River-floodplain Ecosystems Research Articles

Relationships between Floodplain Lake Fish Communities and Environmental Variables in a Large River-Floodplain Ecosystem.

Floodplain lakes of large river systems contain fish habitats that are not found elsewhere within the river, and these lakes have a diversity of environmental conditions that vary in space and time. Our objective was to examine relationships between floodplain lake fish communities and environmental variables associated with lake morphology, water chemistry, and river‐floodplain connectivity in a large river‐floodplain ecosystem. Multivariate direct‐gradient analyses indicated that lake surface area, lake depth, water clarity, and (to a lesser extent) dissolved oxygen were the most important factors in the structuring of lake fish communities. Results further suggested that floodplain lakes could be placed into groups that contained distinctive fish communities. Fish community structure was not strongly related to river‐floodplain connectivity, though fish species richness in individual lakes was positively correlated with degree of flooding in those lakes. Fish species diversity in lakes was positively correlated with linear distance between lakes and the main river channel; lakes that were furthest from the main river channel had more diverse fish communities. The diversity of environmental conditions in floodplain lakes is essential for maintaining net ecosystem diversity in large river ecosystems.

Contrasting the Middle Paraná and Mississippi Rivers to develop a template for restoring large floodplain river ecosystems

Effective rehabilitation of large rivers requires a concept of normal floodplain river behavior ‐ the reference condition ‐ to understand system‐level disturbance history and to develop plans to improve river health. However, reference conditions are difficult to obtain for large rivers. Using Correspondence Analysis of a select subset of the world's great rivers, we show that the Paraná and Mississippi Rivers are relatively similar at the watershed scale based on general geographical and physico‐chemical variables, although these rivers differ substantially in disturbance history. We believe that the less disturbed Paraná River provides reference conditions for the more disturbed Mississippi River for some processes and functions whereas the Mississippi River provides a compelling vision of the future state of the Paraná River unless sustainable development plans are developed and implemented. By integrating information between them, this pair of rivers provides a unique opportunity for scientists to develop more robust conceptual models and improved deterministic models to better guide river management and rehabilitation actions. We suggest that development of large river reference conditions may be better obtained through expanded inter‐hemispheric scientific collaboration on multiple systems than through increased focus on a single impacted system.

Developing a dynamic model to predict the recruitment and early survival of black willow ( Salix nigra) in response to different hydrologic conditions

Disruption of the natural flood regime in floodplain-river ecosystems can decrease plant diversity and lead to dominance by one or a few species that can tolerate the altered conditions. Restoration of native plant communities requires naturalization of the flood regime, to prevent dominance by one species and to promote germination and growth of a variety of native species. Black willow ( Salix nigra), cottonwood ( Populus deltoides) and silver maple ( Acer saccharinum) are native, flood-tolerant trees that often form monocultures and shade out moist-soil plants (e.g., millets) in waterfowl management areas and restoration sites on floodplains along the Illinois River and some portions of the Upper Mississippi River. Since the moist-soil plants provide ecological goods and services that are not provided by pioneering tree species alone, such as food for migratory waterfowl, wetland managers try to limit recruitment of these pioneering tree species with herbicides, disking and mowing. Other natural resource managers value the same species for reducing bank erosion or as potential sources of rapidly growing biomass for generation of electricity. From either perspective, it is important to identify characteristics of flooding regimes that might encourage these tree species or keep them in check. We developed a dynamic simulation model to predict the recruitment and seedling growth of one of these pioneering trees (black willow, Salix nigra) in response to flood timing and duration over three growing seasons following germination. Tree germination and survival respond to capillary water depth, flood duration, length of growing season and timing of seed dispersal. The model was tested over a range of elevations (132.3–132.9 m) that were observed on aerial photos as the tree line in the vicinity of River Km 201 along the Illinois River. The model was validated to the extent that it simulated establishment of black willow at the elevation of the observed tree line and variability of seedling growth in response to the flooding regime (i.e., poor or no survival or growth at elevations below the tree line where floods were longer-lasting and more frequent). Although more experimental or field-based information on seedling physiology should be gathered to refine the model, the prototype model is useful for exploring effects of alternative water level regimes. The model provides a framework for simulating any pioneering tree species that colonizes floodplains, if the required species-specific physiological information is available.

Applicability of the flood-pulse concept in a temperate floodplain river ecosystem: thermal and temporal components

Annual growth increments were calculated for blue catfish (Ictalurus furcatus) and flathead catfish (Pylodictis olivaris) from the lower Mississippi River (LMR) to assess hypothesized relationships between fish growth and floodplain inundation as predicted by the Flood-Pulse Concept. Variation in catfish growth increment was high for all age classes of both species, and growth increments were not consistently related to various measures of floodplain inundation. However, relationships became stronger, and usually direct, when water temperature was integrated with area and duration of floodplain inundation. Relationships were significant for four of six age classes for blue catfish, a species known to utilize floodplain habitats. Though similar in direction, relationships were weaker for flathead catfish, which is considered a more riverine species. Our results indicate the Flood-Pulse Concept applies more strongly to temperate floodplain-river ecosystems when thermal aspects of flood pulses are considered. We recommend that future management of the LMR should consider ways to ‘recouple’ the annual flood and thermal cycles. An adaptive management approach will allow further determination of important processes affecting fisheries production in the LMR. Copyright © 2006 John Wiley & Sons, Ltd.

The role of timing, duration, and frequency of inundation in controlling leaf litter decomposition in a river-floodplain ecosystem (Tagliamento, northeastern Italy)

Despite growing recognition of the importance of a natural flow regime in river-floodplain systems, researchers struggle to quantify ecosystem responses to altered hydrological regimes. How do frequency, timing, and duration of inundation affect fundamental ecosystem processes such as leaf litter decomposition? Along the semi-natural Tagliamento River corridor, located in northeastern Italy, we employed in situ experiments to separate effects of different inundation components on breakdown rates of black poplar (Populus nigra). We used a litter-bag method with two different mesh sizes to investigate how fungi and macroinvertebrates influence leaf breakdown rates. Ten treatments, each representing a specific combination of duration and frequency of inundation, were deployed in two seasons (summer, winter) to mimic complex inundation patterns. After 30 days of exposure, mean percentage of remaining leaf litter (ash free dry mass) ranged between 51% (permanent wet) and 88% (permanent dry). Leaf breakdown was significantly faster in winter than in summer. Duration of inundation was the main inundation component that controlled leaf breakdown rates. Leaf-shredding macroinvertebrates played only a role in the permanent wet treatment. Fungal parameters explained the faster leaf breakdown in winter. Our study suggests that modifications of the inundation regime will directly modify established decomposition processes. Factors reducing duration of inundation will decelerate leaf breakdown rates, whereas a decrease in flow variation will reduce leaf breakdown heterogeneity.

Significance of instream autotrophs in trophic dynamics of the Upper Mississippi River

Trophic dynamics of large river-floodplain ecosystems are still not well understood despite development of several conceptual models over the last 25 years. To help resolve questions about the relative contribution of algal and detrital organic matter to food webs in the Upper Mississippi River, we (1) separated living and detrital components of ultrafine and fine transported organic matter (UTOM and FTOM, respectively) by colloidal silica centrifugation; (2) identified stable isotope signatures (delta(13)C and delta(15)N) for these two portions of transported organic matter and other potential organic matter sources; and (3) employed a multiple source, dual-isotope mixing model to determine the relative contribution of major energy sources to primary consumers and the potential contribution of basal sources to the biomass of secondary consumers. The delta(13)C and delta(15)N of living and detrital fractions of UTOM and FTOM were distinct, indicating clear differences in isotopic composition of the algal and detrital fractions of transported organic matter. Living and detrital transported organic matter also differed from other potential organic matter sources by either delta(13)C or delta(15)N. A six-source mixing model using both delta(13)C and delta(15)N indicated that algal transported organic matter was the major resource assimilated by primary consumers. The contribution of detrital transported organic matter was small in most cases, but there were a small number of taxa for which it could potentially contribute to more than half the assimilated diet. Colloidal dissolved organic matter, which includes heterotrophic bacteria, accounted for only a small fraction of the organic matter assimilated by most primary consumers, indicating that coupling between microbial processes and metazoan production is minimal. Terrestrial C(3) litter from the floodplain forest floor and aquatic macrophytes were also relatively unimportant to the assimilated diet of primary consumers. Application of the mixing model to compare basal source isotopic ratios to secondary consumers revealed that most organic matter moving from primary to secondary consumers originated from algal TOM. Our findings indicate that autochthonous organic matter is the major energy source supporting metazoan production in the main channel of this large river, at least during the summer. This study joins a number of other investigations performed globally that indicate organic matter originating from instream production of sestonic and benthic microalgae is a major driver in the trophic dynamics of large river ecosystems.

Floodplain–river ecosystems: Fragmentation and water resources development

Floodplain–river ecosystems are natural fragmented systems because of periodic hydrological connections. The integrity of these ecosystems is thought to be dependent, in part, upon exchanges of energy and matter between patches, such as the main river channel, adjacent floodplain surface and other morphological features, during periods of connection. Flow regulation and its associated infrastructure change the natural character of fragmentation in floodplain–river ecosystems, and have important consequences for their overall productivity. This paper considers the influence of water resources development on the character of fragmentation in a large lowland river in SE Australia. Large-scale water resources development in the Macintyre River, Australia, has significantly altered the spatial and temporal patterns of hydrological connections. The construction of weirs and other embankments on the lower Macintyre River floodplain prevents water movement through a series of anabranch channels thereby reducing the availability of these floodplain patches by 55%. In addition, because of flow regulation, hydrological connections to these channels occur up to 22% less often. Data are presented showing the impacts of these changes on the potential supply of dissolved organic carbon from the anabranch channels during periods of inundation over a 98-year period. Reductions of up to 98% of potential dissolved organic carbon supply from some anabranch channels were noted.

The role of floods in particulate organic matter dynamics of a southern Appalachian river–floodplain ecosystem

We investigated the effect of a flood on particulate organic matter (POM) dynamics in the floodplain and active channel of the Little Tennessee River in western North Carolina. We measured litterfall, leaf breakdown, and floodplain litter (before and after the flood) at 12 sites. Annual litterfall (256–562 g m−2 y−1) was typical of a temperate deciduous forest but lower than lowland floodplain forests in the eastern US. Leaf breakdown rates of 4 tree species (Acer rubrum, Carpinus caroliniana, Juglans nigra, and Platanus occidentalis) ranged from 0.001 to 0.010/d. We separated the 12 sites into 2 groups (inundated and non-inundated) based on the degree of flooding after a flood on 8 January 1998 and determined POM exchange between the active channel and floodplain. Significant transport of leaves from the floodplain to the river occurred at inundated sites, but transport of herbaceous or woody material did not occur. The flood increased leaf breakdown rates of all 4 tree species. Our measurements of litterfall, leaf breakdown, and floodplain litter standing crop suggest that leaves entrained from the floodplain of Little Tennessee River during floods are a source of POM to the active channel. However, flood input of leaves to the river were a small source of POM compared to direct leaffall.

BIO‐SAFE: assessing the impact of physical reconstruction on protected and endangered species

AbstractAssessing actual and potential biodiversity of river–floodplain ecosystems on the basis of policy and legislation concerning endangered and protected species is necessary for consistency between different policy goals. It is thus a prerequisite to sustainable and integrated river management. This paper presents BIO‐SAFE, a transnational model that quantifies the relevance of species and ecotopes, characteristic of the main channels and floodplains of the rivers Rhine and Meuse, on the basis of international treaties and directives and national Red Data Lists. BIO‐SAFE was developed into a tool for biodiversity assessment with regard to design and evaluation of physical planning projects, Environmental Impact Assessments and comparative landscape–ecological studies. It was conceived to be applicable in Germany, France, Belgium and the Netherlands.Taxonomic groups involved are higher plants, birds, herpetofauna, mammals, fish, butterflies, and dragonflies and damselflies. The linkage of habitat requirements of species to ecotopes allows the user to derive information at the level of several ecotope types and scales. The model requires input data on presence of species and/or surface area of ecotopes. BIO‐SAFE has been applied to flood risk reduction projects along the rivers Rhine and Meuse. Results show that BIO‐SAFE yields quantitative information regarding the degree to which actual situations, reconstruction designs and developments of species and ecotope composition meet national and international agreements on biodiversity conservation.Attuning biodiversity conservation and flood risk reduction measures is a major issue in applied ecology and spatial planning. Assessments with BIO‐SAFE can help find an optimal balance. Because of its policy‐based character, BIO‐SAFE yields information that is complementary to ecological biodiversity indices, single‐species habitat models and ecological network analysis. The development of BIO‐SAFE was based on species characteristic of rivers and floodplains, but the method can easily be applied to other ecosystems as well. Copyright © 2004 John Wiley & Sons, Ltd.

Fish as potential dispersal agents for floodplain plants: first evidence in North America

In river–floodplain ecosystems with seasonal flood pulses, many species display adaptations to take advantage of resources in the aquatic–terrestrial transition zone. Frugivory and seed dispersal by fishes is a clear example of such adaptations, known primarily from South American river–floodplain systems. Here, we document consumption of red mulberry (Morus rubra) and swamp privet (Forestiera acuminata) fruit by channel catfish (Ictalurus punctatus) in the floodplain of the Mississippi River, U.S.A., and demonstrate that red mulberry and swamp privet seeds will germinate after being consumed by channel catfish. In a common-garden experiment, consumption of fruits and seeds by channel catfish improved germination success of these riparian plants relative to a treatment simulating fruit dropped during a flood without being consumed by fish. This is the first study to provide evidence of frugivory and viability of seeds ingested by fish in a North American river–floodplain ecosystem. Frugivory and seed dispersal by fishes may be more widespread than previously thought.

Floodplain–river ecosystems: lateral connections and the implications of human interference

Floodplains are ecotones that form a transition between aquatic and terrestrial environments. These important ecosystems can be described as dynamic spatial mosaics in which water plays an important role in connecting various patches on the floodplain surface. Hydrological connections facilitate the exchange of carbon and nutrients between the river channel and the floodplain and therefore influence the productivity of the entire river system. This paper examines the influence of hydrological connections on the potential exchange of dissolved organic carbon between a large Australian floodplain to a river channel, and the effects of land and water developments on these exchanges. The paper proposes that an understanding of floodplain ecosystems requires an interdisciplinary approach—a recognition of the importance of the three disciplines hydrology, geomorphology and ecology. Large-scale water-resources and floodplain development has significantly altered the spatial and temporal patterns of hydrological characteristics in the Lower Balonne floodplain, Australia. The magnitude, frequency and duration of flooding events have all been reduced. The construction of levees and water storages has also reduced the reactive floodplain surface area. The presented data show the impacts of these changes on the potential supply of dissolved organic carbon from the floodplain surface during periods of inundation. Annual reductions of up to 1293 tonnes of dissolved organic carbon supply were noted and reductions were especially significant for floods with an average recurrence interval of 2 years or less. Some small flood events no longer facilitate the potential supply of dissolved organic carbon from the floodplain to the river channel because of water-resources and floodplain developments.

Blue sucker stock characteristics in the upper Yazoo River basin, Mississippi, USA

AbstractBlue sucker Cycleptus elongatus (L.) in the upper Yazoo River basin, Mississippi, USA was studied using overnight hoop net sets (n = 4093) during 1988 and 1990–1998 to determine the influence of channel dredging. There were 264 blue suckers captured, ranging from 3 to 11 years of age. Length ranged from 265 to 700 mm and weight from 120 to 4700 g. Concurrent studies with smaller mesh hoop nets failed to capture any juvenile blue suckers. Catch‐per‐unit‐effort (CPUE: fish net−1) declined throughout the study in the Yalobusha River, a river subjected to channel dredging during 1988 and 1994. With the exception of limited hydraulic dredging in the Tallahatchie River, there was no dredging in the other Yazoo River tributaries, nor were there declines in blue sucker catch rates in these rivers. Throughout the upper Yazoo River basin, blue sucker stocks were dominated by adult fish, and there was little evidence of reproduction or recruitment. Conservation of the blue sucker in the upper Yazoo River basin should include actions that ensure the functional integrity of this floodplain river ecosystem and elimination of channel dredging throughout the basin.

Public Ownership of US Streambeds and Floodplains: A Basis for Ecological Stewardship

Abstract Larger US river–floodplain ecosystems are severely degraded. We analyze how public proprietary interests in streambeds and floodplains afford enormous untapped opportunity to protect them. We estimate that states hold in trust for the public approximately 500,000 kilometers of streambeds and hundreds of thousands of hectares of island-derived floodplains that continue to form at thousands of hectares annually. We find that although courts in 42 states have enforced public proprietary interests in submerged lands and floodplains, only three states have inventoried public streambeds, and no state has a comprehensive program to find, claim, and manage public streambeds and floodplains. We describe a legally and scientifically sound strategy to limit human interference with fluvial geomorphic processes, thereby regenerating diverse habitats and securing their myriad benefits, and we show how numerous successes in claiming and protecting public submerged lands and floodplains in dozens of states confi...

Long-term perspectives on human impacts on floodplain–river ecosystems, Murray–Darling Basin, Australia

(2002). Long-term perspectives on human impacts on floodplain–river ecosystems, Murray–Darling Basin, Australia. SIL Proceedings, 1922-2010: Vol. 28, No. 2, pp. 710-716.

Importance of flood regime to invertebrate habitat in an unregulated river–floodplain ecosystem

AbstractUnlike most large rivers of the northern hemisphere, several medium-sized rivers in the southeastern USA Coastal Plain remain unregulated. These smaller rivers possess 2 habitat types (snags and floodplain) that were historically important for invertebrate assemblages in many rivers and are strongly dependent on flood regime. I reviewed and compared 2 models of habitat inundation (snags and floodplain) that were developed for the Ogeechee River (Georgia, USA) to understand the ecological significance of these habitats. These models showed that snag habitat surfaces varied from only ∼20 to 50% of channel bottom surfaces, but floodplain inundation varied from 0 to 37 times the width of the channel at 100% inundation. Long-term analysis of inundation patterns from a 58-y record of discharge demonstrated that substantial flooding occurred almost annually for 1 to 2 mo/y. Habitat-specific invertebrate biomass was highest on snags (mostly aquatic insects), followed by the main channel (dominated by Corb...

Conservation of natural ecosystems of poplar and willow

This paper discusses the need and possibilities for conservation of riparian ecosystems with emphasis on two important groups of tree species in these flood plain river ecosystems, poplars and willows. It puts conservation and restoration activities into perspective with the needs and benefits for society and the environment. Natural ecosystems of poplar and willow that still exist are relatively scarce and although much is known about the different components that together make up such an ecosystem, relatively little is known about the system itself. The managers of ecosystems have to deal with many influences that come from outside the ecosystem: not only upstream and downstream, but also perpendicular to the river. The influence of mankind, by means of its actions that reflect the opinion that nature is secondary to economics, is identified as one of the major problems. Key words: riparian ecosystem, flood plain river ecosystem, conservation, restoration, rehabilitation, poplar, willow

MODELING THE EFFECTS OF LAND USE AND CLIMATE CHANGE ON RIVERINE SMALLMOUTH BASS

Anthropogenic changes in temperature and stream flow, associated with watershed land use and climate change, are critical influences on the distribution and abundance of riverine fishes. To project the effects of changing land use and climate, we modeled a smallmouth bass (Micropterus dolomieu) population in a midwestern USA, large river–floodplain ecosystem under historical (1915–1925), present (1977–1990), and future (2060, influenced by climate change) temperature and flow regimes. The age-structured model included parameters for temperature and river discharge during critical seasonal periods, fish population dynamics, and fishing harvest. Model relationships were developed from empirical field data collected over a 13-yr period. Sensitivity analyses indicated that discharge during the spawning/rearing period had a greater effect on adult density and fishing yield than did spawning/rearing temperature or winter discharge. Simulations for 100 years projected a 139% greater mean fish density under a historical flow regime (64.9 fish/ha) than that estimated for the present (27.1 fish/ha) with a sustainable fishing harvest under both flow regimes. Simulations under future climate-change-induced temperature and flow regimes with present land use projected a 69% decrease in mean fish density (8.5 fish/ha) from present and an unstable population that went extinct during 56% of the simulations. However, when simulated under a future climate-altered temperature and flow regime with historical land use, the population increased by 66% (45.0 fish/ha) from present and sustained a harvest. Our findings suggest that land-use changes may be a greater detriment to riverine fishes than projected climate change and that the combined effects of both factors may lead to local species extinction. However, the negative effects of increased temperature and precipitation associated with future global warming could be mitigated by river channel, floodplain, and watershed restoration.

Biodiversity of floodplain river ecosystems: ecotones and connectivity1

A high level of spatio-temporal heterogeneity makes riverine floodplains among the most species-rich environments known. Fluvial dynamics from flooding play a major role in maintaining a diversity of lentic, lotic and semi-aquatic habitat types, each represented by a diversity of successional stages. Ecotones (transition zones between adjacent patches) and connectivity (the strength of interactions across ecotones) are structural and functional elements that result from and contribute to the spatio-temporal dynamics of riverine ecosystems. In floodplain rivers, ecotones and their adjoining patches are arrayed in hierarchical series across a range of scales. At a coarse scale of resolution, fringing floodplains are themselves complex ecotones between river channels and uplands. At finer scales, patches of various types and sizes form habitat and microhabitat diversity patterns. A broad spatio-temporal perspective, including patterns and processes across scales, is needed in order to gain insight into riverine biodiversity. We propose a hierarchical framework for examining diversity patterns in floodplain rivers. Various river management schemes disrupt the interactions that structure ecotones and alter the connectivity across transition zones. Such disruptions occur both within and between hierarchical levels, invariably leading to reductions in biodiversity. Species richness data from the connected and disconnected floodplains of the Austrian Danube illustrate this clearly. In much of the world, species-rich riverine/floodplain environments exist only as isolated fragments across the landscape. In many large rivers, these islands of biodiversity are endangered ecosystems. The fluvial dynamics that formed them have been severely altered. Without ecologically sound restoration of disturbance regimes and connectivity, these remnants of biodiversity will proceed on unidirectional trajectories toward senescence, without rejuvenation. Principles of ecosystem management are necessary to sustain biodiversity in fragmented riverine floodplains. Copyright © 1999 John Wiley & Sons, Ltd.

Hand Grappling Blue Catfish in the Main Channel of a Mississippi River

Blue catfish Ictalurus furcatus were hand grappled, primarily by using scuba gear, from 30.5-cm-square × 122-cm-long wood boxes, May through July 1990–1993 in the Tallahatchie River, a principal tributary of the Yazoo River in Mississippi. During January through August 1990–1994, blue catfish also were collected with hoop nets (4.3 m long, having seven hoops with 1.07 m diameters and 3.81-cm-bar-mesh netting), which are standard gear for riverine fish stock assessments in Mississippi. During the study, 638 grabs yielded 29 blue catfish, and 638 overnight hoop-net sets yielded 71 blue catfish. Hand grappling caught larger blue catfish than did hoop netting. Mean total lengths (TL) of blue catfish captured by hand grappling and hoop netting were 77.3 em (SE, 2.1 cm) and 48.3 cm (SE, 0.8 cm), respectively. There were also differences in length-frequency distributions between methods, which confirmed hand-grappling selectivity for larger blue catfish. Overall. the hand-grappling catch rate for the entire study was about 24–25 grabs per fish captured. The overall hoop-netting catch rate was about 9–10 net sets per fish captured and about 50 net sets per fish captured for fish 60 cm TL or longer, which was the minimum size captured by hand grappling. Siltation in conjunction with strong currents and fluctuations in water level created logistic challenges to hand grappling. Hand grappling, as a specialized fishery, should be viewed as another component in the suite of environmental and sociological concerns linked to conservation initiatives for floodplain river ecosystems in the United States.

Notes: Largemouth Bass Size Distributions under Varying Annual Hydrological Regimes in the Illinois River

Little is known about the population dynamics of largemouth bass Micropterus salmoides in temperate large river–floodplain ecosystems. However, the hydrological regimes in these systems are often similar to those of large reservoirs where fluctuating water levels during spawning have been shown to affect largemouth bass population dynamics. Most backwater lakes of the Illinois River have soft, silty substrates. These substrates are not conducive to nest building, so spawning centrarchids may use annual spring floods to access inundated terrestrial vegetation and previously dry, compacted substrates on the floodplain. We used electrofishing catch data from La Grange Reach of the Illinois River (1990–1995) to assess how spring and early-summer river levels influence the cohort strength of largemouth bass. Strong cohorts were produced during years with high spring floods (1990, 1993, and 1995) when largemouth bass could access the floodplain for spawning and nursery habitat. In both 1990 and 1993, fish had access to the floodplain for extended periods during spring and summer, which resulted in exceptionally strong cohorts. These cohorts probably benefitted from abundant forage and cover on the floodplain, which bolstered survival rates. Weak cohorts were produced in 1992 and 1994 when river levels were low and fluctuating during spawning and in 1991 when river levels declined dramatically during spawning. Length distributions of other centrarchids mirrored those of largemouth bass, indicating that river levels probably affected other nest-building species in La Grange Reach.