Extreme Discharge Research Articles

The Genesis of Mass Carpological Deposits (Bedload Carpodeposits) in the Tertiary of the Lower Rhine Basin, Germany

Large, well-preserved carpofloras from Miocene and Pliocene lignite-bearing sequences of the Lower Rhine Basin in Germany occur in a variety of fluvial and lacustrine facies, including channel, point-bar, cut-off channel (oxbow-lake), crevasse-splay, channel-bar (sand-gravel flat), and lake-delta deposits. Despite their occurrence in a wide spectrum of depositional facies, the accumulations share a suite of distinctive characteristics. They appear as a jumble of woody material dominated by abraded pieces of wood, including conifer cones, woody fruits, seeds, small branches, and charcoal in an unconsolidated matrix of medium- or coarse-grained sand. The key characteristic of the fossil plant remains is woodiness. Close association between the lignified material and these grain sizes indicates that these woody fruits are hydrodynamically equivalent to medium- to coarse-sized sand, and were transported as bedload during flood events. Transport by bedload also is evidenced by the high degree of roundness in associated wood clasts, by abrasion in some diaspores, and by the lack of non-resistant plant parts (e.g., leaves, flowers, fleshy or delicate tissues) in the accumulations. Thus, the lignified plant remains behave as sedimentary clasts in the water column, but only after they have been fully saturated with water. Mass carpological accumulations in very coarse-grained sand on active point bars in the Sieg River near Bonn and their formation during events of extreme discharge serve as a modern analog for the fossil carpofloras. New terminology and concepts pertaining to these mass carpological deposits, or “bedload carpodeposits,” also include the terms “bedload carpobiofacies,” and “bedload carpolithofacies.” A basic depositional model for the genesis of bedload carpodeposits is presented.

Subnanosecond Thomson scattering on a vacuum arc discharge in tin vapor

In a previous series of Thomson scattering (TS) experiments on an extreme ultraviolet producing vacuum arc discharge in tin vapor, background radiation emitted by the plasma was found to make measurements impossible for all parts of the discharge except the prepinch phase. To reduce the level of recorded background radiation, we have built a setup for time and space resolved subnanosecond TS. Results obtained with this new setup are presented for experiments on previously inaccessible parts of the discharge--the ignition phase, pinch phase, and decay phase. For the first two, measurements have been performed at different heights in the plasma. Electron densities for the pinch phase have been derived. For the decay phase, the electron densities confirm previous Stark broadening data. From the overall results, a more complete picture of the plasma evolution can be formed.

The historical and palaeoflood record of Katherine river, northern Australia: evaluating the likelihood of extreme discharge events in the context of the 1998 flood

AbstractThe discharge regimes of the large rivers of northern Australia are characterized by the occurrence of extreme flood events with far‐reaching environmental and societal impacts. In January 1998 the largest flood ever recorded on the Katherine River, northern Australia, resulted in widespread inundation and resultant damage to the town of Katherine. The occurrence of the flood emphasized the unreliability of the then available flood probability estimates and prompted a palaeoflood approach to estimate the recurrence interval of the event. The location of Katherine is ideal for such a study, as the town is located immediately downstream from Katherine Gorge, which provides the necessary bedrock‐confined channel required for such an approach. In addition, previous work in Katherine Gorge had demonstrated that the gorge sections hold suitable deposits for palaeoflood stage reconstruction. The results of the present study show that at least two flow events with discharges similar to the 1998 flood have occurred within the last 600 years, and that high‐magnitude floods are a general feature of the discharge record of the Katherine River over the last c. 2000 years. Furthermore, because the study was undertaken within a few months of the occurrence of the 1998 flood, it provided the opportunity to evaluate the previously obtained flood discharge estimates and draw attention to the general uncertainties associated with palaeoflood studies. Our results emphasize that palaeoflood stage estimates based on slackwater deposits need to be treated as conservative estimates only. More specifically, with respect to the 1998 event, our study demonstrates that the controls of flood peak were more complex than simply flood routing through the gorge sections. It is clear that the areas downstream from Katherine Gorge made an important contribution to the flood peak of the 1998 event. Copyright © 2005 John Wiley & Sons, Ltd.

Seasonal Response of Juvenile Atlantic Salmon to Experimental Hydropeaking Power Generation in Newfoundland, Canada

AbstractVariable hydropower production leads to hydropeaking, which causes discharge fluctuations that are potentially harmful to aquatic organisms. In this study, an experimental approach was used to investigate hydropeaking effects and associated hydraulic and habitat conditions on the home range and movement of juvenile Atlantic salmon Salmo salar. Prior studies examined the responses of Atlantic salmon and brook trout Salvelinus fontinalis to experimental hydropeaking during summer and autumn. The present study focused on Atlantic salmon, involved more rapid and extreme discharge manipulation, and included winter experiments to reflect influences of reduced temperature, ice conditions, and seasonal differences in behavior and habitat selection. Experiments were conducted over a range in discharge (0.5–5.0 m3/s) that resulted in dramatic habitat changes in the wide, shallow, boulder‐strewn study reach. Experiments were repeated in summer and winter; however, the winter range in discharge was narrower due to constraints on water release. Fish response was monitored using manual telemetry in both seasons, and fixed telemetry was used to monitor fine‐scale diel winter movements. Atlantic salmon had larger home ranges and were more mobile during all flow conditions and over diel cycles in summer than in winter, and there was anecdotal evidence of stranding in isolated pools in summer. Stream morphology, in addition to the magnitude of discharge change, was an important determinant of the propensity to move. In our study, there were considerable refugia from increased velocity and dewatering, which may have reduced the need to move. In winter, fish remained relatively sedentary in comparison with the summer foraging period, and this behavior may increase the likelihood for dewatering, stranding, and freezing. A secondary concern with hydropeaking regimes is the energetic cost to fish of moving to find suitable habitats, and during summer this cost could affect stored energy reserves, which could, in turn, affect overwinter survival.

Tree-ring based streamfiow reconstruction for Ashley Creek, northeastern Utah: implications for palaeohydrology of the southern Uinta Mountains

Two ring-width chronologies from Douglas fir (Pseudotsuga menzeisii) and one from pinyon pine (Pinus edulis) were used to reconstruct mean annual discharge of Ashley Creek in the Uinta Mountains, Utah, for the years AD 1637-1970. A backward-elimination multiple linear-regression model identified six statistically significant variables, including annual lags ranging from 1 to +2 years. The final model explains 71% of variability (R2 adjusted for degrees of freedom lost) when regressed against the annual discharges recorded for 1915-72. Statistical analysis of the model reconstruction using the reduction-of-error statistic, the Durbin-Watson statistic and the hypergeometric distri bution all indicate fidelity of the model for reconstructing discharge. The Lilliefors' test indicates that years of extreme discharge (greater than the 90th percentile or less than the 10th percentile) are non-randomly distributed between 1637 and 1970. Persistent years of above-median discharge as well as non-random clustering of years above the 90th percentile occurred from 1692-1704 and 1898-1945. Analysis of the modern gauge data set shows that mean annual discharge on Ashley Creek explains between 81% and 91% of the variations of mean annual discharge on the other main stream in the southern Uinta Mountains, indicating that reconstructed mean annual discharge on Ashley Creek is representative of palaeohydrologic conditions across the south flank of the range. These data suggest that the southern Uinta Mountains experienced persistent below-median discharge for 1741-1897, through the peak of the ‘Little Ice Age’ as identified in numerous surrounding mountain ranges in the Central Rockies. This interpretation agrees with lichenometric evidence for extremely limited ‘Little Ice Age’ glacial activity in the Uintas, indicating that the range experienced unusually warm/dry conditions at this time.

Fluxes and sources of suspended organic matter in an estuarine turbidity maximum region during low discharge conditions

Water column concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured at three different depths in four different locations bracketing the estuarine turbidity maximum (ETM) along the main channel of a temperate riverine estuary (Winyah Bay, South Carolina, USA). Measurements were carried out over full tidal cycle (over 24 h). Salinity, temperature, current magnitude and direction were also monitored at the same time throughout the water column. Tidally averaged net fluxes of salt, TSS, POC and PN were calculated by combining the current measurements with the concentration data. Under the extreme low river discharge conditions that characterized the study period, net landward fluxes of salt were measured in the lower part of the study area, suggesting that the landward transport through the main channel of the estuary was probably balanced by export out through the sides. In contrast, the net fluxes of salt in the upper reaches of the study area were near zero, indicating a closed salt balance in this part of the estuary. In contrast to salt, the net fluxes of TSS, POC and PN in the deeper parts of the water column were consistently landward at all four sites in Winyah Bay indicating the non-conservative behavior of particulate components and their active transport up the estuary in the region around the ETM. The carbon contents (%POC), carbon:nitrogen ratios (org[C:N]a) and stable carbon isotopic compositions ( δ 13C POC) of the suspended particles varied significantly with depth, location and tidal stage. Tidally averaged compositions showed a significant increase up the estuary in the %POC and org[C:N]a values of suspended particles consistent with the preferential landward transport of carbon-rich particles with higher vascular plant debris content. The combination of tidal resuspension and flood-dominated flow appeared to be responsible for the hydrodynamic sorting of particles along the estuary that resulted in denser, organic-poor particles being transported landward less efficiently. The elemental and isotopic compositions indicated that vascular C 3 plants and estuarine algae were the major sources of the particulate organic matter of all the samples, without any significant contributions from salt marsh C 4 vegetation ( Spartina alterniflora) and/or marine phytoplankton.

A methodology for regional flood frequency estimation based on scaling properties

There is a persisting need for extreme value assessments of discharge for ungauged catchments for design purposes, flood protection and flood risk analysis. A methodology is put forward for estimating flood quantiles in ungauged catchments, taking into account the scaling behaviour of discharge in a homogeneous region. From an assumption that the spatial distribution of specific discharge generation is a shifted exponential distribution, theoretical expressions of the maximum specific discharge values relative to catchment size are developed and a regional scaling equation is presented. The homogeneity constraint refers to climatic conditions, whereas catchment heterogeneity due to physiographic features of catchments is incorporated in the models using multiple regression analysis. The methodology is tested for a region in Southern Norway with homogeneous climatic conditions but with varying physiographic catchment characteristics. It is investigated if the extreme discharge values estimated with traditional methods are adequately reproduced with the proposed methodology. For the tested region, the agreement between flood quantiles estimated with the proposed methodology and flood quantiles estimated from observed data is good. The effective lake percentage and the river gradient proved to be significant descriptors of the deviation from the scaling procedure. Copyright © 2004 John Wiley & Sons, Ltd.

Impact of climate change on river flooding assessed with different spatial model resolutions

The impact of climate change on flooding in the river Meuse is assessed on a daily basis using spatially and temporally changed climate patterns and a hydrological model with three different spatial resolutions. This is achieved by selecting a hydrological modelling framework and implementing appropriate model components, derived in an earlier study, into the selected framework (HBV). Additionally, two other spatial resolutions for the hydrological model are used to evaluate the sensitivity of the model results to spatial model resolution and to allow for a test of the model appropriateness procedure. Generations of a stochastic precipitation model under current and changed climate conditions have been used to assess the climate change impacts. The average and extreme discharge behaviour at the basin outlet is well reproduced by the three versions of the hydrological model in the calibration and validation, the results become somewhat better with increasing model resolution. The model results with synthetic precipitation under current climate conditions show a small overestimation of average discharge behaviour and a considerable underestimation of extreme discharge behaviour. The underestimation of extreme discharges is caused by the small-scale character of the observed precipitation input at the sub-basin scale. The general trend with climate change is a small decrease of the average discharge and a small increase of discharge variability and extreme discharges. The variability in extreme discharges for climate change conditions increases with respect to the simulations for current climate conditions. This variability results both from the stochasticity of the precipitation process and the differences between the climate models. The total uncertainty in river flooding with climate change (over 40%) is much larger than the change with respect to current climate conditions (less than 10%). However, climate changes are systematic changes rather than random changes and thus the large uncertainty range will be shifted to another level corresponding to the changed average situation.

Towards sustainable flood risk management in the Rhine and Meuse river basins: synopsis of the findings of IRMA‐SPONGE

Recent flood events in western Europe have shown the need for improved flood risk management along the Rhine and Meuse rivers. In response, the IRMA-SPONGE research programme was established, consisting of 13 research projects, in which over 30 organizations from six countries co-operated. The aim of IRMA-SPONGE was the development of methods and tools to assess the impact of flood risk reduction measures and of land-use and climate change scenarios, in order to support the spatial planning process for the Rhine and Meuse River Basins. Several important conclusions were agreed upon by participants in the programme; they emphasize the role of spatial planning, especially in areas at risk of flooding, as an important component of flood risk management: 1 Flood risk, defined as a function of both flood probability and potential damage, is increasing not only due to climate change (likely to cause an increase in the probability of extreme discharges) but also due to continued investment in areas at risk of flooding (resulting in an increase in potential damage). 2 Water retention areas and land-use adaptations far upstream may be useful in lowering the frequency of floods in small basins. However, extreme floods far downstream in the lower Rhine and Meuse basins, where the probability of design discharges is less than 1/200 per year, cannot be prevented by such upstream measures. 3 The most effective and sustainable reduction of flood risks could be achieved by reducing the potential damage (vulnerability) in flood-prone areas through adapted land use and spatial planning. 4 Flood risk management organizations should not only aim at a reduction of flood risk but should also seek to enhance the ecological quality of rivers and floodplains.

Towards estimation of extreme floods: examination of the roles of runoff process changes and floodplain flows

This paper presents the development and application of a distributed rainfall–runoff model for extreme flood estimation, and its use to investigate potential changes in runoff processes, including changes to the ‘rating curve’ due to effects of over-bank flows, during the transition from ‘normal’ floods to ‘extreme’ floods. The model has two components: a hillslope runoff generation model based on a configuration of soil moisture stores in parallel and series, and a distributed flood routing model based on non-linear storage–discharge relationships for individual river reaches that includes the effects of floodplain geometries and roughnesses. The hillslope water balance model contains a number of parameters, which are measured or derived a priori from climate, soil and vegetation data or streamflow recession analyses. For reliable estimation of extreme discharges that may extend beyond recorded data, the parameters of the flood routing model are estimated from hydraulic properties, topographic data and vegetation cover of compound channels (main channel and floodplains). This includes the effects of the interactions between the main channel and floodplain sections, which tend to cause a change to the rating curve. The model is applied to the Collie River Basin, 2545 km 2, in Western Australia and used to estimate the probable maximum flood (PMF) from probable maximum precipitation estimates for this region. When moving from normal floods to the PMFs, application of the model demonstrates that the runoff generation process changes with a substantial increase of saturation excess overland flow through the expansion of saturated areas, and the dominant runoff process in the stream channel changes from in-bank to over-bank flows. The effects of floodplain inundation and floodplain vegetation can significantly reduce the magnitude of the estimated PMFs. This study has highlighted the need for the estimation of a number of critical parameters (e.g. cross-sectional geometry, floodplain vegetation, soil depths) through concerted field measurements or surveys, and targeted laboratory experiments.

Extreme discharge events in the Paraná River and their climate forcing

The largest discharge anomalies of the Paraná River were examined focusing on the contribution from the sub-basins and on the climate forcing of these events. Major discharge anomalies at Corrientes originated in the central and southern Upper Paraná basin with relatively small contributions from the Paraguay river and the northern Upper Paraná basin. About two thirds of the major discharge anomalies in Corrientes occurred during El Niño events while none was registered during La Niña events. Major discharge anomalies related to El Niño occurred either in the spring of the year of El Niño onset or in autumn of the following year (autumn (+)) accompanying the precipitation signal of El Niño in eastern subtropical South America. The signal during autumn (+) is the most relevant as five out the six top discharges of the Paraná River at Corrientes occurred in this season. The remaining third of the major discharges not related to El Niño took place during the austral spring or austral summer of neutral periods. In each of these seasons, they share a common sea surface temperature anomaly pattern in the proximity of the South American coasts.

Longterm evaluation of land use changes on catchment water balance––a case study from North-East Germany

In this paper, the longterm effects of an afforestation scenario on catchment water balance are analyzed taking into account the results of a regional case study. The analysis is based on the GIS-coupled conceptual hydrological simulation model T heseus for the spatially distributed calculation of water balance and discharge. In the regional case study, longterm model runs were performed using a detailed spatial data set from a mesoscale catchment together with time series of measured daily discharge rates and meteorological data from the period 1953–1997. The catchment is located at the moraine landscape of North-East Germany. Based on this data set and the spatially distributed modelling results from T heseus, the impact of the actual land use and of an afforestation scenario on water balance dynamics are analyzed. For the water balance in the catchment within the afforestation scenario, model calculations predict a decrease in discharge of the order of 5–48% with the average of 24% and an increase in evapotranspiration of the order of 3–31% with the average of 14% in comparison with the actual land use. The decrease in extreme peak discharges due to the afforestation scenario was of the order of 4–5%.

Laboratory Detection and Pure Rotational Spectrum of the C[CLC]a[/CLC]C Radical ([ITAL]X[/ITAL] [TSUP]3[/TSUP]Σ[TSUP]−[/TSUP

The calcium carbide radical, CaC, has been observed in the laboratory for the first time using millimeter/submillimeter direct absorption spectroscopy. The species was created in the gas phase by the reaction of calcium vapor and methane under extreme DC discharge conditions. Eleven rotational transitions of CaC in its X 3Σ- ground state were recorded in the frequency range 247-536 GHz. Each transition of CaC was found to consist of triplets due to fine-structure interactions that indicate that the ground state is 3Σ-, as opposed to 5Σ-, both of which have been suggested by theory. The data were analyzed in a Hund's case (b) basis, and rotational, spin-spin, and spin-rotation constants have been accurately determined. CaC may be detectable in circumstellar envelopes of asymptotic giant branch stars, in particular those that are carbon-rich.

Modelling the effects of spatial and temporal resolution of rainfall and basin model on extreme river discharge

Important characteristics of an appropriate river basin model, intended to study the effect of climate change on basin response, are the spatial and temporal resolution of the model and the rainfall input. The effects of input and model resolution on extreme discharge of a large river basin are assessed to give some indication on appropriate resolutions. A simple stochastic rainfall model and a river basin model with uniform parameters and multiple rainfall input have been developed and applied to the River Meuse basin in northwestern Europe. The results show that the effect of model resolution on extreme river discharge is much greater than that of input resolution. The highest model resolution seems to be quite accurate in determining extreme discharge. Although the results should be interpreted with caution, they may give some indication of appropriate input and model resolutions for the determination of extreme discharge of a large river basin.

Predicting channel patterns

The proposed distinction between meandering and braided river channel patterns, on the basis of bankfull specific stream power and bed material size, is analysed and rejected. Only by using regime-based estimates of channel widths (rather than actual widths) has discrimination been achieved, and it is argued that this procedure is unacceptable. An alternative is to explore the patterning processes underlying the marked pattern scatter on bankfull stream power/bed material size plots. Of the five sets of patterning processes, large-scale bedform development and stability is seen as especially important for meandering and braiding. For gravel-bed rivers, bedforms developed at around or above bankfull stage appear important for pattern generation, with braiding relating to higher excess shear stress and Froude number. There seems to be an upper threshold to both meandering and braiding which is achieved at extreme discharges and steep gradients, as on steep alluvial fans, rather than for the rivers with available flow data here considered. For sand-bed rivers with greater excess shear stress, the equivalent upper plane bed threshold may occur below bankfull, with bed material mobility and bedform modification occurring over a wider range of sub-bankfull discharges. Sand-bed channel margin outlines appear to be less perturbed by bedform effects than gravel bed planforms, and they may have naturally straight or sinuous planforms. Bedform relief may nevertheless lead to some being designated as braided when viewed at low flows. It is concluded that the use of a single-stage stream power measure and bed material size alone is unlikely to achieve meandering/braiding discrimination.

Flood frequency estimation by continuous simulation for a gauged upland catchment (with uncertainty)

This paper explores the possibility of deriving frequency distributions of extreme discharges by continuous simulation. The rainfall-runoff model TOPMODEL is applied within the Generalised Likelihood Uncertainty Estimation (GLUE) framework to the River Wye catchment, Plynlimon, Wales, using a 21-year period of rainfall and discharge observations. Rejection of non-behavioural parameter sets is achieved through an evaluation of both annual maximum discharge and continuous hydrograph simulation. Annual maximum peak timings and rankings are also considered. It is demonstrated that, within the prescribed limits, TOPMODEL can adequately achieve both flood frequency and continuous simulation modelling goals. Extension of the flood frequency estimations beyond the upper limit of the observed series is attained through the coupling of behavioural TOPMODEL sets with those of a stochastic rainfall generator for 1000-year simulation periods using hourly time steps. The rainfall model is conditioned on the observed rainfall frequency statistics for different storm duration classes, also within the GLUE framework.

The exceptional Oder flood in summer 1997 — distribution patterns of the oder discharge in the Pomeranian Bight

The exceptional Oder flood in summer 1997 was a unique event in order to investigate the impacts on and the consequences for the ecosystem of the Baltic Sea of about 6.5 km3 additional water loaded with nutrients and contaminants and discharged within only 5 weeks. About 15 institutions participated in this investigation in both the Szczecin Lagoon and the Pomeranian Bight. The Baltic Sea Research Institute Warnemunde studied the water and nutrient inflow, the spreading of the Oder discharge, and the impact of the discharge on the ecosystem. The main topic of the presented investigations is a detailed study of the spatial and temporal spreading of the extreme river discharge in the Pomeranian Bight and the southern Baltic Sea by satellite data, ship observations and continuous buoy measurements as well as numerical modelling.

Geographic Information System Use for Hydrologic Data Development for Design of Highway Drainage Facilities

A significant part of the cost of most highway projects is attributable to drainage facilities such as storm drains, highway culverts, bridges, and water quality and quantity control structures. A geographic information system (GIS) for hydrologic data development for design of drainage facilities is presented. The system was developed to reduce the analysis time and improve accuracy by integrating spatial data describing the watershed with hydrologic theory. A grid-based GIS to estimate potential extreme peak discharges, watershed parameters, peak discharges for different frequencies, isochrone lines, and runoff curve numbers is presented. Data from the State of Texas were used in an example application.

A physically-related regional model for extreme discharges in Israel

Israel is a small country which experiences wide variations in magnitudes of extreme discharges. A consistent model has been constructed for prediction of such discharges throughout the country. Extreme flow characteristics, geographical proximity, lithology, soils, and rainfall properties are the major factors in the delineation of relatively homogeneous regions within the country. For each region, discharge-area relationships are formulated in association with low exceedance probabilities. These relationships follow at-site predictions which have been prepared by fitting the log Pearson type III distribution to annual maxima series of peak discharges. For catchments larger than 100 km2 in area, the differences between the regional and the at-site predictions are small. Relatively high extreme discharges are found for the arid area where rainfall depth is low, for an area of steep slopes, for areas of low permeable lithology and soils, and for areas where the fraction of intense rainfall in the total depth of precipitation is high. For large arid catchments, the discharge- area relationships exhibit a negative trend. The model is simply applicable and appears suitable for other semiarid and arid areas.

Sedimentology of the Upper Burdekin River of North Queensland, Australia—an example of a tropical, variable discharge river

ABSTRACTThe Burdekin River is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge, in response to seasonal but erratic monsoonal rainfall. Floods of the Burdekin rise abruptly, reaching peak discharges of up to 40,000 m3 s‐1 in less than 24 h; maintain peak flow for up to a few days, and recede exponentially. The geomorphology and deposits of these rivers reflect the extreme discharge fluctuations, and have not previously been described.A stretch of the upper Burdekin River comprising four bends and one straight reach was examined near the town of Charters Towers. The river bed is largely exposed for most of any year, with a small, misfit perennial channel carrying low stage flow. Major geomorphic elements of bends include point bars with ridge‐and‐swale topography, three distinct types of chute channels, avalanche slipfaces up to 5 m or more high around the downstream edges of bars, and on the outer part of one point bar an elevated, vegetated ridge. Straight reaches are flat or gently inclined, sand‐ and gravel‐covered surfaces. Much of the river bed is covered by well sorted, in places gravelly, coarse to very coarse‐grained sand with local accumulations of pebble to boulder gravel. Lower parts of the river bed are periodically draped by mud which is desiccated on exposure. Dunes and plane beds are the most commonly occurring bedforms, with local development of gravelly antidunes. Most bank tops and upper, vegetated bars are covered by silt and fine‐grained sand. The river bed also hosts a low‐diversity but locally high‐abundance, flood‐tolerant flora dominated by the paperbark tree Melaleuca argentea, which plays an important role in controlling the distribution of sediment.The gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major (bankfull or near bankfull) flows, which have a recurrence of about 18 years (based on 65 years hydrographic data). The initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact, whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent, non‐bankfull discharge events.