least five distinct Mississippi Rivers based on geomorphology and hydraulics. (1) The far upper reach, from the river’s source at Lake Itasca, Minnesota, to St. Anthony’s Falls (Minneapolis), Minnesota (793 km length) is characterized as a typical boreal stream with a calcareous, cobble streambed. (2) From Minneapolis to the confluences with the Missouri and Illinois Rivers (near St. Louis, Missouri; 1069 km), the Upper Mississippi River (UMR) historically was a very shallow river with a main channel and numerous paleochannels in a braided configuration. (3) Below St. Louis to the confluence with the Ohio River (Cairo, Illinois; 310 km), the Middle Mississippi River (MMR) starts to develop a deeper, broader cross-section with finer sediments. (4) The Lower Mississippi River (LMR) begins at Cairo; the Ohio provides most of the water in the Mississippi system, so the LMR is extremely broad and deep (the Mississippi Alluvial Valley) and is characterized by a sinuous course with many oxbows, chutes and floodplain lakes (1027 km). While these floodplain features often appear to be disconnected from the main channel of the river, as the river stage rises these backwaters and chutes frequently become reconnected to the main river channel (Marks-Guntren et al., 2013). (5) At the confluence with the Red and Atchafalaya Rivers (Simmsport, Louisiana) the Mississippi River changes from one with tributaries entering to one with distributaries flowing out; the Mississippi system begins to form the extensive delta in coastal Louisiana (507 km). Interestingly, the Atchafalaya, the largest distributary, has a much shorter length (220 km), and hydraulically would become the new Mississippi River were it not for human intervention (Old River Control Complex). As with all large river systems, the primary ecological driver is hydraulics (longitudinal flow) and hydrology (vertical/lateral flow). The entire Mississippi River system (including the Ohio, Missouri and other tributaries) exhibits high flow in spring (March–June) leading to flood pulses that provide an annual subsidy of sediment, nutrients, and energy to drive primary, and subsequently secondary, productivity (Ahearn et al., 2006; Preiner et al., 2008; Roach et al., 2009; McGinness and Arthur, 2011; Meyer et al., 2013). However, on the Mississippi, changes to riverine hydraulics and hydrology have led to a radically altered system. The management of stressed river and floodplain ecosystems is a major challenge for water managers worldwide in the near future. It is incumbent to understand causes and effects of anthropogenic changes in order to better these large systems. Over half of the world’s large river systems have been impacted by dams for navigation, flood control, and hydroelectricity (Nilsson et al., 2005). Additionally, management approaches need to be adaptive and embedded within a catchment-wide concept to cope with upcoming pressures originating from global change (Tockner et al., 2010). 3. Ecosystem alterations 3.1. Navigation and flood risk management

Fig. 1. The Mississippi River catchment (excluding the Ohio and Missouri subcatchments). The Mississippi River and Tributaries Project is highlighted in red.

Perhaps the most pronounced change to the Mississippi River system has been the extensive alterations to river

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hydraulics and hydrology due to navigation and flood risk management (‘‘flood control’’). Long before there were railroads or interstate highways, the Mississippi River was the major avenue of commerce in the central United States. Barges are the most efficient form of commercial shipping; water transportation moves 16% of the nation’s freight for

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2% of the freight cost (Institute for Water Resources, 2008). Currently, barge traffic accounts for 500-million tonnes of goods shipped annually down the Mississippi to the Gulf (DuBowy, 2010). Additionally, deep-draft navigation provides important international shipping opportunities along the Louisiana Chemical Coast, and navigation

Fig. 2. The Mississippi River and Tributaries Project showing its extensive levee system (tan) and floodways and diversions (red).

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provides an important societal service. Likewise, protection of infrastructure and property from the perils of annual river floods is a major concern of all citizens of this region. Since 1928 the U.S. Government has funded US$14.5 billion on the LMR toward the Mississippi River and Tributaries (MR&T) Project (Figs. 1 and 2) alone and has received an estimated US$478 billion return on that investment, including savings on transportation costs and flood damages (Camillo, 2012). Levees play the most important role in Mississippi River flood control by preventing or eliminating most of the historical overbank flooding during high water events. In MR&T 5998 km of levees designed to withstand a Project Flood have been authorized through the Flood Control Act of 1928; this levee system is now 95% complete (Camillo, 2012). Additional levees exist on the UMR, MMR and nearly all tributaries. However, by eliminating these periods of extensive, shallow flooding, most of the historical floodplain has now been eliminated by hydrologically disconnecting the floodplain from these flood pulses (Franklin et al., 2009). The geological Mississippi Alluvial Valley is a broad, flat floodplain (Saucier, 1994). Between Memphis, Tennessee, and Little Rock, Arkansas, the floodplain is more than 200 km wide; between Vicksburg, Mississippi, and Monroe, Louisiana, it is 125 km wide. The constructed levees have now constricted the active floodplain (the batture) to less than 10 km and frequently much less; where once flood pulses were wide and shallow, they are now narrow and deep, constrained by levees. In the Mississippi River system, navigation improvements fall into two different categories. In the UMR (above St. Louis), the Ohio River, and other important tributaries (e.g., Red, Illinois, Arkansas, Ouachita Rivers), the principal navigation structures are locks and dams. The dams pool water behind them, raising the surface elevation of the water, and allow for safe navigation for a longer portion of the year or during other times of low water (above Minneapolis the mainstem dams of the Mississippi Headwater Project hold water to be released in late summer and fall to provide additional water for downriver navigation below the Twin Cities). The locks allow towboats and barges to pass from one river reach to the next (from pool to pool). The MMR and LMR (below St. Louis), the Lower Missouri River (Sioux Falls, South Dakota – St. Louis) and the Atchafalaya River (a principal distributary of the Mississippi in Louisiana) are characterized as ‘‘open rivers,’’ where dams and locks are not required to raise water levels for navigation (there is a lock in the Old River Control Complex at the head of the Atchafalaya to regulate the mandated 70:30 division of water between the Mississippi and Atchafalaya Rivers and to prevent capture of the Mississippi by the Atchafalaya, but it is not necessary for navigation). In the MMR, LMR and Lower Missouri Rivers the principal navigations structures are wing dikes, closing structures and revetment (articulated concrete mats [ACM] or rock). Revetment functions to eliminate the dynamic, undulating sinuosity of the river; locking the river channel in place provides an established navigation channel, reduces dredging, and protects adjacent riverside infrastructure, notably flood-control levees, docks and boat launches, and bargeloading facilities (Smith and Winkley, 1996). Every reach of

Fig. 3. Aerial view of wing dikes along Mississippi River.

the LMR from the confluence of the Ohio River at Cairo to the Gulf has long stretches of ACM or rock revetment to facilitate navigation and protect infrastructure. Wing dikes are long, linear berms of large rock constructed perpendicularly from the riverbank toward the main channel of the river. Often dikes are constructed in a series, known as a dike field, and are used to deflect or direct water flows toward the navigation channel of the river at medium to low river stages (Fig. 3). This increases current velocity in the navigation channel, thereby increasing transport of sediments and maintaining open and safe navigation. Slack water between dikes also facilitates the deposition of sand and mud, thus further reducing sediment volume and accretion in the channel. The equivalent of over 500 km of dikes has been constructed along the LMR as part of MR&T; each dike ranges from 50 to 500 m in length (DuBowy, 2010). Additional dikes and related structures are also found in the MMR. However, wing-dike construction severs the hydrological connections between the main river and side channels (in the batture) as sand and other deposits fill the chute. Closing structures, placed within or at the lower end of side channels, further reduce connectivity. There has been a marked decrease in the number of side channels as the channel improvement program has progressed and the number of dikes has increased (Marks-Guntren et al., 2013). These backwater habitats are important feeding, spawning and nursery areas for many important fish species, as well as providing habitat for other environmentally sensitive wildlife and invertebrate species. 3.2. Agricultural nutrients/industrial chemicals Another radical change that has occurred in the Mississippi River Basin (including the Ohio and Missouri River sub-basins) has been the conversion to intense rowcrop agriculture throughout what is commonly known as the ‘‘Corn Belt.’’ This process has been exacerbated in recent times due to increases in corn prices as a result of ethanol production. Corn yields of 200 bushel/acre (12,713 kg/ha) are not uncommon (Iowa State University, 2012) due to (in large part) the intensive use of nitrogen-based (nitrate, nitrite and ammonium) fertilizers. In many locales, agricultural practices (e.g., tillage, drainage) allow for the loss of

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excess nutrients from crop fields as point-source (drain tiles) or non-point-source runoff (McIsaac et al., 2002). Nitrate/nitrite concentrations frequently exceed 3 mg (N) per liter in the Mississippi River (Meade, 1995), and this excess nitrogen (and phosphorus) eventually makes its way into the Mississippi River drainage and ultimately into the Gulf of Mexico. These high amounts of nitrogen are the principle cause of the Gulf of Mexico hypoxic zone adjacent to the coast of Louisiana and Texas (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, 2004). Hydraulic alterations which now decouple the floodplain from the river have had major effects on water quality as well (Nilsson and Reno¨fa¨lt, 2008; Schramm et al., 2009). Industrial chemicals, including agricultural pesticides and legacy PCBs, are likewise found in excessive concentrations along various reaches of the Mississippi River ecosystem (Scribner et al., 2006; National Research Council, 2008). 3.3. Sediment and salinity changes The Mississippi River is a dynamic system that responds to geomorphic features such as landforms, sediment loads and stream velocities. Hydraulic changes, especially dams on the Missouri River (flood control) and the UMR and Ohio Rivers (navigation) and dredging on the MMR and LMR have altered sediment dynamics throughout the river system (National Research Council, 2010). Dams create reservoirs or pools for water storage behind them; these extensive areas of slack water lead to a rapid settlement of sediment that historically flowed to the Gulf Coast where it was the primary mechanism for land building and the prevention of marsh loss. The historical sediment load of the Mississippi River has been reduced by over 50% (Meade, 1995; Smith and Winkley, 1996), leading to a rapid subsidence of coastal marsh ecosystems (Morton et al., 2010). In contrast, in some areas of the LMR, dredging and other navigation remediation, such as cutoffs and channel connections (Camillo, 2012), have led to a radical change in the grade (slope) of the river bottom (Grenfell et al., 2012; Marks-Guntren et al., 2013). This change in profile has led to extensive areas of head-cutting, where rapid and pronounced erosion attempts to bring the river back to a more normal hydraulic grade. The Mississippi Delta in Louisiana has experienced pronounced reductions and shifts in flow (as well as sediment load) due to the extensive levee system and coastal canals which now allow salt water to extend landward due to a reduction in the hydraulic head (Barras, 2009). These salt water incursions have been exacerbated by hurricanes and tropical storms and have led to rapid changes in marsh salinity with concomitant shifts from fresh-water or brackish ecosystems to more intermediate or saline systems. These ecosystem changes are exemplified by extensive mortality of historical vegetation (marsh die-off) and similar loss of sessile marsh fauna, oyster (Crassostrea virginica) beds being the most obvious (Klinck et al., 2002). 3.4. Non-native species Like many other ecosystems world-wide, the Mississippi River system has been plagued by rapid increases in

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non-native species. These invasive species run the gamut from plants (e.g. Hydrilla and reed canary grass Phalaris arundinacea) to fish (northern snakeheads Channa argus and round gobies Neogobius melanstomus). Two of the more noteworthy invasive taxa are Zebra mussels (Dreissena polymorpha) and Asian carp (Cyprinidae). Zebra mussels are presumed to have arrived in the Mississippi River ecosystem after passing through the Chicago Sanitary and Ship Canal (CSSC) which connects the Illinois River to Lake Michigan, where it is believed the mussels became established after arriving in bilge water from ships that passed up the St. Lawrence Seaway and entered the Great Lakes (Briski et al., 2012). Similar to their cousins, the Common Carp (Cyprinus carpio), four additional species, Grass Carp (Ctenopharyngodon idella), Silver Carp (Hypophthalmichthys molitrix), Bighead Carp (H. nobilis), and Black Carp (Mylopharyngodon piceus), either escaped during flood events or were deliberately released from aquaculture ponds in the Mississippi Alluvial Valley and quickly became established in the Mississippi River system. Ironically, the major concern now is the potential for these carp species to move in the opposite direction through the CSSC and to become established in Lake Michigan and then the other Great Lakes. Like Zebra mussels, the latter three carp species are filter feeders that significantly reduce the amount of phytoplankton in oxbow lakes and other floodplain features; phytoplankton is the base of the food chain for many economicallyimportant fish species, and carp have the potential to radically alter the aquatic fauna of the system (Kolar et al., 2007). 3.5. Climate change Floods and droughts have been a great concern to civilizations and societies for thousands of years, influencing the locations of cities, agricultural activities, and transportation (Krysanova et al., 2008). Additionally, global climate shifts have been cited as a possible cause of increased flooding in some parts of the world (Pall et al., 2011). The Mississippi River system has experienced pronounced extremes in weather patterns, particularly rainfall events, in the past few years. In 2011 UMR, MMR and LMR all experienced record or near-record flood stages along most reaches of the system (Camillo, 2012). In Vicksburg the highest stage recorded was +57 LWRP (Low Water Reference Plain – defined as the level that is exceeded 97% of the time; 57 ft  17.1 m) in May 2011 which was 4 m above flood stage and was the highest recorded stage ever (the 1927 flood stage may have been higher had the upstream levees not failed). Likewise, many towns on the UMR, MMR and tributaries were inundated by record levels of flood water. These record flood levels follow on high water events in 2009 as well. These flood events are caused by cyclonic activities that push moisture north from the Gulf of Mexico resulting in wide-spread heavy rainfall events. Ironically, little winter snowpack followed by a hot summer and record drought in northern portions of the system during the following year (2012) resulted in some of the lowest Mississippi River stages ever recorded. The river gage at Vicksburg recorded a reading of

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2 LWRP ( 60 cm) during August 2012 (as LWRP is defined as 97% exceedance, gage readings can, in fact, be negative). This is a nearly 18 m change from the preceding year. How global climate change will further impact future rainfall and snowfall events is yet to be determined. However, the possibility exists that more summer droughts and more spring/summer rainfall events may lead to additional record high-water or low-water events in the future. Additionally, climate change already has had impacts on aquatic flora and fauna; more vulnerable communities, such as montane systems, have shown the greatest turnover (Bush et al., 2012), but impacts on Mississippi River catchment are also expected. 4. Ecohydrological remediation 4.1. Improved hydraulics Mississippi River Basin ecosystem rehabilitation will require extensive reconstruction of historical hydraulics and hydrology. Hydrological connectivity is essential for most riparian ecological processes, especially nutrient dynamics (Meyer et al., 2013), plant propagule colonization (Gurnell et al., 2008) nursery areas for many fish species (Miranda, 2005; Fullerton et al., 2010; Go´rski et al., 2011; Tonkin et al., 2011) and biodiversity (Paillex et al., 2009). Reconnection of historical bottomland features through the reestablishment of historical hydrology, or the creation of floodplain habitats de novo (Gallardo et al., 2012) is critical to developing more resilient and dynamic ecosystems. As there are several different sections of the Mississippi River, itself, (plus the Missouri and Ohio and other sub-basins) the hydroengineering required may be radically different in different stretches of the river system. The obvious solution to restoring the hydraulics would be to remove all flood control and navigation structures that impede water flow. Clearly, this cannot happen without societal costs via threats to human safety, infrastructure security and economic benefits. Rather, improved hydraulics and hydrology must be accomplished within the context of navigation and flood risk management (DuBowy, 2010; Miller and Kochel, 2010; Radspinner et al., 2010). In the MMR and LMR immediate hydraulic improvements can be accomplished within the batture (active floodplain). Here, navigation structures restrict flow into side channels and other floodplain features. Engineering features have been considered to provide flow into side channels and other floodplain features while continuing to provide flow in the main channel for navigation; several engineering features (dike notches, chevrons) have been developed and implemented. Within existing dike fields the best environmental engineering feature found for this has been the dike notch. A notch is a trapezoidal opening in a dike that typically has a 30-m top width, sloping sides and a 90-m bottom width; the bottom elevation of the notch is typically at 0 LWRP to +5 LWRP (ft  1.5 m) (roughly between 4.5 and 9 m below the top elevation of the wing dike). Some notches are larger or smaller, being adjusted to the specific channel conditions. Notches are made either by removing rock during maintenance work on an existing dike or by leaving an open, low section when

Fig. 4. Newly notched wing dike at rising stage, Lower Mississippi River, Robinson Crusoe dike field, Tennessee.

a new dike is built (Fig. 4); this low section permits lower river stages to pass through the notch and down the side channel 90–97% of the time. Notches reduce sedimentation in old chute channels and behind sandbars and maintain flowing water conditions at lower stages in secondary channels. Additionally, low water stages flowing through a notch result in a diversity of current velocities at the notch that increase substrate diversity (both in composition and topography/bathymetry), thereby increasing aquatic habitat and aquatic species diversity downstream of the notch. Some notches have been constructed to elevations higher than +5 LWRP. This is unfortunate as +10 LWRP (for example) is rated at only 80% exceedance, meaning 20% of the time (2.4 months) there is no flow through the side channel, usually during the hottest time of year when conditions are physiologically most stressful for aquatic fauna. For new construction, developing navigation structures that divide flow, providing ample flow for navigation while providing environmental flows for floodplain enhancement, are being planned and implemented. Chevrons (U- or Jshaped rock structures pointing upriver) and rootless dikes (not tied into the riverbank thus providing flow like an enlarged notch) have been constructed in many locations in the MMR and LMR. Not only do chevrons divide the flow, water flowing over the middle of the structure at high river stages scours the bottom and provides deep-water habitat in the center of the chevron, creating additional habitat diversity (Davinroy et al., 1996). Additional floodplain hydrologic improvement by countering the desired consequences of levees and flood-risk management is more problematic. The key would be to establish improved hydrology while continuing to provide a similar level of safety and security for people, property and infrastructure. One alternative would be to adopt a strategy of levee setbacks – moving the levees farther apart to both provide more hydrological connectivity to the floodplain as well as establishing improved flood water conveyance during critical times of year (Opperman et al., 2009). This would require protection of infrastructure and property with the new, wider batture,

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probably by means of ring levees around critical elements, similar to the construction of ring levees around small towns in the wide valley of the Red River of the North in Manitoba, Minnesota and North Dakota. The principal drawback to this concept is that farmsteads and personal property would not be protected unless the landowner took the responsibility to construct a ring levee or elevate structures above the high-water mark; consequently, these non-structural solutions have been met with resistance on the part of landowners. 4.2. Nutrient reductions Much deliberation has occurred recently concerning the role that the Mississippi River and its floodplain could play in water quality remediation. In particular, nitrogen and phosphorus removal via biogeochemical processes has been proposed as an objective for the restored Mississippi River floodplain (Mitsch et al., 2005, 2008; Schramm et al., 2009). The length of the LMR is almost 1600 km; the average flow rate is about 6 km h 1. Consequently, retention time along the LMR is approximately 250 h or more than 10 days. This would prove to be more than adequate time for water treatment and nutrient removal. However, with the construction of mainline levees the cross-sectional channel profile has been radically altered. Where once the floodplain was 100 km miles or more wide and flooded to an average depth of <0.5 m deep (Saucier, 1994), the current configuration of the system is one where the river is 2-3 km wide and 15 m deep at flood stage; river velocities correspondingly increase during floods in the modified channel. On less-regulated rivers riparian wetlands only reduce annual nitrogen flux by about 50%, and nitrogen flux may be as little as 10% during peak flow conditions (Montreuil et al., 2010). This level of nitrogen reduction, while favorable, would not be sufficient to eliminate the Gulf hypoxia zone. In its current configuration, most Mississippi River water would flow with little treatment even with the entire revegetation of the batture. The existing LMR floodplain system exhibits reduced channel and floodplain hydrological processes due to channelization and levee construction (Franklin et al., 2009). Current nitrogen removal in bottomland soils is estimated to be 542 kg N ha 1, nearly a 50% reduction from historical hydrological conditions (Schramm et al., 2009). The highest levels of N reduction are normally found on 1st–3rd order streams, i.e., headwater streams far removed from the mainstem of the Mississippi River (Craig et al., 2008). Moreover, hypoxic conditions even exist in low-order streams in the Mississippi River catchment during high flow events (Shields and Knight, 2012). As most nutrient contribution is by 1st-order streams (Alexander et al., 2007) efforts for water quality remediation should, likewise, occur in headwater areas and other regions where agricultural practices lead to high nutrient loads. For over 25 years numerous constructed wetland studies have shown that shallow water (<50 cm) and concomitant broad surface area are necessary for adequate treatment (Hammer, 1989; Moshiri, 1993; Etnier and Guterstam, 1997). As much of these excess nutrients are agricultural in nature, a far better solution would be the development of

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comprehensive measures for the treatment and remediation of agricultural runoff (especially cropland and dairy operations) at its source (DuBowy and Reaves, 1994; DuBowy, 1999). Small constructed wetlands or other treatment systems could be constructed onsite before the water is released into adjacent surface waters or groundwater. Numerous studies have shown the efficacy of these small-scale projects, even in areas with cold winter temperatures, especially when treatment facilities are coupled with existing farm lagoons or other holding facilities. Small-scale constructed wetlands employing anoxic limestone drains particularly to remove mobile iron, have been used to remediate acid mine drainage in many parts of Appalachia (Ohio River drainage; Johnson and Hallberg, 2005). For larger volumes of wastewater (municipal or industrial), constructed wetland systems could be scaled up to meet the necessary treatment demands and used as tertiary treatment for water polishing (Vermaat et al., 2012). Examples include Cattail Marsh (municipal, Beaumont, Texas), Orlando Easterly Wetlands Reclamation Project (municipal, Florida) and the Amoco Oil Refinery (industrial, Mandan, North Dakota). 4.3. Sediment and freshwater diversions Historically, the farthest downriver reaches of the LMR in Louisiana functioned as a web of distributaries which spread (fresh) water and sediment to maintain coastal wetland ecosystems. Construction of flood control levees obliterated most of this network by preventing flow through most of these natural channels. Loss of sediment and freshwater has resulted in marsh subsidence and salt water intrusion (Barras, 2009). Moreover, the sediment load in the Mississippi River has been reduced by more than 50% due to sediment capture by large flood control dams on the Missouri River and navigation dams on the Upper Mississippi and Ohio Rivers (Meade, 1995). To remediate the loss of natural crevasses and outflow channels, a series of freshwater and sediment diversions are in the planning, construction or operational phases. The Old River Control Complex, which sends 30% of the combined flow of the Mississippi and Red Rivers down the Atchafalaya River, provides freshwater and sediments to the western portion of the coastal Louisiana delta (Fig. 2). Sedimentation and land building have been observed near the Wax Lake Outlet along the Atchafalaya Waterway (Barras et al., 2008). Additional diversions currently in operation include Bonnet Carre´ (floodwater), Davis Pond (freshwater), Caernarvon (freshwater), Naomi Siphon (freshwater), West Point a la Hache (freshwater), West Bay Sediment Diversion, and Delta Crevasses (sediment). Numerous other diversions are proposed or planned to replace the historical crevasses along this entire stretch of river (Falcini et al., 2012; Kenney et al., in press). 4.4. Invasive species control Biological control of non-native species is fraught with controversy. While billions of dollars in economic losses are expected with the spread of Zebra mussels and Asian carp, control currently revolves around eliminating, or at

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least reducing, the spread of these species into new bodies of water rather than eradication. As Zebra mussel adults are sessile and attach to various structures, many states have embarked upon programs to insure that boaters do not translocate mussels when moving watercraft from one lake or stream to another (Leung et al., 2006). This program has been only partially successful as mussels can move through other means – adults can attach to large ships and barges that are not regularly taken out of the water or inspected, and free-swimming veligers may passively move downstream in the water column (unlike native mussel species, Zebra mussel veligers do not attach to fish; Bossenbroek et al., 2007; Bunnell et al., 2009). Asian carp present a different suite of problems as they are strong swimmers and extremely mobile. Since the early 1990s Asian carp have made their way from the southern portions of the Mississippi River all the way north to St. Anthony’s Falls in addition to moving into the Ohio and Missouri Rivers and tributaries (Chick and Pegg, 2001). Currently the greatest concern is to prevent carp moving from the Illinois River through the Chicago Ship and Sanitary Canal (CSSC) into Lake Michigan and eventually the other Great Lakes (there is some evidence that Bighead Carp are already in the Calumet River and Lake Erie notwithstanding). The CSSC is an artificial channel dug to link the Great Lakes to the Mississippi system in the late 1800s. Some agencies and government bodies have advocated closing this link by decommissioning the canal. Others have objected on the grounds that prevention of shipping would be financially ruinous. Currently, an alternate mechanism is being developed – an electrical barrier (perhaps coupled with bioacoustic and bubbler features) that block carp (and other fish) from passing through the CSSC. The likelihood that this barrier will be 100% effective in preventing carp passage into Lake Michigan is conjectural at this point. A similar barrier system also has been proposed on the Mississippi River at Minneapolis. 4.5. Climate change Like other wetlands, floodplains and bottomland ecosystems can play an important role in the remediation of carbon dioxide emissions by means of belowground carbon sequestration (peat) and aboveground biomass accumulation (timber; Schoch et al., 2009). However, by eliminating the annual flood pulse in bottomland systems, flood control levees also have allowed extensive land clearing for agriculture behind the levees, thus reducing carbon storage in these areas (Franklin et al., 2009). Bottomland carbon sequestration should not be discounted; however, it will require a major paradigm shift to return bottomland systems to pre-development sequestration levels. In particular, hydrological reconnection of the historical floodplain is a necessary first step in meaningful carbon storage. 4.6. Hydrokinetic energy production Besides remediating climate change through carbon sequestration, the Mississippi River system potentially can reduce our carbon footprint by providing for alternate

hydrokinetic energy systems (National Research Council, 2013). The promise of hydrokinetic turbines is in their ability to produce clean, sustainable energy that takes advantage of the river’s current (flow and velocity). Several large-scale hydrokinetic projects have been proposed for various reaches of the UMR, MMR, and LMR. However, there are questions about unintended consequences on aquatic organisms, wildlife and surrounding areas. Various agencies including the U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, National Park Service, state agencies, and non-governmental entities such as navigation associations, levee boards, Trout Unlimited, American Rivers, and others have raised concerns about specific aspects of these projects. Impacts on fish, mussels and recreational boating and angling are of particular concern. Studies of fish impacts proposed by power applicants may not be adequate to assess true impacts, especially cumulative impacts over time. Several threatened or endangered species, including the pallid sturgeon (Scaphirhynchus albus) may be affected by these structures. Impacts may multiply if multiple hydrokinetic arrays are deployed as expected. Likewise, mussel impacts are unknown, but of concern. Several federally-endangered species are found in the Mississippi River in the general areas of proposed hydrokinetic kinetic arrays. Impacts to diving birds are also unknown, but if turbines entrain many fish these sites will become magnets for birds in the middle of one of North America’s most important flyways for migratory birds. Especially in the LMR impacts to navigation, channel maintenance and river hydraulics are also of concern. Large static arrays of pylons with attached turbines will undoubtedly present logistical problems for dredging, dredge material disposal, and revetment. As these turbines will reduce the kinetic energy of the river impacts to river hydraulics, sediment transport, river stages, and flood risk management must also be considered. Additionally, impacts on recreational boating and plans for emergency maintenance have not been adequately addressed. 5. Planning and evaluation Ecohydrology can be used throughout the river restoration process, from planning, through implementation and construction, to evaluation and monitoring. Comprehensive understanding of Ecohydrological principles is required during the planning process (Hermoso et al., 2011, 2012a,b); without understanding the linkages between ecology and hydrology, including the spatial extent of hydraulic and hydrological processes project failure is likely. Likewise, these same principles can be employed during the evaluation process by using the metrics developed pre-project to assess project success (Lake et al., 2007; Bunn et al., 2010). An inherent problem of many current ecosystem rehabilitation projects is the inability to measure success (Woolsey et al., 2007). While most projects conduct postproject monitoring, such monitoring only evaluates change (physical and biological responses) and not success; in order to evaluate success a project must a priori develop Objectives, Goals, Targets and Metrics to be incrementally

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measured both during and after the life of the project. Such targets and metrics allow for Adaptive Management (AM) to get projects ‘‘back on track’’ in the event of deviations from the prescribed project objectives (National Research Council, 2004; King et al., 2010; Mika et al., 2010). To date, many Mississippi River rehabilitation projects, especially most on the LMR, do not have the ability to measure success due to an absence of pre-defined Objectives, Goals, Targets and Metrics (DuBowy, 2010). Oftentimes the ability to simply measure project performance (D pre-/post-project; Miller et al., 2010) cannot even be accomplished due to a lack of systematic data collection. The spending hundreds of thousands of dollars on projects that cannot be adequately evaluated is undoubtedly the most egregious aspect of Mississippi River rehabilitation. Even U.S. Army Corps of Engineers Implementation Guidance for Section 2039 of Water Resources Development Act (WRDA) 2007 (31 AUG 2009) clearly reads that projects include plans for monitoring the success of ecosystem restoration projects. Yet, not one LMR environmental project has ever had its level of success measured; in fact, there are few data to even corroborate change of most projects. This has led to a lack of scientific accountability on these projects (DuBowy, 2010: 440). Using Ecohydrology principles it would be easy to formulate Goals, Objectives, Targets and Metrics for future projects that comprise the rehabilitation program. Goals would be fairly neutral, comprising the overall structure of the Ecohydrology concept (reducing input and controlling pathways of excess nutrients and pollutants in aquatic ecosystems; enhancing ecosystem carrying capacity, resilience, biodiversity, ecosystem services for society; etc.) and would complement the overarching program. Objectives, on the other hand, would be specific to each project and would reflect the ecological and (specifically) hydrological functions that describe the historical system. In contrast, Targets would reflect expectations of the realized, rehabilitated system, recognizing that often systems can never be restored to a historical ‘‘pristine’’ condition. Metrics would be the specific numerical criteria (hydrological, physical and biological) expected from the outcome of each project. In this way, each (linked) project would meet the overall objectives of the program while reflecting individual site characteristics and likelihood of success. In the future, AM plans (i.e., contingency plans) must be developed for all riverine (and other aquatic) ecosystem restoration projects. Each AM plan must be appropriately scoped to the scale of the project. If the need for a specified adjustment is anticipated due to high uncertainty in achieving desired outputs/results, the nature and costs of such actions should be explicitly described in the decision document for the project. If the results of the monitoring program support the need for physical modifications to the project, the cost of the changes should be cost-shared among all sponsors and partners via established guidelines. 6. Conclusions The Mississippi River system is not unlike other global large-river systems which have undergone radical hydraulic and hydrological alterations. As with many current river

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programs, rehabilitation of the Mississippi River and its associated floodplain is fraught with controversy as environmental goods and services are balanced against socioeconomic constraints such as navigation, flood control, agriculture and water supply. It will take much work and critical ‘‘thinking outside the box’’ to accomplish the optimization of all these factors while creating a more resilient dynamic ecosystem. Ecohydrology presents a framework for the establishment of evaluation metrics that link environmental resiliency, biodiversity and ecosystem goods and services for the goal of sustainable riverine and aquatic ecosystems. Given that the ability to measure project success and, consequently, to implement adaptive management is lacking in many Mississippi River rehabilitation projects, it is critical that agencies and organizations currently engaged in these endeavors change their philosophy and protocols to integrate Ecohydrology into a framework to develop Goals, Objectives, Targets and Metrics before any new projects are initiated. Without a clear vision of Adaptive Management, program success, especially in the LMR, is unlikely. Conflict of interest None declared. Financial disclosure statement The Contributing Author as no conflict of interest, including specific financial interests and relationships and affiliations relevant to the subject matter or materials discussed in the manuscript. Acknowledgements Development of the framework for using the Ecohydrology concept as a platform for restoration evaluation was refined while I was a visiting professor in the Erasmus Mundus Master of Science Programme in Ecohydrology at the European Regional Centre for Ecohydrology (University of Ło´dz´, Poland) and the International Centre for Coastal Ecohydrology (University of Algarve, Portugal) through the Fulbright Specialists Program. I thank my colleagues, Prof. M. Zalewski (Ło´dz´) and Prof. L. Chı´charo (Algarve), and their respective institute associates for their hospitality and stimulating discussions during my tenure. I also thank Prof. Zalewski for the invitation to present at the Ecohydrology for River Basins symposium at EcoSummit 2012 in Columbus, Ohio. References Ahearn, D.S., Viers, J.H., Mount, J.F., Dahlgren, R.A., 2006. Priming the productivity pump: flood pulse driven trends in suspended algal biomass distribution across a restored floodplain. Freshwater Biology 51, 1417–1433. Alexander, R.B., Boyer, E.W., Smith, R.A., Schwarz, G.E., Moore, R.B., 2007. The role of headwater streams in downstream water quality. Journal of the American Water Resources Association 43, 41–59. Barras, J.A., 2009. Land area change and overview of major hurricane impacts in coastal Louisiana, 2004–08. U.S. Geological Survey Scient. Invest. Map 3080, scale 1:250,000, 6 pp.

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