River Discharge Rates Research Articles

Reconstructing a 180 yr record of natural and anthropogenic induced low-oxygen conditions from Louisiana continental shelf sediments

Hypoxia on the Louisiana continental shelf is tied to nutrient loading and freshwater stratification from the Mississippi River. Variations in the relative abundance of low-oxygen-tolerant benthic foraminifers in four sediment cores from the Louisiana shelf provide a proxy record of low-oxygen events. Core chronologies are obtained using 2 1 0 Pb dating techniques. The foraminiferal data are consistent with previous studies indicating that the intensity of hypoxic events (oxygen <2 mg/L) has increased over the past 50 yr owing to the higher nutrient loading associated with the use of commercial fertilizer, and also reveal several low-oxygen events between A.D. 1817 and 1910, prior to the widespread use of fertilizer. The pre-1910 low-oxygen events are associated with high Mississippi River discharge rates, indicating that these low-oxygen episodes are related to natural variations in river drainage that enhance transport of nutrients and freshwater to the continental shelf. Our data show that the low-oxygen events of the past few decades were more extreme than any that occurred in the previous ∼180 yr, and support the interpretation that the increased use of fertilizer has amplified an otherwise naturally occurring process.

Fluvial mixing during floods

Mixing in fluvial systems, as characterised by longitudinal dispersion, may be thought intuitively to scale with the river discharge or flow rate. Here, this tenet is demonstrated theoretically to be well‐founded during normal flows but to be perfidious during flood conditions. The results suggest that during floods longitudinal mixing rates in rivers can be several orders of magnitude larger than during normal flows and can actually decrease with further increases in flow magnitude. The very large longitudinal dispersion coefficients found during extreme flows cannot be predicted using existing empirical equations; neither can the rapid decrease with increasing flow rate. This is not surprising because these equations do not cater for the extremely strong transverse velocity shear that exists in these flows that contain zones of quite different velocity and depth. The conclusions are important because it appears that the great majority of observations of fluvial mixing have been made during normal flows when a scaling with flow rate occurs. During floods, however, this behaviour switches and longitudinal dispersion scales inversely and non‐linearly with flow rate.

Biogeochemical behavior of organic carbon in the Trinity River downstream of a large reservoir lake in Texas, USA

Dissolved and particulate organic carbon concentrations were measured and annual loads estimated for the Trinity River, the main freshwater input source to Galveston Bay, which lies on the upper Gulf coast of Texas, USA, during 2000–2001. This river drains the forested lowlands south of a relatively large reservoir lake, Lake Livingston. A weak relationship between dissolved organic carbon (DOC) and Q TR indicated hydrologic control but separation of the data, based on individual discharge events, was necessary to improve interpretation. For instance, the first rain of the season resulted in only a modest increase in DOC concentrations and led to an inverse relationship with discharge, due to decreased lateral flow and increased infiltration of rainwater, with the lower flows being more efficient at DOC leaching from soils. In contrast, a long duration high discharge river crest event resulted in an opposite trend, i.e. a linear increase in DOC with increasing discharge rates. A short duration high discharge tropical storm showed reduced Trinity River DOC concentrations and the highest POC concentrations measured, likely resulting from the relatively short duration, and minimal contact time, of this event. In contrast to DOC, the concentrations of particulate organic carbon, POC (mg C l −1) were linearly correlated to suspended particulate matter (SPM) concentrations and accounted for between 10 and 12% of the total suspended load at low discharge but decreased to ∼2% at high discharge. This suggests dilution by larger particles with a reduced organic carbon content, possibly silicate minerals, more readily resuspended at elevated levels of discharge. The annual total organic carbon (TOC) load to Galveston Bay, estimated from the slope of the daily load vs. discharge relationship, was 11.2×10 10 g C and calculated export coefficients (g C m −2 year −1) were in good agreement with previous results. Using this relationship, accurate assessments of TOC flux inputs to Galveston Bay over the past quarter-century and in the future are possible by obtaining annual Trinity River discharge rates, which are readily available from the USGS. Comparing DOC riverine inputs to benthic sources in Trinity Bay, measured directly on the same day, indicates that the sediments contribute approximately 20% of total inputs of DOC to Trinity Bay. However, assuming a constant benthic source during low-flow conditions, which can occur for periods of up to 14 months in this region of Texas, benthic fluxes would account for >80% of the total inputs into Trinity Bay. At high levels of discharge, the Trinity River discharges ∼1.0×10 9 g C day −1 and dominates DOC inputs to Trinity Bay.

Long-term changes in hydrographic conditions in northern Adriatic and its relationship to hydrological and atmospheric processes

Abstract. A very long data series (1921–2000) of hydrographic parameters (temperature, salinity and density) collected in surface and bottom layers at a station in the coastal zone in the northeastern Adriatic was analyzed. Seasonal and interannual variability of the three parameters is described on the basis of monthly and yearly averages. The seasonal cycles obtained were found to be typical for the northern Adriatic. Interannual analysis showed that there was no trend in temperature, salinity and density in the region during the interval analyzed. The long-term series of yearly means of hydrographic parameters were related to the long-term series of yearly means of several hydrological/atmospheric parameters using different time lags of between 0 and 4 years. These parameters are surface heat flux gained by insolation Qs computed for Trieste, precipitation P in Trieste, Po River discharge rate R and two atmospheric oscillation indexes, a Mediterranean Oscillation (MO) index and North Atlantic Oscillation (NAO) index. At zero time lag the interannual changes at the surface were found to be significantly related to changes in the Qs and NAO index (temperature) and R (salinity and density). At the same time lag the changes in the bottom were found to be related to the MO index (temperature, salinity and density). Additionally, bottom salinity and density were significantly related to R at a time lag of 1 year. Key words. Oceanography: Physical (air-sea interactions; hydrography)

Unusual upwelling event and current reversal off the Italian Adriatic coast in summer 2003

An exceptional upwelling event occurred near the Conero Promontory along the Italian Adriatic coast between 11 and 23 July 2003, as a result of dominant southeasterly sirocco winds. Contemporarily, surface currents off central and southern Italy reversed to the northwest direction. This peculiar dynamic situation happened when the Adriatic river discharge rates were extremely low due to a prolonged dry season. Satellite images of sea surface temperature and chlorophyll concentration and satellite‐tracked drifters revealed that the upwelling event first appeared northwest of the promontory and then progressed to the southeast. Its offshore extension (20–30 km) includes convoluted filament structures. Surface temperature and ocean color signatures are correlated, indicating that the cold‐upwelled water is rich in nutrients. Coldest temperatures are as low as 18–19°C (8°C lower than the open sea). The northwestward currents reached maximum speeds larger than 40 cm/s inshore of the 20 m‐isobath (&lt;10 km from shore).

Variability of environmental factors of an eastern Mediterranean Sea river influenced coastal system

Spercheios River discharge rates of nutrients, suspended particulate matter (SPM) and particulate organic carbon (POC) and seawater concentrations of these parameters as well as chlorophyll a(chl a) were measured in the Maliakos Gulf, Greece, on a monthly basis between 1992-1993. From all the nutrients measured, silicate showed the highest discharge rates followed by nitrate, phosphate and nitrite. The river introduced significant amounts of SPM (8.5 – 35.5 Kg d -1 ) with very low POC content (&lt;3%). Most of the variables measured showed no gradient from the river to the outer gulf, which could be attributed to fast mixing of the incoming water. However, chl a had higher concentrations in the inner (0.3 – 4.9 Μg l -1 ) and lower in the outer gulf (0.05 – 2.5 Μg l -1 ). It is suggested that the nutrients introduced by the river are consumed faster in the inner gulf and that a number of temporal streams and non-point sources at the periphery of the gulf prevent the formation of a gradient.

Classification of Australian Clastic Coastal Depositional Environments Based Upon a Quantitative Analysis of Wave, Tidal, and River Power

A statistical assessment of wave, tide, and river power was carried out using a database of 721 Australian clastic coastal depositional environments to test whether their geomorphology could be predicted from numerical values. The geomorphic classification of each environment (wave- and tide-dominated deltas, wave- and tide-dominated estuaries, lagoons, strand plains, and tidal flats) was established independently from remotely sensed imagery. To our knowledge, such a systematic numerical analysis has not been previously attempted for any region on earth. The results of our analysis indicate that a relationship exists between the ratio of annual mean wave power to mean tidal power and the geomorphic development of clastic coastal depositional environments. Deltas and estuaries are associated with statistically significant differences in mean wave and tidal power. Statistically significant distinctions between populations of deltas, estuaries, strand plains, and tidal flats are also associated with river discharge and river flow rate (defined as discharge divided by open water area). Our results support the hypotheses of previous workers that wave, tide, and river power exert the principal control over the gross geomorphology and facies distribution patterns in clastic coastal depositional environments. Mean values and confidence limits of wave power, tide power, and river flow for the coastal depositional environments targeted in this study may provide a basis for the comparison of modern environments as well as constraints for paleo-reconstructions.

Seasonal changes of water mass structure and shelf slope exchanges at the Ebro Shelf (NW Mediterranean)

Among several field efforts undertaken in the framework of the EU MAST-III FANS project, three oceanographic cruises covering the Ebro shelf and slope regions (NW Mediterranean) were carried out on board R/V Garcı́a del Cid between November 1996 and July 1997. A major aim of these three cruises was to provide insight into the seasonal variability of the distribution of physico-chemical oceanographic parameters and the shelf/slope circulation. In this paper we discuss the observed changes in the hydrographic structure, the spatial distribution of nutrients, chlorophyll and suspended particulate matter and the local circulation in relationship to the seasonal variability of the Ebro river discharge rates and the water column stratification. The added effects of mesoscale circulation structures and of the exchanges across the shelf edge are also addressed.

Istrian Coastal Countercurrent and its Year-to-Year Variability

In this paper we demonstrate the existence of a descending surface current which usually appears in August along the Istrian coast line and runs counter to the general Adriatic-wide cyclonic flow. This is achieved by computing the relative (with respect to the 30-dbar surface) geostrophic currents as well as absolute currents and net transports from hydrographic data collected monthly to seasonally at six stations of the Po-Rovinj profile in the northern Adriatic. We name it the Istrian Coastal Countercurrent (ICCC). Its mean relative speed is high, 7cms−1(relative currents at the profile on average stay below 5cms−1in winter and below 8cms−1in summer), inducing a net outflow near the Istrian coast.The ICCC was strong in 1968, 1977, 1983, 1984, 1988, 1989, 1991, 1992, 1996 and 1997, weak or absent in 1971–1973, 1982, 1986, 1993–1995. Whenever the ICCC was well pronounced, specific dynamic and hydrographic features—pointing to a possible existence of an anticyclonic gyre off Rovinj—were observed. The existence of the ICCC is confirmed by direct current measurements, which show that the descending coastal current was intense in 1977 and 1983, and that it did not develop in 1976, 1978 and 1987.The intensity of the ICCC in August over the 1966–1992 interval was significantly correlated to the air–sea heat flux observed with the −7 months time lag in combination with the Po River discharge rate with the −1 month time lag and to the sea surface temperature recorded in the area with the −5 months time lag. On the correlation basis a weak or non-existent ICCC is hindcasted for 1976, 1978 and 1987. It seems that high intensity of the ICCC coincides with oceanographic conditions which favour near anoxia or mucilage events, observed in northern Adriatic in 1977, 1988, 1989, 1991 and 1997.

Satellite-Based Studies of the 1997 Oder Flood Event in the Southern Baltic Sea

In summer 1997 the largest Oder River flood of this century occurred from drainage area in Poland, the Czech Republic, and Germany. The spread of floodwater, which reached the Pomeranian Bight of the Baltic Sea end of July, was monitored and investigated by satellite data, shipborne measurements, and coupled biophysical modeling to document impacted coastal waters and to assess changes in water quality and influences on the ecosystem. This paper presents the spatial and temporal floodwater distribution patterns as determined from satellite data with different spectral ranges and spatial resolutions and the relationships of these flooding patterns to meteorological conditions and river discharge rates. The flooding patterns were observed using approximately 80 sea surface temperature (SST) maps, derived from infrared data of the Advanced Very High Resolution Radiometer (AVHRR) operating on weather satellites of the National Oceanic and Atmospheric Administration (NOAA). The flood period was characterized by dominant easterly winds transporting the floodwater along the German coast into the Arkona Sea. In situ measurements and data of the visible spectral range from NOAA weather satellites and the Indian satellites IRS-P3 and IRS-1C with ocean color sensors were used to study the distribution of water constituents in the Szczecin Lagoon and Pomeranian Bight and to verify results from the SSTs. The maximum horizontal extent of discharged river water covered an area larger than 3,000 km 2 in the Pomeranian Bight and the southern Arkona Sea by 20 August. The eastern part of the Szczecin Lagoon was directly influenced by the floodwaters, leading to lower concentrations of water constituents and verifying the diluting effect. The in situ measurements in combination with the satellite observations provided an added assessment of potential impacts from floodwaters on the affected coastal ecosystem.

Freshwater flushing time scales of the Vashishti estuary, west coast of India

Results are presented for the Vashishti estuary to evaluate its freshwater flushing time scales based on 8 sets of observations of longitudinal salinity distributions. The results of the flushing time using the fraction of freshwater method indicated that the load retained in the estuary after infinite number of tidal cycles was 44 to 54 times the load introduced per tidal cycles during premonsoon and 47 to 61 times during postmonsoon periods. Calculations on longitudinal diffusion coefficient (K) showed values ranging from 12 to 271 m2 sec‐1 with smaller values occurring in the mouth region. Results of the effective diffusivity indicated that changes in K and mixing length together can cause a decrease in flushing time which is in general agreement with the observed dependence of the flushing time from the river discharge rate.

Changes in chemical and [formula omitted]/ [formula omitted] signature distribution patterns of suspended matter and bed sediments in the upper Loire river basin (France)

The mineralogy of the suspended particulate matter (SPM) transported by the upper Loire river consists mainly of quartz and K-feldspar during periods of high river flow, with an increase in calcite concentration during periods of low flow. Concurrently, large fluctuations are observed in the levels of CaO and Fe 2O 3, the main oxides present in the SPM along with SiO 2. The analysed trace elements also fluctuate significantly. The bed sediments (BS) have a similar oxide content to the SPM, but lower Zn and Pb levels and higher Zr levels. Fluctuations in the chemical-element concentrations with river discharge are related to fluctuation in the mineral assemblages present in the SPM. Thus K, Ti and Rb levels increase with increasing in K-feldspar abundance during high flow, and Ca and Sr levels decrease with increasing discharge due to a decrease in calcite abundance. These mineralogical and chemical variations relate to different sources of sediments under different flow conditions—sources that can be distinguished by Sr isotopic study. The Sr isotopic composition fluctuates according to the rate of river discharge; i.e., the 87 Sr / 86 Sr ratio increases with increasing discharge and reaches a maximum with peak flow, and vice versa. The 87 Sr / 86 Sr ratio also increases with the combined increase in K-feldspar abundance and decrease in calcite abundance. These similarities suggest the existence of at least two particulate-matter reservoirs, one with detrital silicates and the other with carbonates. Finally, the relationship between the 87 Sr / 86 Sr ratios in the suspended and dissolved loads suggests the coexistence of authigenic calcite in the carbonate reservoir. The SPM flow is related to a specific mechanical erosion rate over the whole Loire watershed ranging from 9 to 23 t km −2 yr −1. During the survey at Orleans, the SPM flow of the Loire river was 37·10 4 t yr −1, providing an estimate of the specific erosion rate of around 9.8 t km −2 yr −1. From the shift in the SPM flux between the sampling point and the mouth of the Loire river and the divergence in the mean annual discharge, a calculation was made of the input of solid matter by the tributaries and by supplementary erosion processes along the river.

An approach and preliminary model of integrating ecological and economic constraints of environmental quality in the Guayas River estuary, Ecuador

Abstract We describe conceptual and simulation models of land use within the intertidal zone of the Guayas River estuary to quantify the contribution of mangrove wetlands to maintaining environmental quality of a tropical estuary. The goal of this exercise is to demonstrate the important consideration of ecological constraints in determining economic and management decisions; and how modeling can be used to quantify impacts of land use such as loss of mangrove wetlands on environmental quality. Our conceptual model treats solar energy, river flow, and tides as forcing functions that control the properties of estuarine ecosystems, but also describes market forces and cultural policies as constraints on properties of socioeconomic systems. The controversy of coastal resource management in Ecuador centers around the relative impacts of shrimp pond construction and management as negative feedbacks to the environmental quality of the Guayas River estuary. Unique oceanographic processes and land use changes contribute to complex issues of water and habitat quality in this tropical estuary, the largest estuarine complex on the Pacific coast of South America. A dynamic box model was developed for the estuary and calibrated with data collected from a 14 month survey of water quality parameters throughout the estuary. Scenarios included conversion of mangroves to shrimp ponds in three regions of the estuary, and the construction of a dam by varying three different rates of river discharge at 100, 50 and 10% of 1989 base flow. Good water quality is maintained by the low residence time of water in the estuary (11 d) because of seasonally high river flow and tidal exchange. With a 90% reduction of mangrove forests in the estuary caused by shrimp pond construction, total nitrogen concentrations increased 5 fold. However, as river discharge decreased to 10%, the same construction caused a 60 fold increase in nitrogen concentrations to 250 μ M. Increases in nitrogen concentrations were higher in the upper estuary region, with much less change in the lower estuary. Thus the sensitivity of environmental quality to changes in land use in the intertidal and upland zone are linked to the hydrography of the estuary and is site specific. In the future, the combinations of these ecological models together with economic analyses of the goods and services of mangroves may provide better techniques to evaluate the economic impacts of specific coastal zone management decisions.

Acid mine drainage composition and the implications for its impact on lotic systems

Intensive sampling of acid mine drainage from adits draining the abandoned copper and sulphur mines at Avoca, south-east Ireland, has confirmed a seasonal variation in the Zn:Cu ratio observed previously. The variation in the Zn:Cu ratio leads to extreme variations in the toxicity of the drainage, and linked with increased adit flows and surface runoff during wetter months, often results in higher river toxicity, even during high river discharge rates. This seasonal variation in toxicity has important ramifications for the control of AMD to surface waters. Sampling over a three-year period shows that the rate of discharge from the adits follows a seasonal cycle being high in the spring, declining through summer to reach lowest flows in autumn and rising again in the winter. During the intensive sampling study over a 13-month period, the discharge rate in the Deep Adit varied from 8.5 to 37.3 l/s compared to 6.3 to 35.2 l/s in the Ballymurtagh Adit. This represents a total discharge rate for the two adits of between 1265 and 6270 m 3/d. Significant weights of cations were discharged from the two adits, ranging from 169–1279 kg/d for Fe, 69–459 kg/d for Zn, 1.5–34.7 kg/d for Cu, and 0.167–0.875 kg/d for Cd, with the Deep Adit contributing on average 40.4, 70.4, 66.7 and 80.9% of each metal respectively over the sampling period of 13 months. The impact of AMD on the river biota is severe with a major decrease in the number of taxa and faunal abundance. The species deficit at all sites below the mines is in excess of 87%. Dipterans dominate impacted sites with Chironomidae abundant. Fish are eliminated except for juvenile eels close to the estuary.

Water exchange in the Pomeranian Bight investigated by satellite data and shipborne measurements

The Pomeranian Bight is characterized by the main freshwater input into the western Baltic Sea coming from the Oder and Peene rivers. The rate of river discharge is driven by varying sea level in the southern Baltic, and the distribution patterns of the river plume are dominated by the local wind. This study uses daily available sea surface temperature (SST) maps (4–6 times a day), derived from Advanced Very High Resolution Radiometer (AVHRR) data of the NOAA satellites, to investigate these patterns in relation to the meteorological conditions. Comparisons of SST maps with shipborne measurements for different wind situations confirm the usability of satellite data for investigation of the river discharge. On the basis of about 125 available NOAA scenes scattered over all seasons in the period of 1991–1994, typical horizontal distribution patterns were derived for wind directions of the eight octants. Chlorophyll concentration maps derived from Coastal Zone Colour Scanner (CZCS) data of the NIMBUS-7 satellite are, in special cases, more detailed than corresponding SST patterns (channel 6). They confirm the patterns and the transport of suspended material. Long-term wind records from the Arkona weather station indicate the dominant wind directions and the seasonal variations in wind forcing leading to seasonal preferred patterns and transport directions of the river plume into and out of the bight. Dominant westerly winds in summer, autumn and winter produce an eastward transport along the Polish coast into the Bornholm Sea and further into the Gdansk Bay over more than 300 km. In the period of the main freshwater inflow in spring, easterly winds are dominant and transport the river water along the German coast into the Arkona Sea. This is confirmed by high accumulation rates at the bottom of the Arkona Basin. The results of these investigations allow a prediction of distribution patterns and main transport directions in relation to the prevailing wind situation.

Seasonal Variations in Suspended Particulate Matter in the Marennes-Oléron Bay, France, using Lipids as Biomarkers

In the most important oyster-producing area in France, an investigation was conducted to determine the origin of particulate organic matter (POM) of the size, <100 μm, available to oysters. Suspended particulate matter (SPM) was sampled monthly over 1 year at one estuarine station in the Bay, Fort Boyard, and at one river station in the Charente at St. Savinien. Lipid-class and fatty-acid profiles were determined in the SPM. At Fort Boyard, variations in amounts of SPM and its ratio of both POC:Chlorophyll aand C:N were associated with changes in river discharge rates, local resuspension and phytoplankton proliferation. High concentrations of mineral and detrital particles were observed in winter, and low levels during the rest of the year, when phytoplankton blooms occur frequently. Lipid-class and fatty-acid profiles varied in relation to seasonal changes in both weather and hydrology. The terrestrial contribution to SPM was highest in winter, when river outflow and resuspension in the Bay were also maximal. High bacterial biomass occurred in association not only with river detritus, but also with early-spring blooms. Microalgal fatty acids were predominant from spring to autumn and, by using compounds shown to be of taxonomic significance, the presence of diatoms could be discerned from that of other microalgae. In winter, the estuarine and freshwater stations showed similar lipid compositions, with dominance by terrestrial and bacterial biomarkers. In summer, however, fatty-acid profiles were very different at the two sites, showing clear temporal variations, which suggest that the Bay and the River then function as separate systems and follow different biological patterns.

The behavior of particle-reactive tracers in a high turbidity environment: 234Th and 210Pb on the Amazon continental shelf

Excess 234Th and 210Pb seabed inventories were measured in cores collected from the Amazon continental shelf to examine particle scavenging and seabed dynamics. Typical excess 210Pb inventories range from 100 to 300 dpm cm −2, and the total excess 210Pb inventory for the Amazon shelf was determined to be 2.7 × 10 17 dpm. The 210Pb measurements indicate that particle-reactive species are scavenged not only from the Amazon River but also from the lateral advection of offshore water. In order to sustain the 210Pb inventories, the volume of water supplied by the lateral advection from offshore must be approximately five to ten times the water discharge of the Amazon River. This lateral advection supplies about 67% of the total excess 210Pb to the Amazon continental shelf with relatively small contributions from riverine input (31%), atmospheric fallout (2.3%), and in-situ production (0.1%). The 234Th inventories were measured on four cruises, which occurred during periods of differing river discharge, wind stress, and flow rates of the North Brazil Current. The 234Th excess seabed inventories show large spatial and seasonal variability, with a range from 0 to 22 dpm cm −2, Ratios of predicted 234Th inventories over observed 234Th inventories indicate that a large portion of the inventories on the inner and mid shelf occur in the fluid mud layer or the suspended sediments. The 234Th excess seabed inventories are controlled by resuspension, cycling the inventories between the water column and the seabed. 210Pb and 234Th excess seabed inventories are compared directly using corrections for the different time scales and offshore concentrations. This approach indicates that for most of the shelf, the inventories of the shorter-term tracer 234Th are less than predicted by the inventories of the longer-term tracer ( 210Pb). There are two explanations for this trend. The first is that a larger portion of the 234Th inventory occurs in the fluid muds or the water column relative to 210Pb. The second is that the supply of offshore water, scavenging efficiency, and/or deposition have been lower over the two year study period relative to the last one hundred years.

The relationship between river flow and entry to the Aberdeenshire Dee by returning adult Atlantic salmon

One hundred and nine returning adult salmon were radio‐tagged in the estuary of the Aberdeenshire Dee, Scotland between February and August 1985 to 1989 and the times when 62 fish entered the river were recorded. Elapsed times between tagging and river entry were significantly greater during periods of lower than average river flows in all months where there were sufficient data to allow statistical comparison. The degree of association between river entry and particular levels of river discharge rate varied seasonally. The proportion of days associated with river entry declined at the lower end of the range of flows available to tagged fish in the summer months. Absolute levels of river discharge played a significant role in modifying the response of salmon to changing flows. During periods of lower than average seasonal flow, river entry was closely associated with days when flow had increased since the previous day. During periods of higher than average flow, river entry was not significantly associated with such periods of increased flow. The results suggest that models which relate river entry by salmon to absolute discharge rates alone are unlikely to be generally reliable.

Postglacial Recession of Niagara Falls in Relation to the Great Lakes

The recessional history of Niagara Falls in the present Niagara Gorge during the postglacial period has been a focus of study throughout this century. Radiocarbon ages of clam shells suggest that Niagara Falls migrated very slowly around the narrowed gorge section at Niagara Glen from 10,500 to 5500 yr B.P., when upper Great Lakes water bypassed Lake Erie and flowed to the Ottawa River via the outlet at North Bay, Ontario. Prior to that interval, river discharge and recession rates were similar to those at present, and similar rates resumed after 5200 yr B.P. By about 4500 yr B.P., the present gorge had intersected a buried gorge of the pre-late Wisconsinan Niagara River (St. Davids Gorge). The sediment derived from the excavated buried valley fill may be present as a distinct marker horizon in the sediments in southwestern Lake Ontario.

Satellite Thermal Observation of the River Shannon Plume

Thermal infra-red images from National Oceanic and Atmospheric Administration (NOAA) satellites have been used to obtain synoptic patterns of surface temperature off the west coast of Ireland. As natural tracers of surface circulation, patterns of surface temperature reveal the movement of the River Shannon plume. The plume frequents the mid-western coastal waters north of the River Shannon estuary with a length of the order of 100 km and an area of about 100 km 2. The plume turns nearly 90° and travels towards the north after it leaves the river mouth. When approaching the Aran Islands, the plume usually turns to the northwest. Sometimes it divides into two parts, one going northeast towards Galway Bay and the other going northwest and meeting the open sea. The effect of the river discharge rate, of wind and of tide on the plume dynamics have been studied. The amount of the river discharge affects the area of the plume but does not change the direction of the plume movement. On the other hand, the winds are effective in dispersing and extending the plume. The tidal effect on the plume appears very weak.