River Discharge Rates Research Articles

Modeling river discharge rates in California watersheds

River discharge rates across all California's watershed have been modeled using the NASA version of the Carnegie-Ames-Stanford Approach (CASA) ecosystem model coupled with a surface hydrologic routing scheme previously called the Hydrological Routing Algorithm (HYDRA). To assess CASA-HYDRA's capability to estimate actual water flows in extreme and non-extreme precipitation years, we have organized hundreds of California river gauge records for comparison to monthly model predictions. Previously, CASA-HYDRA snowmelt algorithms were modified with equations from the USDA Snowmelt Runoff Model, which has been designed to predict daily stream flow in mountain basins where snowmelt is a major runoff factor. Based on model predictions of monthly flow rates across 336 stream gauges statewide, the multi-year model-to-measurement correlation between monthly river flow rates was R2=0.72. The model output was 15% higher across all these stream gauges than the measured monthly flow records for 1982–1990. It is plausible that the model would predict higher flow rates statewide than was measured at many gauge locations, due mainly to extensive water diversions for power generation and crop irrigation in the valley growing regions of the state. Predictions for gauges located on the state's North Coast and Sierra regions showed errors distributed fairly evenly throughout the seasons, whereas results for Central Coast and Southern regions showed higher errors mainly during the summer and fall. Future model applications for land cover and climate change in California are outlined.

Tracking stormwater discharge plumes and water quality of the Tijuana River with multispectral aerial imagery

Spatial–temporal characteristics and environmental factors regulating the behavior of stormwater runoff from the Tijuana River in southern California were analyzed utilizing very high resolution aerial imagery, and time-coincident environmental and bacterial sampling data. Thirty nine multispectral aerial images with 2.1-m spatial resolution were collected after major rainstorms during 2003–2008. Utilizing differences in color reflectance characteristics, the ocean surface was classified into non-plume waters and three components of the runoff plume reflecting differences in age and suspended sediment concentrations. Tijuana River discharge rate was the primary factor regulating the size of the freshest plume component and its shorelong extensions to the north and south. Wave direction was found to affect the shorelong distribution of the shoreline-connected fresh plume components much more strongly than wind direction. Wave-driven sediment resuspension also significantly contributed to the size of the oldest plume component. Surf zone bacterial samples collected near the time of each image acquisition were used to evaluate the contamination characteristics of each plume component. The bacterial contamination of the freshest plume waters was very high (100% of surf zone samples exceeded California standards), but the oldest plume areas were heterogeneous, including both polluted and clean waters. The aerial imagery archive allowed study of river runoff characteristics on a plume component level, not previously done with coarser satellite images. Our findings suggest that high resolution imaging can quickly identify the spatial extents of the most polluted runoff but cannot be relied upon to always identify the entire polluted area. Our results also indicate that wave-driven transport is important in distributing the most contaminated plume areas along the shoreline.

Sea‐tide effects on flows in the lower reaches of the Amazon River

AbstractThe physical influence of sea tides on the lower reaches of the Amazon River can be identified more than 1000 km upstream from the sea. Analysis of upstream tidal wave propagation on the Amazon River contributes to the understanding of the hydrodynamics of the lower reaches and to the quantification of the influence of sea level on sedimentation rates. Continuous over‐the‐year recording of water levels has been undertaken at eight locations along the lower reaches of the Amazon River and flow measurement campaigns have been organized for different river discharge rates (100 000 m3 s−1, 190 000 m3 s−1 and 250 000 m3 s−1) and sea‐tide amplitudes (2–3·5 m). Damping, time‐delay and deformation of the tidal wave signals, depending on water regimes and distances from the sea, have been analyzed. The physical processes of wave propagation and cyclic water storage and release have been investigated both through data analysis and the application of mathematical hydrodynamic models. The impact of tidal pulses on sediment transport to the sea has been assessed. Copyright © 2009 John Wiley & Sons, Ltd.

Route choices, migration speeds and daily migration activity of European silver eels Anguilla anguilla in the River Rhine, north‐west Europe

Downstream migration of Anguilla anguilla silver eels was studied in the Lower Rhine, Germany, and the Rhine Delta, The Netherlands, in 2004-2006. Fish (n = 457) released near Cologne with implanted transponders were tracked by remote telemetry at 12 fixed detection locations distributed along the different possible migration routes to the North Sea. Relatively more A. anguilla migrated via the Waal than the Nederrijn, as would be expected from the ratio of river discharges at the bifurcation point at Pannerden. Downstream migration from the release site to Rhine-Xanten, close to the German-Dutch border, generally occurred in the autumn of the year of release but migration speeds tended to be low and variable and unaffected by maturation status or river discharge rates. Detection frequencies were not significantly related to discharge peaks or lunar cycles, but there was a minor detection peak 1-6 h after sunset. Between 2004 and 2009, 43% of the 457 A. anguilla released were never detected and of the 260 detected entering the Netherlands, 83 (32%) were detected escaping to the sea, 78 (94%) via the Nieuwe Waterweg and three (4%) and two (2%) via the sluices in the Haringvlietdam and Afsluitdijk, respectively. Possible causes of non-detections are discussed and it is suggested that many A. anguilla temporarily ceased migration, but that fishing mortality could have been important during passage through the Dutch parts of the Rhine. Practical implications of the results for predicting emigration routes, timings and magnitudes and use in management initiatives to promote escapement of A. anguilla silver eels to the sea are critically discussed.

Limited behavioural thermoregulation by adult upriver-migrating sockeye salmon (Oncorhynchus nerka) in the Lower Fraser River, British Columbia

The objective of this study was to combine radio telemetry with individual thermal loggers to assess the extent to which adult migrating sockeye salmon ( Oncorhynchus nerka (Walbaum in Artedi, 1792)) behaviourally thermoregulate during their migration through the Fraser River mainstem, British Columbia. The Fraser mainstem represents a region of the migration route that contains some of the highest mean temperatures encountered by sockeye salmon during their life history. We found that throughout the study area, individual sockeye salmon body temperatures occasionally deviated from ambient temperatures (ΔT), yet individuals maintained a ΔT of –1 °C or cooler for only 5% of their migration through the study region. There were moderate mean deviations of ΔT in two segments that are known to contain thermally stratified waters. In one of the study segments with the greatest ΔT, mean body temperatures decreased as river temperatures increased and ΔT became increasingly positive with higher river discharge rates, but these relationships were not observed in any of the other study segments. No relationship existed between ΔT and migration rate. While periodic associations with cool water were evident, mean body temperatures were not significantly different than mean river temperatures throughout the lower Fraser mainstem. This finding raises further conservation concerns for vulnerable Fraser River sockeye stocks that are predicted to encounter increasing peak summer river temperatures in the coming decades.

Effects of the Changjiang (Yangtze) River discharge on planktonic community respiration in the East China Sea

Planktonic communities tend to flourish on the western margins of the East China Sea (ECS) fueled by substrates delivered largely from the Changjiang River, the fifth largest river in the world. To study the effects of the Changjiang River discharge on planktonic community respiration (CR), physical‐chemical variables and key processes were measured in three consecutive summers in the ECS. Results showed that concentrations of nitrate and Chl a, protozoan biomass, bacterial production, as well as CR in the surface water were all negatively correlated with sea surface salinity, reflecting the strong influence of river discharge on the ECS shelf ecosystem. Moreover, mean values of nitrate, Chl a concentrations, and CR rates were proportionally related to the area of Changjiang diluted water (CDW; salinity ≤31.0 practical salinity units (psu)), an index of river discharge rate. Presumably, higher river flow delivers higher nutrient concentrations which stimulate phytoplankton growth, which in turn fuels CR. CR exhibited significant monthly and interannual variability, and rates appear to be dominated by bacteria and phytoplankton. Although the plankton community was relatively productive (mean = 0.8 mg C m−2 d−1) in the CDW, the mean ratio of production to respiration was low (0.42). This suggests that the heterotrophic processes regulating CR were supported by riverine organic carbon input in addition to in situ autotrophic production.

Modern and late Quaternary clay mineral distribution in the area of the SE Mediterranean Sea

The present-day clay mineral distribution in the southeastern Levantine Sea and its borderlands reveals a complex pattern of different sources and distribution paths. Smectite dominates the suspended load of the Nile River and of rivers in the Near East. Illite sources are dust-bearing winds from the Sahara and southwestern Europe. Kaolinite is prevalent in rivers of the Sinai, in Egyptian wadis, and in Saharan dust. A high-resolution sediment core from the southeastern Levantine Sea spanning the last 27 ka shows that all these sources contributed during the late Quaternary and that the Nile River played a very important role in the supply of clay. Nile influence was reduced during the glacial period but was higher during the African Humid Period. In contrast to the sharp beginning and end of the African Humid Period recorded in West African records (15 and 5.5 ka), our data show a more transitional pattern and slightly lower Nile River discharge rates not starting until 4 ka. The similarity of the smectite concentrations with fluctuations in sea-surface temperatures of the tropical western Indian Ocean indicates a close relationship between the Indian Ocean climate system and the discharge of the Nile River.

Modelling the possible effects of climate change on an Australian multi-fleet prawn fishery

The relationship between fisheries and climate has been given renewed emphasis owing to increasing concern regarding anthropogenically induced climate change. This relationship is particularly important for estuarine fisheries, where there are documented correlations between river discharge and productivity. The commercial catch of school prawns (Metapenaeus macleayi) has been shown to be positively correlated with the rates of river discharge in northern New South Wales, Australia. In the present study, a simulation model was developed to analyse the dynamics of the stock for 10 years under alternative river discharge scenarios, and the effectiveness of a series of management strategies under these scenarios was examined. A size-based metapopulation model was developed that incorporated the dynamics of school prawn populations in three habitats being harvested by three different fishing methods. The model indicated that both the growth and movement of prawns were affected by the rates of river discharge, and that higher rates of river discharge usually generated increased commercial catches, but this outcome was not certain. It was concluded that the population does not appear to be overexploited and that none of the three alternative management strategies performed better within the model than the current spatio-temporal closures, even under a wide range of river discharge scenarios.

Evidence for the inherent unsteadiness of a river plume: Satellite observations of the Niagara River discharge

We present satellite data from the Niagara River plume showing that a large eddy, or bulge, forms and grows in the plume near the mouth of the river. The plume consists of a semicircular bulge region immediately offshore of the mouth and a narrow current that propagates east along the shoreline. During the low‐wind period from 27 to 29 May 1999, the width of the bulge more than doubled and the current width increased only slightly. Approximately one‐third of the Niagara River water accumulated in the bulge near the mouth during this period, implying that the transport rate in the shore‐parallel current was reduced to two‐thirds of the river discharge rate. The effective radius and the growth rate of the bulge computed from the satellite images was in good agreement with previous laboratory results and a numerical model of the Niagara River plume. This is the first field evidence of the unsteadiness of the bulge.

Annual variability in upstream migration of glass eels in a southern USA coastal watershed

We investigated the environmental factors that affected temporal variability of eel recruitment and upstream migration in a freshwater coastal river along the southeastern US. Glass eels Anguilla rostrata were collected through ichthyoplankton sampling in the lower Roanoke River, North Carolina. Monthly samples were taken from fixed stations from May 2001 through June 2003. There was no evidence of consistent seasonal migration patterns for glass eels in Roanoke River. From May through December in 2001, glass eels were captured only during August. In 2002, glass eels arrived in February and remained in ichthyoplankton samples through October, with the exception of samples from September. Peak catch occurred in March at 4.02 ± 1.2 and declined through June to 0.18 ± 0.07 (#/1,000 m3). By August, the mean density increased to 0.96 ± 0.82 and to 3.59 ± 2.77 by October. In 2003 from January through June, glass eels were captured only during February and March. Glass eels were routinely collected when river discharge rates were 650 m−3 s−1 resulted in no glass eels in our samples. Upstream migration during 2002 was not correlated with water temperature or related to lunar phase. Glass eel freshwater upstream migration was initiated when water temperatures exceeded a threshold range of 10°C to 15°C; however, glass eels continued to migrate when water temperatures approached 30°C. The overall negative effect of river discharge suggests that changes in the water release schedules of upstream hydroelectric facilities during glass eel migration could strongly influence their recruitment success.

Annual variation of throughflow in the Seto Inland Sea

The latitude of the Kuroshio stream axis is closely related to the intrusion of the Kuroshio water mass and the throughflow offshore from two open boundaries. The water densities at entrances depend on the intrusion difference of the Kuroshio water mass. A new data processing method, which accounts for various data of water quality measured at different observation points under different river discharge rates, is proposed to let the model be applicable to long-term and wide-area simulations. The throughflow depends on the boundary sea level difference and the density gradient between boundaries caused by the intrusion of the Kuroshio water.

MODIS chlorophyll variability in the northern Adriatic Sea and relationship with forcing parameters

The variability of the MODIS chlorophyll concentration in the northern Adriatic Sea is described between July 2002 and January 2005 and related to the Po River discharge rates and local wind forcing. After interpolation, the MODIS data were decomposed into EOF modes. Significant variability (∼61%) is explained by the first three modes in which mode 1 includes a strong seasonal cycle with enhanced signal in winter for the open sea waters and a second maximum in April–May, and mode 2 separates the coastal and river‐influenced waters, from the open sea area. The temporal amplitude of the modes is significantly correlated with the Po River runoff. The interpolated MODIS data were also correlated with the Po River discharge rates, resulting in significant positive correlation north and south of the delta and off the Croatian Islands. When correlated with wind speed, the MODIS chlorophyll shows positive correlation in the area off Istria, off Croatia and at the edge of the coastal layer north of Ancona. Using wind regimes, similar patterns are found for the Bora‐wind like winds. In contrast, for Sirocco‐like winds, the correlation is significantly positive in the vicinity and north of the Po River delta. The regions off Ancona and Istria have poor correlation with the Po and wind because the MODIS chlorophyll variability is associated with mesoscale instability features not directly related to these forcing parameters. Poor (and even slightly negative) correlations are found off the Po delta. In most areas, correlations reach maximum values after 1–2 days.

Transport of organic carbon from the Mogot Experimental Watershed in the southern mountainous taiga of eastern Siberia

More than 60% of river runoff from the Lena River basin originates in the southern mountainous region of eastern Siberia within the permafrost zone. We studied the transport of dissolved organic carbon (DOC) and particulate organic carbon (POC) within the Mogot Experimental Watershed, which is close to the drainage divide between the Lena and Amur River basins in the southern mountainous taiga region, from 1 August 2000 to 12 November 2001. DOC concentration was strongly related to thawing depth at the bottom of the main valley when thawing depth was less than 20 cm during snowmelt runoff. When thawing depth was equal to or greater than 20 cm, DOC concentration was more closely related to the rate of river discharge in summer runoff. On the basis of our observations, we extrapolated the annual transport of DOC and POC to be 4.75 g C m−2 yr−1 and 0.03 C kg C m−2 yr−1, respectively. Transport of organic carbon from the catchment was about 4.78 g C m−2 yr−1 during 2001. DOC is the main form of organic carbon flux in the study area.

Nitrogen and silicon limitation of phytoplankton communities across an urban estuary: The East River-Long Island Sound system

To understand how nutrient loading may impact phytoplankton community growth, structure, and photosynthetic efficiency in Long Island Sound (LIS), nutrient enrichment (N, P, or Si) experiments were conducted at stations along the longitudinal gradient of the East River-Long Island Sound system during high and low river flow conditions (spring and summer). During summer, East River (ER) phytoplankton showed no response to N, P, or Si additions. In contrast, N enrichment significantly increased growth rates of all algal size classes (pico- (<3 μm), nano- (3–20 μm) and microphytoplankton (>20 μm)) and most major groups (dinoflagellates, diatoms) beyond control treatments in all regions of LIS (western, WLIS, central, CLIS and eastern, ELIS). Nitrogen enrichment also increased particulate organic carbon production and the photosynthetic efficiency of the phytoplankton community. During spring, ER phytoplankton remained nutrient replete, while larger phytoplankton (nano- and microphytoplankton; diatoms, dinoflagellates) within WLIS and CLIS algal communities were stimulated by N and/or Si additions (1.5- to 7-fold increased growth rates). An absence of response in ELIS phytoplankton following nutrient enrichment during spring together with elevated dissolved inorganic nutrients concentrations and river discharge rates suggests that seasonal variation in river flow can impact the degree to which ELIS phytoplankton are nutrient-limited. A 14-year record of nutrient concentrations and ratios in LIS suggested that our experimental results were representative of long-term seasonal trends, with N-limitation most likely to occur during the spring, summer, and fall in CLIS, during summer and fall in ELIS, and during late spring and summer in WLIS. The existence of low silicate levels relative to inorganic nitrogen in WLIS during spring months suggested that anthropogenic nutrient loading in this region may promote the seasonal Si-limitation of diatoms at this time.

Surface drifter derived circulation in the northern and middle Adriatic Sea: Response to wind regime and season

More than 120 satellite‐tracked drifters were deployed in the northern and middle Adriatic (NMA) Sea between September 2002 and November 2003, with the purpose of studying the surface circulation at mesoscale to seasonal scale in relation to wind forcing, river runoff, and bottom topography. Pseudo‐Eulerian and Lagrangian statistics were calculated from the low‐pass‐filtered drifter velocity data between September 2002 and December 2003. The structure of the mean circulation is determined with unprecedented high horizontal resolution by the new data. In particular, mean currents, velocity variance, and kinetic energy levels are shown to be maximal in the Western Adriatic Current (WAC). Separating data into seasons, we found that the mean kinetic energy is maximal in fall, with high values also in winter, while it is significantly weaker in summer. High‐resolution Local Area Model Italy winds were used to relate the drifter velocities to the wind fields. The surface currents appear to be significantly influenced by the winds. The mean flow during the northeasterly bora regime shows an intensification of the across‐basin recirculating currents. In addition, the WAC is strongly intensified both in intensity and in its offshore lateral extension. In the southeasterly sirocco regime, northward flow without recirculation dominates in the eastern half of the basin, while during northwesterly maestro the WAC is enhanced. Separating the data into low and high Po River discharge rates for low‐wind conditions shows that the WAC and the velocity fluctuations in front of the Po delta are stronger for high Po River runoff. Lagrangian covariance, diffusivity, and integral time and space scales are larger in the along‐basin direction and are maximal in the southern portion of the WAC.

Dense water characteristics in the northern Adriatic in the 1967–2000 interval with respect to surface fluxes and Po river discharge rates

Winter thermohaline properties of the northern Adriatic are analysed here with the aim of getting a better insight into dense water formation on the shelf. The hydrographic parameters collected in February in the 1967–2000 interval at two stations, the first located close to the eastern shore (station 1), and the second positioned near the Po river mouth (station 2), are compared. Two types of winter hydrographic conditions are distinguished: type A when bottom salinity and density are higher at station 1 than at station 2 and type B when these parameters are higher at station 2 than at station 1. Type A is more likely to occur in warmer and type B in colder winters. Both A and B distribution types can occur in periods when the Adriatic is under the influence of very saline waters of Mediterranean origin. Interannual changes in density are, at both stations, more dependant on haline than on thermal variations. At both stations temperature was somewhat higher in the early seventies than during the eighties and nineties, while salinity and density were lower in the early seventies and early nineties than in other years of the analysed period. By comparing the 1967–2000 changes in hydrographic conditions in February to monthly values of northern Adriatic surface fluxes and Po river discharge rates, it is shown that winter thermohaline characteristics in the region depend on processes which occur much earlier, i.e. during the previous autumn and late in spring of the preceding year, and even during the previous winter, 12 months before.

Po River plume on the Adriatic continental shelf: Dispersion and sedimentation of dissolved and suspended matter during different river discharge rates

Suspended matter and dissolved nutrients were studied in the prodelta of the Po River (northern Adriatic Sea) immediately after one of the greatest floods of the last century, which occurred in October 2000, and in four successive seasonal conditions. Hydrology, total suspended matter (TSM), particulate organic carbon (POC) and particulate nitrogen (PN) content, organic carbon stable isotopes ( δ 13C POC), particle grain size spectra, phytoplankton, and dissolved inorganic nutrients (i.e., nitrate, nitrite, ammonium, phosphate, and silicate) were measured in different discharge conditions to describe processes occurring in the Po River delta. The main effect of high river discharge was a large volume of low salinity water at the surface that extended over the entire study area. The concentration of most sampled variables decreased along a dilution gradient from the Po River distributaries’ mouths to the offshore. For TSM at the surface, riverine input and dilution were the main factors affecting its distribution. At the bottom, however, increased TSM in the nearshore was due to rapid sinking of suspended matter at the river mouth, while resuspension and advection dominated the transport of near bottom sediments offshore at 15–20 m depth. In the offshore where resuspension was active, an increase in ammonium and decrease in oxygen supported the presence of regeneration processes occurring at the bottom. During the October 2000 flood event, high nutrient concentrations promoted new production, triggering an algal bloom in the southern region of the study area. The origin of particulate organic carbon in the river delta was then estimated from a mixing model based on δ 13C POC values and carbon : nitrogen (C / N) molar ratios, and as the contribution of three distinct sources of organic carbon were determined (i.e., marine, riverine, and terrestrial). The riverine end-member accounted for ∼50% of the surface POC and generally decreased towards the offshore. The terrestrial carbon contribution was generally the least abundant (20–28% at the surface and at depth, respectively) and exhibited peak values (30–40%) during the October 2000 flood. Last, the marine contribution was the most abundant at the bottom (∼40%) and generally increased offshore. During the October 2000 flood, the fluvial contribution of particulate organic carbon, that is the terrestrial and riverine fractions, represented approximately 80% of the total particulate organic carbon in the Po River delta.

CDOM and its contribution to the underwater light climate of a shallow, microtidal estuary in south-western Australia

Light attenuation ( K d) of photosynthetically active radiation (PAR) by chromophoric dissolved organic matter (CDOM), total suspended solids (TSS) and chlorophyll a (Chl a) were measured at nine stations along an estuarine gradient in the Swan River, Western Australia, over 15 months. There were strong spatial gradients associated with the marine-freshwater transition along the 32 km of estuary sampled, as well as seasonal gradients mainly associated with rainfall, ∼80% of which occurs between May and September. CDOM absorbances at 440 nm reached a maximum of 10.9 m −1 with the freshwater inflow but concentrations of suspended matter remained low throughout the sampling period (1.0–21.0 mg l −1) under the diurnal tides of the estuary. CDOM was the dominant constituent of K d and a stepwise multiple regression showed that 66% ( p < 0.0001) of the variation in K d can be explained by CDOM and an additional 8% ( p < 0.0001) by TSS. As a consequence of this result, analysis into the influence of river discharge rates on CDOM absorbance levels was examined for 2002 using data collected during this study, and for 2000 and 2001 using historical dissolved organic carbon (DOC) and river discharge data. The outcome of this analysis infers that greater river discharge rates result in increased CDOM absorbances in the Swan River estuary.

Importance of seasonal variability and coastal processes on estuarine manganese and barium cycling in a Pacific Northwest estuary

Dissolved and particulate manganese (Mn) and barium (Ba) within the Tillamook Bay estuary are examined over seasonal cycles under a range of river discharge conditions from January to December 1999. Based on estuarine distributions and box model calculations, Mn and Ba behavior within the estuary is best quantified by considering two dominant factors: interaction with suspended particulate material (adsorption/desorption reactions) and input from benthic sources. Seasonal differences in elemental concentration patterns are dominated by variations in the relative importance of these two variables. While Mn exhibits estuarine behavior typical for this element, Ba shows an uncharacteristic lower estuary source. We hypothesize that this source results from the benthic dissolution of ocean-derived particulate Ba. The fluxes of dissolved Mn and Ba from the river to the coastal ocean are related to river discharge rates with the high fluxes observed under winter conditions. These winter flux maxima occur despite estuarine removal processes that also occur under winter conditions. During all other seasons, box model calculations indicate that the total (dissolved plus particulate) river fluxes of Mn and Ba to the coastal ocean are enhanced by internal estuarine processes—i.e., by transport across the sediment–water interface. On an annual basis, dissolved Mn and Ba fluxes to the coastal ocean are enriched by 390% and 170%, respectively, over what is expected from riverine inputs alone. The results of this research underscore the importance of estuarine processes in modifying the riverine flux of these elements to the ocean and the necessity of temporal sampling in evaluating processes controlling elemental distributions and delivery to the coastal ocean. Measurements of both the dissolved and particulate phases are also essential for describing and modeling these elemental budgets.

Chironomids (Diptera) in river floodplains: their status and potential use for palaeoenvironmental reconstruction purposes

The chironomid subfossils from a single sediment core in the floodplain of the Aa river (Saint-Omer basin, Northern France) were analysed. This study was a pioneering attempt to use sedimentary chironomid assemblages and lithostratigraphic analysis to reconstruct the palaeoenvironment and the past hydrodynamic conditions in a river system during the mid-Holocene period. In this paper, a chironomid subfossil typology is proposed, based on the current scientific knowledge about the present distribution of chironomid species through the cross-section of a floodplain (from lotic to more lentic habitats). A hydrological status is attributed to 27 subfossil chironomid taxa (limnophilous taxa and rheophilous taxa) from the Saint-Omer sediment sample. Based on these chironomid hydrological statuses and in regard with the lithostrati-graphic data, the dynamics of chironomid subfossil assemblages showed the occurrence of some fluctuations between periods with lower (lentic periods SMch-1, SMch-3 and SMch-5) and higher (lotic periods SMch-2 and SMch-4) Aa river discharge rates during the Late-Atlantic and Subboreal periods. The lotic periods were correlated with several climatic events known to have occurred during these periods in Europe. Several hypotheses are put forward and discussed, as to the potential relationships between the levels of precipitation and the hydrological conditions in the Saint-Omer basin. Although chironomid analysis in river systems is still in its developmental phase, the conclusions drawn from the present findings on subfossil chironomids show that these can provide a useful tool for palaeoenvironmental reconstruction purposes.