.[1] Along >4000 km of the Mississippi River system, we document that climate, land-use change, and river engineering have contributed to statistically significant increases in flooding over the past 100-150 years. Trends were tested using a database of >8 million hydrological measurements. A geospatial database of historical engineering construction was used to quantify the response of flood levels to each unit of engineering infrastructure. Significant climate-and/or land use-driven increases in flow were detected, but the largest and most pervasive contributors to increased flooding on the Mississippi River system were wing dikes and related navigational structures, followed by progressive levee construction. In the area of the 2008 Upper Mississippi flood, for example, about 2 m of the flood crest is linked to navigational and flood-control engineering. Systemwide, large increases in flood levels were documented at locations and at times of wing-dike and levee construction.

The Upper Mississippi River System (UMRS) is a geographically diverse basin extending 10°north temperate latitude that has produced fishes for humans for millennia. During European colonization through the present, the UMRS has been modified to meet multiple demands such as navigation and flood control. Invasive species, notably the common carp, have dominated fisheries in both positive and negative ways. Through time, environmental decline plus reduced economic incentives have degraded opportunities for fishery production. A renewed focus on fisheries in the UMRS may be dawning. Commercial harvest and corresponding economic value of native and non-native species along the river corridor fluctuates but appears to be increasing. Recreational use will depend on access and societal perceptions of the river. Interactions (e.g., disease and invasive species transmission) among fish assemblages within the UMRS, the Great Lakes, and other lakes and rivers are rising. Data collection for fisheries has varied in intensity and contiguousness through time, although resources for research and management may be growing. As fisheries production likely relies on the interconnectivity of fish populations and associated ecosystem processes among river reaches (e.g., between the pooled and unpooled UMRS), specieslevel processes such as genetics, life-history interactions, and migratory behavior need to be placed in the context of broad ecosystem-and landscape-scale restoration. Formal communication among a diverse group of researchers, managers, and public stakeholders crossing geographic and disciplinary boundaries is necessary through peer-reviewed publications, moderated interactions, and the embrace of emerging information technologies.

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The goal of this study was to construct a large, data-rich model to test hydrological responses to engineering modifications on over 3200 km of the Mississippi and Lower Missouri Rivers. We compiled model explanatory variables from a geospatial database quantifying construction of all bridges, wing dikes, bendway weirs, levees, artificial meander cutoffs, channel constriction and navigational dams over the past 100-150 years. Response variables were derived from 68 rated and un-rated hydrologic stations in the study area, with responses analysed across a range of discharges from within-channel flows up to moderate floods. Correlation analysis, multiple linear regression and stepwise regression analyses document strong and consistent responses to construction history, both in individual reach-scale models and systemwide. Meander cutoffs are associated with degradation and acceleration of flow that has reduced stages across the full discharge range. Navigational dams on the Upper Mississippi River increased low-flow stages and flood levels to a lesser extent, with little or no post-dam change. One of the strongest signals was the hydrologic response to wing-dike construction, which resulted in large back-water increases in stage upstream of wing dikes and mixed effects downstream, including the overlapping effects of incision and velocity losses. Levees were associated with local flow concentration, overbank storage loss and floodplain conveyance loss depending on reachscale conditions. The results presented here (1) quantify incremental and cumulative hydrologic responses to a range of engineering activities and (2) provide an empirical tool for verifying and assessing hydraulic and other models of river-system change.

Understanding trends in the diverse resources provided by large rivers will help balance tradeoffs among stakeholders and inform strategies to mitigate the effects of landscape scale stressors such as climate change and invasive species. Absent a cohesive coordinated effort to assess trends in important large river resources, a logical starting point is to assess our ability to draw inferences from existing efforts. In this paper, we use a common analytical framework to analyze data from five disparate fish monitoring programs to better understand the nature of spatial and temporal trends in large river fish assemblages. We evaluated data from programs that monitor fishes in the Colorado, Columbia, Illinois, Mississippi, and Tallapoosa rivers using non-metric dimensional scaling ordinations and associated tests to evaluate trends in fish assemblage structure and native fish biodiversity. Our results indicate that fish assemblages exhibited significant spatial and temporal trends in all five of the rivers. We also document native species diversity trends that were variable within and between rivers and generally more evident in rivers with higher species richness and programs of longer duration. We discuss shared and basin-specific landscape level stressors. Having a basic understanding of the nature and extent of trends in fish assemblages is a necessary first step towards understanding factors affecting biodiversity and fisheries in large rivers.

Fish community assessments are often based on sampling with multiple gear types. However, multivariate methods used to assess fish community structure and composition are sensitive to differences in the relative scale of indices or measures of abundance produced by different sampling methods. This makes combining data from different sampling gears and methods a serious challenge. We developed a method of combining catch per unit effort data that standardizes catch per unit effort data across gear types, which we call multigear mean standardization (MGMS). We evaluated how well MGMS and other types of standardization reflect underlying community structure through a computer simulation that generated model riverine-fish communities and simulated sampling data for two gears. In these simulations, combining sampling observations from two gears with MGMS produced community structure estimates that were highly correlated with true community structure under a variety of conditions that are common in large rivers. Our simulation results indicate that the use of MGMS to combine data from different sampling gears is an effective data manipulation method for the analysis of fish community structure.Résumé : Les évaluations de communautés de poissons reposent souvent sur un échantillonnage fait avec différents types d'engins. Les méthodes multivariées utilisées pour évaluer la structure et la composition de communautés de poissons sont toutefois sensibles aux différences d'échelle relative des indices ou mesures d'abondance produits par différentes méthodes d'échantillonnage, de sorte que le groupement de données issues de différents engins et méthodes d'échantillonnage constitue un important défi. Nous avons mis au point une méthode pour combiner des données de prises par unité d'effort qui normalise les données de prises par unité d'effort issues de différents types d'engins, méthode que nous appelons la normalisation moyenne multiengins (NMME). Nous avons évalué la mesure dans laquelle la NMME et d'autres approches de normalisation reflètent la structure sous-jacente des communautés en utilisant des simulations par ordinateur qui ont généré des communautés modèles de poissons de rivière et simulé des données d'échantillonnage pour deux engins. Dans ces simulations, le groupement d'observations d'échantillonnage issues de deux engins à l'aide de la NMME a produit des estimations de la structure des communautés fortement corrélées à la structure réelle des communautés dans différentes conditions couramment observées dans les grandes rivières. Les résultats des simulations indiquent que l'utilisation de la NMME pour combiner des données issues de différents engins est une méthode de manipulation de données efficace pour l'analyse de la structure des communautés de poissons. [Traduit par la Rédaction]

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