World's Largest River Research Articles

Incorporating DNA barcodes into a multi-year inventory of the fishes of the hyperdiverse Lower Congo River, with a multi-gene performance assessment of the genus Labeo as a case study

Background and aims: Here we describe preliminary efforts to integrate DNA barcoding into an ongoing inventory of the Lower Congo River (LCR) ichthyofauna. The 350 km stretch of the LCR from Pool Malebo to Boma includes the world's largest river rapids. The LCR ichthyofauna is hyperdiverse and rich in endemism due to high habitat heterogeneity, numerous dispersal barriers, and its downstream location in the basin.Materials and methods: We have documented 328 species from the LCR, 25% of which are thought to be endemic. In addition to detailing progress made to generate a reference sequence library of DNA barcodes for these fishes, we ask how DNA can be used at the current stage of the Fish Barcode of Life initiative, as a work in progress currently of limited utility to a wide audience. Two possibilities that we explore are the potential for DNA barcodes to generate discrete diagnostic characters for species, and to help resolve problematic taxa lacking clear morphologically diagnostic characters such as many species of the cyprinid genus Labeo, which we use as a case study.Results: Our molecular analysis helped to clarify the validity of some species that were the subject of historical debate, and we were able to construct a molecular key for all monophyletic and morphologically recognizable species. Several species sampled from across the Congo Basin and widely distributed throughout Central and West Africa were recovered as paraphyletic based on our molecular data.Conclusion: Our study underscores the importance of generating reference barcodes for specimens collected from, or in close proximity to, type localities, particularly where species are poorly understood taxonomically and the extent of their geographical distributions have yet to be established.

Seasonal Variation in the Mineralogy of the Suspended Particulate Matter of the Lower Changjiang River at Nanjing, China

AbstractThe source and temporal changes of minerals transported by the world's large rivers are important. In particular, clay minerals are important in evaluating the maturity of suspended sediments, weathering intensity, and source area. To examine seasonal changes in mineralogical compositions of the Changjiang River (CR), suspended particulate matter (SPM) samples were collected monthly for two hydrological cycles in Nanjing city and then were studied using X-ray diffraction (XRD), diffuse reflectance spectrophotometry (DRS), X-ray fluorescence spectrometry (XRF), and chemical analyses. The results indicate that the concentration of CR SPM ranges from 11.3 to 152 mg/L and is highly correlated to the rate of water discharge, with a greater concentration in flood season and lower concentrations during the dry season. CaO, MgO, and Na2O increase with increasing discharge whereas Al2O3 decreases sharply with increasing discharge. Dolomite, calcite, and plagioclase show strikingly similar seasonal variations and increase with increasing discharge with maximum concentrations in the flood season. In contrast, the clay mineral content exhibits the opposite trend with the lowest concentrations in the flood season. Illite dominates the clay minerals of the CR SPM, followed by chlorite, kaolinite, and smectite. Illite and kaolinite show distinctly seasonal variations; SPM contains more illite and less kaolinite during the flood season than during the dry season. The illite chemistry index and crystallinity, as well as kaolinite/illite ratio, all indicate intense physical erosion in the CR basin during the rainy season. Total iron (FeT) and highly reactive iron (FeHR) concentrations display slight seasonal changes with the smallest values observed during the flood season. Goethite is the dominant Fe oxide mineral phase in the CR SPM and hematite is a minor component, as revealed by DRS analyses. The FeT flux and FeHR flux are 2.786×106 T/y and 1.196×106 T/y, respectively.

Biogeochemical weathering under ice: Size matters

The basal regions of continental ice sheets are gaps in our current understanding of the Earth's biosphere and biogeochemical cycles. We draw on existing and new chemical data sets for subglacial meltwaters to provide the first comprehensive assessment of sub‐ice sheet biogeochemical weathering. We show that size of the ice mass is a critical control on the balance of chemical weathering processes and that microbial activity is ubiquitous in driving dissolution. Carbonate dissolution fueled by sulfide oxidation and microbial CO2 dominate beneath small valley glaciers. Prolonged meltwater residence times and greater isolation characteristic of ice sheets lead to the development of anoxia and enhanced silicate dissolution due to calcite saturation. We show that sub‐ice sheet environments are highly geochemically reactive and should be considered in regional and global solute budgets. For example, calculated solute fluxes from Antarctica (72–130 t yr−1) are the same order of magnitude as those from some of the world's largest rivers and rates of chemical weathering (10–17 t km−2 yr−1) are high for the annual specific discharge (2.3–4.1 × 10−3 m). Our model of chemical weathering dynamics provides important information on subglacial biodiversity and global biogeochemical cycles and may be used to design strategies for the first sampling of Antarctic Subglacial Lakes and other sub‐ice sheet environments for the next decade.

Calcium isotope ratios in the world's largest rivers: A constraint on the maximum imbalance of oceanic calcium fluxes

The oceanic mass balance of calcium (Ca) is defined by a balance between the inputs (rivers and hydrothermal) and outputs (bulk carbonate) of Ca. Large rivers were analyzed for Ca isotope ratios (44Ca/42Ca, expressed as Ca) to investigate the source and cycling of riverine Ca, and to add an isotopic mass balance constraint to the oceanic budget of Ca. The new data account for approximately one‐third of the total Ca supplied to the oceans by rivers. Inter‐sample and seasonal variability was assessed by analyzing more than one sample for many rivers. The range in the Ca of large rivers at high water stand is extremely narrow at 0.27‰. Variations in Ca do not correlate with proxies for carbonate, silicate or evaporite derived Ca, and are more likely related either to inherent variability in the lithological sources of Ca or to process related fractionation. The spread in riverine Ca overlaps with the spread in marine limestone Ca consistent with most riverine Ca coming from the recycling of limestones. The Ca isotope composition of continental runoff has an average Ca value of 0.38 ± 0.04‰, identical to recent (5 M.yr) bulk carbonate ooze (0.33 ± 0.13‰, 2S.D.). Isotopic mass balance constrains that the input and output fluxes of Ca to and from the oceans, are balanced to within 15% over time‐scales similar to the residence time of Ca in the oceans (1 M.yr). A greater imbalance between the fluxes would result in a detectable difference between the Ca value of bulk carbonate and the riverine input at the current level of uncertainty. The input and output fluxes could be imbalanced over much shorter time‐scales (such as glacial‐interglacial cycles), in which case the ocean‐carbonate system will not yet have responded, because of the long residence time of Ca. The maximum current flux imbalance of 15% would be sufficient to account for the total variations in Ca concentration over the Tertiary. Such an interpretation is not unique, but is the simplest interpretation given the similarity between the input and output isotopic compositions, and rules out hypotheses of extreme imbalance in the recent global biogeochemical cycle of Ca.

Northern Hemisphere geography of ice‐covered rivers

AbstractAlthough river ice is a major component of the cryosphere and is particularly important to many river processes, including extreme events, its full geographical coverage has never been documented. Recognizing that the freeze‐up and break‐up of river ice is closely linked to the timing of 0 °C air temperatures, this study analyses the spatial extent of river networks relative to the location of three 0 °C isotherm periods. These were defined to represent a suite of ice‐affected conditions that would be experienced for 6, 3 or 0·5 month periods, the briefest interval possibly leading only to a very thin and transient ice cover or simply border/frazil ice formation. Four different GIS databases were used to represent the river networks. The percentages of the total Northern Hemisphere land mass (average river network) influenced by cold temperatures conducive to ice formation were 52, 45 and 25 (56, 47 and 28), respectively. The related southern position of the isotherms ranged from 33°N, 35°N and 50°N in central North America to a nearly consistent 27°N for Eurasia, reflecting the influence of the high‐elevation central plateau region. Also identified are the lengths of major rivers that fall within the three 0 °C isotherm boundaries. Included are some of the world's largest rivers including the Lena, Mackenzie, Ob, Yellow, Yukon and Yenisey rivers, although their percentage of ice‐affected coverage varied for the three isotherm periods from a consistent 100% for the Lena and Yukon rivers to as little as 23% at the 6‐month interval for the Yellow River. Copyright © 2009 Her Majesty the Queen in right of Canada. Published by John Wiley & Sons, Ltd.

Evaluation of GRACE filter tools from a hydrological perspective

Approximately seven years of time-variable gravity data from the satellite mission Gravity Recovery and Climate Experiment (GRACE) are available to quantify present-day mass variations on and near the Earth's surface. Mass variations caused by the continental water cycle are the dominant signal component after subtracting contributions from atmosphere and oceans. This makes hydrology a primary area of application of GRACE data. To derive water storage variations at the scale of large river basins, appropriate filter techniques have to be applied to GRACE gravity fields given in a global spherical harmonic representation. A desirable filter technique minimises both GRACE data error and signal leakage across the border of the region of interest. This study evaluates the performance of six widely used filter methods (isotropic filters, anisotropic filters and decorrelation methods) and their parameter values to derive regionally averaged water mass variations from GRACE data. To this end, filtered time series from GRACE for 22 of the world's largest river basins were compared to continental water mass variations from a multimodel mean of three global hydrological models (WGHM, GLDAS and LaD). Filter-induced biases for seasonal amplitudes and phases of water storage variations, as well as satellite and leakage error budgets, were quantified for each river basin and explained in terms of storage variations in and around the basin. The optimum filter types and filter parameters were identified for each basin. The best results were provided by a decorrelation method that uses GRACE orbits for the filter design. Our ranking between all filter types and parameters depended on the geographical location, shape and signal characteristics of the specific river basin. Based on a multicriterial evaluation of satellite and leakage error, as well as an error assessment of the hydrological data, the filter selection and parameter optimisation results were shown to be reliable for 17 river basins. The results serve as a guideline for the optimal filtering of GRACE global spherical harmonic coefficients for hydrological applications.

A comparative overview of weathering intensity and HCO 3− flux in the world's major rivers with emphasis on the Changjiang, Huanghe, Zhujiang (Pearl) and Mississippi Rivers

In this paper, general relationships of riverine bicarbonate concentrations and fluxes as a function of drainage basin mineral content and runoff are examined using a database of the 25 largest rivers in the world. Specific HCO 3 − flux normalized to unit basin area, which peaks in the mid latitudes, was found to be strongly correlated with the carbonate mineral content of river basins, while river HCO 3 − concentration was related to the balance of precipitation and evaporation. Within this global context, the weathering patterns of CO 2 in a few large rivers (Changjiang, Huanghe, Pearl, and Mississippi rivers) were examined in further detail. The Zhujiang (Pearl River), especially its largest branch (Xijiang), was characterized by the highest specific weathering rate among all the world's large rivers due to an exceptionally high carbonate mineral content (over 80%) in its drainage basin and its warm and wet environment. It has a moderate level of HCO 3 − concentration, however, due to dilution by relatively high precipitation in the watershed. In stark contrast, the Huanghe (Yellow River) has one of the lowest specific weathering rates because of low carbonate mineral content and a dry climate. However, it has a high HCO 3 − concentration due largely to the concentrating effects of high evaporative water loss, as a result of arid weather and the agricultural use of water through irrigation systems, as well as carbonate-containing surficial deposits (i.e., loess). The strong correlation between specific HCO 3 − fluxes and discharge in all four rivers with different discharge seasonality suggests that higher precipitation in drainage basins promotes higher weathering rates. For the 25 large rivers studied here, rivers in low (<30°), mid (30–60°) and high (>60°) latitudes have an average HCO 3 − concentration of 0.584, 1.649, and 1.154 mM, respectively, and they account for 42.6%, 47.3% and 10.1%, respectively, of the total global dissolved inorganic carbon flux to the ocean. Thus the mid-latitude rivers carry a disproportionally high dissolved inorganic carbon flux with a relatively small (26%) amount of freshwater discharge. The discharge-averaged global river HCO 3 − concentration was estimated to be 1.1 mM.

Sustaining the Flow of the World's Rivers

This article reports on the 2007 International Riversymposium and Environmental Flow Conference held in Brisbane, Australia. The conference featured more than 150 presentations describing the ongoing degradation of rivers and the socioeconomic effects the degradation is having on the people who depend on rivers for their livelihoods. Some of the presentations also offered some field‐tested solutions for better protecting or restoring rivers. Particular focus was given to maintaining adequate river flows (referred to as environmental flows or instream flows). Some of the world's largest rivers now dry up on a regular basis before reaching the sea because their waters are heavily used for irrigation and as urban water supplies. Conference participants reached a consensus that there is no single, scientifically correct answer to the question of how much water a river needs to remain healthy. They concluded that the environmental flow needs of a particular river should instead be determined through an integrated social and scientific dialogue in which societal goals for the river are first defined. The article provides copies of The Brisbane Declaration and Global Action Agenda (summary findings and a global action agenda that addresses the urgent need to protect rivers globally).

Wavelet analysis of the annual discharge records of the world's largest rivers

Wavelet analysis of the annual discharge records of the world's largest rivers

Influence of runoff parameterization on continental hydrology: Comparison between the Noah and the ISBA land surface models

A comprehensive set of hydrological parameterizations without any basin‐scale calibration was recently introduced into the global ISBA land surface model in order to improve the simulation of the hydrological impacts of both seasonal climate anomalies and global warming. In this study, the same approach is introduced into the Noah land surface model, in order to improve the representation of surface runoff. The Dunne runoff is computed using topographic information via a TOPMODEL approach and the Horton runoff using an explicit distinction between frozen and unfrozen soils, while land surface heterogeneities are introduced via a tile approach. The validation is conducted at a 1° by 1° horizontal resolution using global off line simulations that were driven by Global Soil Wetness Project atmospheric forcing. The simulated runoff is converted into discharges using the TRIP river routing model and compared to observations from a dense network of gauging stations distributed over the world's largest river basins. Results confirm that the relationships between the Dunne runoff and the topography, as well as between the Horton runoff and the frozen soils, are among the main hydrological processes to simulate realistic global river discharges. The comparison between the old and the new versions of Noah and ISBA reveal that the model disparity, in term of water flux production, is globally reduced due to the use of similar surface hydrology that lead to comparable improvements of discharge scores. This confirms the relevance of these surface hydrological processes for regional and global applications.

Land use change in Asia and the ecological consequences

AbstractViewed within a historical context, Asia has experienced dramatic land transformations, and currently more than 50% of Asian land area is under agriculture. The consequences of this transformation are manifold. Southeast Asia has the highest deforestation rate of any major tropical region. Many of the world's large rivers and lakes in Asia have been heavily degraded. About 11 of 19 world megacities with more than 10 million inhabitants are in Asia. These land use activities have resulted in substantial negative ecological consequences, including increased anthropogenic CO2 emissions, deteriorated air and water quality, alteration of regional climate, an increase of disease and a loss of biodiversity. Although land use occurs at the local level, it has the potential to cause ecological impact across local, regional and global scales. Reducing the negative environmental impacts of land use change while maintaining economic viability and social acceptability is an major challenge for most developing countries in Asia.

Land use and climatic impacts on the Rhine system (RheinLUCIFS): Quantifying sediment fluxes and human impact with available data

The RheinLUCIF project investigates the effects of land use and climatic impacts on sediment fluxes in the river Rhine catchment. Among the world's large rivers the Rhine system has been particularly affected by land use changes for 7500 years, resulting in its transformation to an entirely man-controlled environment. Geomorphologically this transformation is performed by largely man-induced sediment fluxes since the mid-Holocene. As the large scale of the Rhine catchment inhibits a renewed recording of stratigraphic records, the main goal is to synthesize quantitative data on sediment flux, climatic and human impacts from already available data from various fields of Earth surface and social sciences (particularly history and archaeology). This paper presents the first steps and results of methodological groundwork exploring how to derive sediment budgets from available spatially distributed data (soil maps, geological maps, DEMs). Furthermore, a new basically statistical approach utilising archaeological data is outlined. This approach provides time series of quantified human impact, i.e. information on population densities and open land / woodland ratios. The first results derived from four different sediment budget approaches are helpful in order to assess the quality of available data for budget calculation purposes. Although some estimates on erosion and alluvial deposition seem to broadly agree, a considerable disparity of Holocene erosion and colluviation values occurs when these values are compared to the outcome of an independently recorded soil mapping campaign in a pilot area. However, in general the results suggest that all applied approaches chiefly require an improved estimation of sediment storage. On the one hand, this accounts for the need of more precise information on Holocene floodplain deposits. On the other hand, as the larger portion of man-induced sediments still resides in the sloping parts of catchments, realistic estimates on man-induced colluvial deposition on slopes are essential. Apart from budgeting issues all data on Holocene erosion and sedimentation in the Rhine catchment lacked a sufficient dating control. Therefore further investigations on fluxes between budget components necessitate additional datings. Quantified data on human impact is indispensable to the modelling of sediment fluxes because human impact is deemed to be the major driving force of sediment production and re-distribution. Although the archaeological approach was developed and tested by utilising well documented data about the Neolithic era, the promising statistical evaluation suggests that this method can be applied to even less well documented proto-, prehistoric and historic periods.

Persistent disturbance by commercial navigation alters the relative abundance of channel-dwelling fishes in a large river

We provide the first evidence for chronic effects of disturbance by commercial vessels on the spatial distribution and abundance of fishes in the channels of a large river. Most of the world's large rivers are intensively managed to satisfy increasing demands for commercial shipping, but little research has been conducted to identify and alleviate any adverse consequences of commercial navigation. We used a combination of a gradient sampling design incorporating quasicontrol areas with Akaike's information criterion (AIC)-weighted model averaging to estimate effects of disturbances by commercial vessels on fishes in the upper Mississippi River. Species density, which mainly measured species evenness, decreased with increasing disturbance frequency. The most abundant species — gizzard shad (Dorosoma cepedianum) and freshwater drum (Aplodinotus grunniens) — and the less abundant shovelnose sturgeon (Scaphirhynchus platorhynchus) and flathead catfish (Pylodictis olivaris) were seemingly unaffected by traffic disturbance. In contrast, the relative abundance of the toothed herrings (Hiodon spp.), redhorses (Moxostoma spp.), buffaloes (Ictiobus spp.), channel catfish (Ictalurus punctatus), sauger (Sander canadensis), and white bass (Morone chrysops) decreased with increasing traffic in the navigation channel. We hypothesized that the combination of alteration of hydraulic features within navigation channels and rehabilitation of secondary channels might benefit channel-dependent species.

Fish assemblages and biotic integrity of a highly modified floodplain river, the Upper Mississippi, and a large, relatively unimpacted tributary, the Lower Wisconsin

The Upper Mississippi River is a dynamic floodplain river that has been largely transformed by navigational levees and dams since the 1930s. The pools upstream of each dam are lake-like and only about the upper third of each reach retains a riverine character. In contrast, the Wisconsin River is not managed for commercial navigation and today its lower 149 km represent one of the least-degraded large river reaches in central North America. Riverine reaches in both the Mississippi and Wisconsin rivers have similar macro-habitats including numerous islands, large side channels, and connected backwaters and floodplain lakes. In this study, shoreline electrofishing samples were collected during summer 2002 and 2003 to characterize resident fish assemblages. We compared fish species abundance, biomass, and biotic integrity along main and side channel borders between the Upper Mississippi River and the Lower Wisconsin River. We expected that, in the absence of environmental degradation, fish composition and structure would be similar between the Mississippi and Wisconsin rivers, and between channel types within each river. Nonmetric multidimensional scaling and redundancy analysis revealed that fish species in the Mississippi River, unlike in the Wisconsin River, were characteristic of non-riverine habitats. We consider non-riverine fish assemblages indicative of environmental impairment. The main and side channel sites in the Mississippi River had more variable fish assemblages than the Wisconsin River. Analyses of fish index of biotic integrity scores showed that environmental condition was excellent for both channel types in the Wisconsin River, whereas in the Mississippi River the side channel was rated good and the main channel only fair. We conclude that differences between the two rivers and between channel types of the Mississippi River are consistent with direct and indirect effects of navigation. This study demonstrates the utility of a fish index of biotic integrity, an inexpensive and rapid bioassessment tool, for detecting change in ecological health on one of the world's largest rivers. Copyright © 2006 John Wiley & Sons, Ltd.

Use of remote sensing and ANN in assessment of erosion activities in Majuli, the world's largest river island

Majuli, the world's largest river island, is situated to the north of Jorhat in India and at the confluence of the Subansiri and Brahmaputra rivers. Flooding ravages the population of 0.13 million every year, robbing them of the basic requirements of life. This island, which was over 1245 km2 in the 1950s, was reduced to 376.93 km2 in 2002 due to continuous erosion of banks by the Brahmaputra river, wiping out scores of villages and displacing thousands of people. The heritage of the people of Majuli, the civilization they have nurtured for the last five centuries, is about to be buried. Burial of this civilization will be a loss to the whole of humanity. Therefore, a global approach to this problem of flooding and subsequent erosion has become inevitable, and is urgently needed to save the world's largest river island as well as its glowing cultural heritage. This paper focuses on the study of erosion problems of the decaying island, deploying remote sensing and an artificial neural network (ANN) technique.

A Global Dataset of Palmer Drought Severity Index for 1870–2002: Relationship with Soil Moisture and Effects of Surface Warming

AbstractA monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical precipitation and temperature data for global land areas on a 2.5° grid. Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r = 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt plays an important role. Basin-averaged annual PDSI covary closely (r = 0.6 to 0.8) with streamflow for seven of world's largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Niño– Southern Oscillation (ENSO)-induced mode of mostly interannual variations as the two leading patterns. The global very dry areas, defined as PDSI &amp;lt; −3.0, have more than doubled since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to surface warming, while global very wet areas (PDSI &amp;gt; +3.0) declined slightly during the 1980s. Together, the global land areas in either very dry or very wet conditions have increased from ∼20% to 38% since 1972, with surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures and increased drying.

Anabranching and maximum flow efficiency in Magela Creek, northern Australia

Anabranching is the prevailing river pattern found along alluvial tracts of the world's largest rivers. Hydraulic geometry and bed material discharge are compared between single channel and anabranching reaches up to 4 times bank‐full discharge in Magela Creek, northern Australia. The anabranching channels exhibit greater sediment transporting capacity per unit available stream power, i.e., maximum flow efficiency (MFE). Simple flume experiments corroborate our field results showing the flow efficiency gains associated with anabranching, and highlight the prospect of a dominant anabranch, which is found in many anabranching rivers. These results demonstrate that anabranching can constitute a stable river pattern in dynamic equilibrium under circumstances in which a continuous single channel would be unable to maintain sediment conveyance. We propose the existence of a flow efficiency continuum that embraces dynamic equilibrium and disequilibrium (vertically accreting) anabranching rivers.

Historic Island Threatened

The world's largest river island will disappear within the next few decades thanks to the rapid erosion of its banks by the mighty Brahmaputra River, scientists say. In the first detailed analysis of the problem, scientists at the Regional Research Laboratory in Jorhat say the river has already swallowed more than 50% of the island, called Majuli. With a population of more than 130,000, Majuli is culturally important for eastern India. It has 22 Vaishnavaite monasteries, some of which date back to the 15th century, where traditional Hindu arts, music, and dance forms are kept alive.

Fluvial landscape models and catchment-scale sediment transport

The need for the ability to make quantitative predictions regarding the transport of sediment within watersheds and from watersheds to the sea has never been more important than it is today. Sediment transport is at the heart of a surprising number of practical problems, ranging from the prevention of soil loss to the tracking of contaminants, and this has resulted in a rich but scattered literature. In many of these contexts, observational data has been used to develop empirical sediment transport laws. For example, the Revised Universal Soil Loss Equation (RUSLE) has been developed for use at the farm-plot scale, and the equation of Syvitski et al. [Water Resour. Res. 36(9) (2000) 2747] was developed for the world's largest river basins. In the context of fluvial geomorphology, several different but closely related sediment transport laws have been used to construct fluvial landscape models. The sediment transport formulas used by these models are a generalized form of several physically based bed-load transport formulas. Based on studies of fluvial landscape models, we now know that many measurable attributes of channelized landscapes, such as junction angles, Horton ratios, hydraulic geometry exponents, valley geometry, and longitudinal profiles, vary in a predictable manner with a few well-constrained, physical parameters. One important consequence of this success in forward modeling is that it is now feasible to solve the inverse problem of calculating regional sediment transport parameters from measurements (such as long profiles) that can easily be made from digital elevation models (DEMs). This may also make it possible to make a rational selection between alternate sediment transport laws. Since fluvial landscape models represent the state of the art in catchment sediment transport modeling, the three-fold purpose of this paper is to: (1) explain the key concepts and simplifying assumptions that are common to these models; (2) explain how several current models differ from one another; and (3) highlight a few of the significant results that have been obtained in recent years. While serving mostly as an expository or overview paper, some new results for a steady state fluvial landscape model are also included.

Global patterns of dissolved N, P and Si in large rivers

The concentration of dissolved inorganic nitrogen (DIN), dissolved nitrate-N, Total-N (TN), dissolved inorganic phosphate (DIP), total phosphorus (TP), dissolved silicate-Si (DSi) and their ratios in the world's largest rivers are examined using a global data base that includes 37% of the earth's watershed area and half its population. These data were compared to water quality in 42 subbasins of the relatively well-monitored Mississippi River basin (MRB) and of 82 small watersheds of the United States. The average total nitrogen concentration varies over three orders of magnitude among both world river watersheds and the MRB, and is primarily dependent on variations in dissolved nitrate concentration, rather than particulate or dissolved organic matter or ammonium. There is also a direct relationship between the DIN:DIP ratio and nitrate concentration. When nitrate-N exceeds 100 μg-at l−1, the DIN:DIP ratio is generally above the Redfield ratio (16:1), which implies phosphorus limitation of phytoplankton growth. Compared to nitrate, the among river variation in the DSi concentration is relatively small so that the DSi loading (mass/area/time) is largely controlled by runoff volume. The well-documented influence of human activities on dissolved inorganic nitrogen loading thus exceeds the influences arising from the great variability in soil types, climate and geography among these watersheds. The DSi:nitrate-N ratio is controlled primarily by nitrogen loading and is shown to be inversely correlated with an index of landscape development – the “City Lights” nighttime imagery. Increased nitrogen loading is thus driving the world's largest rivers towards a higher DIN:DIP ratio and a lower DSi:DIN ratio. About 7.3 and 21 % of the world's population lives in watersheds with a DSi:nitrate-N ratio near a 1:1 and 2:1 ratio, respectively. The empirical evidence is that this percentage will increase with further economic development. When the DSi:nitrate-N atomic ratio is near 1:1, aquatic food webs leading from diatoms (which require silicate) to fish may be compromised and the frequency or size of harmful or noxious algal blooms may increase. Used together, the DSi:nitrate-N ratio and nitrate-N concentration are useful and robust comparative indicators of eutrophication in large rivers. Finally, we estimate the riverine loading to the ocean for nitrate-N, TN, DIP, TP and DSi to be 16.2, 21, 2.6, 3.7 to 5.6, and 194 Tg yr−1, respectively.