Large Magnitude Floods Research Articles

Estimating long term sediment yields from sediment core analysis

Estimating long term sediment yields from sediment core analysis Sediment cores from lakes and reservoirs can be used to estimate sediment yields. In proglacial lakes, the bed sediment typically accumulates as varves, reflecting changes in seasonal and annual sedimentation. This report compares the results from two different methods of measuring suspended sediment transport in a Norwegian glacial river and lake. Sediment yields obtained from a study of sediment cores in the proglacial lake Nigardsvatn along with measurements of the delta topset were compared with a 25-year record of sediment transport based on automatic water sampling and water discharge measurements at a monitoring station at the inflowing river. During the period from 1980 to 2005, analyses of sediment cores taken from the lake bed along with measurements and grain size distribution analysis of the delta indicated that a volume of 175,670 to 202,697 m3 was deposited in the lake Nigardsvatn and corresponding river delta. The year 1980 was selected as a convenient starting point because a large-magnitude flood with a 100-year recurrence interval occurred at the end of 1979, leaving an easily recognizable sediment layer and accurate reference point. Sediment cores were taken at a total of 24 locations throughout the lake and 25 locations in the delta. The densities of the sediment cores were found to vary between 1.3 and 1.5 g/cm3 during the period examined in this study, giving a total suspended sediment load of between 175,670 and 202,697 tons. The measurements carried out at the sediment station in the glacier melt-water river gave a value of 294,800 tons during the same period. A final value of 211,100 tons is calculated from the monitoring station results after deduction of the sediment fraction which passes through the lake without being deposited. This gives a discrepancy between the two methods of 8,403 to 35,430 tons (4-20%). This can be partially attributed to the difficulties of measuring the water discharge in the unstable glacier meltwater river. In conclusion, sediment cores may be used to extrapolate or correct measurements from sediment monitoring stations over longer periods but caution should be made when considering single years.

The impact of late Holocene climatic variability and land use change on the flood hydrology of the Guadalentín River, southeast Spain

The Guadalentín River, located in southeast Spain, is considered one of the most torrential rivers in Spain, as indicated by catastrophic events such as the 1879 flood that caused 777 fatalities in the Murcia region. In this paper, flood frequency and magnitude of the upper Guadalentín River were reconstructed using geomorphological evidence, combined with one-dimensional hydraulic modelling and supported by records from documentary sources at Lorca in the lower Guadalentín catchment. Palaeoflood studies were conducted along a 2.5-km reach located at the confluence of the Rambla Mayor (162 km 2) and Caramel River (210 km 2). These tributaries join at the entrance of a narrow bedrock canyon, carved in Cretaceous limestone, which is 15–30 m wide and 40 m deep. Six stratigraphic profiles were described, the thickest and most complete corresponding to flood benches deposited upstream of the canyon constriction. The stratigraphic and documentary records identify five main phases of increased flood frequency. Phase 1, based on sedimentary palaeoflood evidence alone, occurred at c. AD 950–1200 with at least ten floods with minimum discharge estimates of 15–580 m 3 s − 1 . Phases 2–5, identified through combined sedimentary and documentary evidence occurred at: (a) AD 1648–1672, with eight documentary floods and two palaeofloods exceeding 580–680 m 3 s − 1 (most probably the AD 1651 and 1653 events); (b) AD 1769–1802, comprising seven documentary floods, of which at least two events (> 250 m 3 s − 1 ) are preserved in the sedimentary record; (c) AD 1830–1840, with four documentary floods, and at least two events recorded in the stratigraphy (760–1035 m 3 s − 1 ); and finally (d) the AD 1877–1900 period that witnessed seven documentary floods, with three palaeofloods exceeding 880 m 3 s − 1 . The palaeoflood and historical flood information indicate an anomalous increase in the frequency of large magnitude floods between AD 1830 and 1900, which can be attributed to climatic variability accentuated by intensive deforestation and land use practices during the first decades of the nineteenth century.

The impact of environmental changes on the sediment loads of Norwegian rivers

Abstract The impact of unusual events on the sediment dynamics in rivers is discussed in this paper. The increase in the number of extreme precipitation events and other unusual weather events in Norway strongly suggests that weather conditions are changing. It is difficult to monitor sediment transport during large magnitude floods. It is however well known that they may deliver substantial amounts of sediment. The suspended sediment load in the upper Atna river during the springflood in 1995 was 44 times the mean load measured during the preceding years. The recurrence interval of this flood was estimated to be 100–200 years. The conditions during the occurrence of a particular flood event may also be of considerable importance and measurements during rainfall on frozen ground have indicated unusually high runoff and erosion rates. The seasonal distribution of runoff in several parts of Norway has changed in recent years and higher winter temperatures have produced an increase in the frequency of flood events. A very long lasting flood caused by a prolonged period of rain occurred in the Vikka stream during September to December 2000. The availability of sediment increased with the increasing soil moisture content in the area. As the ground became saturated, more active slope processes caused erosion rates to increase markedly. The combined effects of climate change and human impact on sediment transport in rivers appear to enhance downstream sediment delivery. Flood protection works along river channels prevent sediment from being deposited on the floodplain. River channels that have been lowered in order to reduce groundwater levels in agricultural floodplain areas will experience the same effect. During the last decade extensive ecological changes have taken place in the coastal areas of southern Norway as 90% of the sugar kelp forest has been lost. This change has been attributed to an apparent increase in sediment delivery to the sea along with a rise in sea temperatures.

CLIMATE, HYDROLOGIC DISTURBANCE, AND SUCCESSION: DRIVERS OF FLOODPLAIN PATTERN

Floodplains are among the world's most threatened ecosystems due to the pervasiveness of dams, levee systems, and other modifications to rivers. Few unaltered floodplains remain where we may examine their dynamics over decadal time scales. Our study provides a detailed examination of landscape change over a 60-year period (1945-2004) on the Nyack floodplain of the Middle Fork of the Flathead River, a free-flowing, gravel-bed river in northwest Montana, USA. We used historical aerial photographs and airborne and satellite imagery to delineate habitats (i.e., mature forest, regenerative forest, water, cobble) within the floodplain. We related changes in the distribution and size of these habitats to hydrologic disturbance and regional climate. Results show a relationship between changes in floodplain habitats and annual flood magnitude, as well as between hydrology and the cooling and warming phases of the Pacific Decadal Oscillation (PDO). Large magnitude floods and greater frequency of moderate floods were associated with the cooling phases of the PDO, resulting in a floodplain environment dominated by extensive restructuring and regeneration of floodplain habitats. Conversely, warming phases of the PDO corresponded with decreases in magnitude, duration, and frequency of critical flows, creating a floodplain environment dominated by late successional vegetation and low levels of physical restructuring. Over the 60-year time series, habitat change was widespread throughout the floodplain, though the relative abundances of the habitats did not change greatly. We conclude that the long- and short-term interactions of climate, floods, and plant succession produce a shifting habitat mosaic that is a fundamental attribute of natural floodplain ecosystems.

Landscape expressions of late Quaternary climate change and large flood events, Flinders Ranges, South Australia

SUMMARY A fundamental problem in earth sciences is how landscapes respond to the climatic and meteorological processes imposed upon them. Sedimentary features such as alluvial fans are commonly used to reconstruct paleoclimatic I paleo-meterological conditions. Alluvial fans in the central Flinders Ranges provide a Late Pleistocene to Holocene record of cyclic sedimentation and incision that may be interpreted to reflect climatic oscillations and episodic large magnitude flooding events. Debris flow deposits ranging in depositional age kom - 71 to 55 thousand years before present (ka) are interpreted to reflect high-energy deposition of sediment derived fiom soil-mantled catchments, implying more humid climates and higher soil production rates than present. These deposits are capped by a -30 ka paleosol and incised by a series of clast-supported fluvial gravels, indicating a change to bedrock dominated landscapes and more arid climates during early stages of the Last Glacial Maximum (LGM). Although localized aeolian deposition I sediment remobilisation occurred on distal portions of the fans, fanhead trenches were deeply eroded and incised during this time (-30 to 18 ka). A series of alluvial terraces were deposited at - 1000 year intervals in the middle to late Holocene, interpreted to reflect infrequent, large magnitude flood events that periodically flush bedrock dominated catchment systems. We suggest that alluvial fan sequences in the Central Flinders Ranges provide meaningful information about Late Pleistocene to Holocene catchment paleogeography, climate change and return intervals of large magnitude floods.

Bedrock erosion and relief production in the northern Flinders Ranges, Australia

AbstractCosmogenic 10Be concentrations in exposed bedrock surfaces and alluvial sediment in the northern Flinders Ranges reveal surprisingly high erosion rates for a supposedly ancient and stable landscape. Bedrock erosion rates increase with decreasing elevation in the Yudnamutana Catchment, from summit surfaces (13·96 ± 1·29 and 14·38 ± 1·40 m Myr−1), to hillslopes (17·61 ± 2·21 to 29·24 ± 4·38 m Myr−1), to valley bottoms (53·19 ± 7·26 to 227·95 ± 21·39 m Myr−1), indicating late Quaternary increases to topographic relief. Minimum cliff retreat rates (9·30 ± 3·60 to 24·54 ± 8·53 m Myr−1) indicate that even the most resistant parts of cliff faces have undergone significant late Quaternary erosion. However, erosion rates from visibly weathered and varnished tors protruding from steep bedrock hillslopes (4·17 ± 0·42 to 14·00 ± 1·97 m Myr−1) indicate that bedrock may locally weather at rates equivalent to, or even slower than, summit surfaces. 10Be concentrations in contemporary alluvial sediment indicate catchment‐averaged erosion at a rate dominated by more rapid erosion (22·79 ± 2·78 m Myr−1), consistent with an average rate from individual hillslope point measurements. Late Cenozoic relief production in the Yudnamutana Catchment resulted from (1) tectonic uplift at rates of 30–160 m Myr−1 due to range‐front reverse faulting, which maintained steep river gradients and uplifted summit surfaces, and (2) climate change, which episodically increased both in situ bedrock weathering rates and frequency–magnitude distributions of large magnitude floods, leading to increased incision rates. These results provide quantitative evidence that the Australian landscape is, in places, considerably more dynamic than commonly perceived. Copyright © 2006 John Wiley & Sons, Ltd.

Floodplain sedimentation in the Upper Mississippi Valley: Natural versus human accelerated

Understanding the time scales and pathways for response and recovery of rivers and floodplains to episodic changes in erosion and sedimentation has been a long standing issue in fluvial geomorphology. Floodplains are an important component of watershed systems because they affect downstream storage and delivery of overbank flood waters, and they also serve as sources and temporary sinks for sediments and toxic substances delivered by river systems. Here, 14C and 137Cs isotopic dating methods are used along with ages of culturally related phenomena associated with mining and agriculture to determine rates of sedimentation and morphologic change for a reach of the upper Mississippi River and adjacent tributaries in southwestern Wisconsin and northwestern Illinois. The most important environmental change that influenced fluvial activity in this region during last 10,000 years involved the conversion of a late Holocene mosaic of prairie and forest to a landscape dominated by cropland and pastureland associated with Euro-American settlement. Results presented herein for the Upper Mississippi Valley (UMV) show that the shift from pre-agriculture, natural land cover to landscape dominance by agricultural land use of the last 175–200 years typically increased rates and magnitudes of floodplain sedimentation by at least an order of magnitude. Accelerated overbank flooding led to increased bank heights on tributary streams and, in turn, contributed to more frequent deep flows of high energy. These high energy flows subsequently promoted bank erosion and lateral channel migration, and the formation of a historical meander belt whose alluvial surface constitutes a new historical floodplain inset against the earlier historical floodplain. The new historical floodplain serves as a “flume-like” channel that provides efficient downstream transport of water and sediment associated with moderate and large magnitude floods. Floodplains on lower tributaries, however, continue to experience rates of overbank sedimentation that are of anomalously high magnitude given improved land cover and land conservation since about 1950. This lower valley anomaly is explained by minimal development of historical (agriculture period) meander belts because of relatively low stream power in these channel and floodplain reaches of relatively low gradient. In general, long-term pre-agriculture rates of vertical accretion between about 10,000 and 200 years ago averaged about 0.2 mm yr − 1 in tributary watersheds smaller than about 700 km 2 and about 0.9 mm yr − 1 on the floodplain of the upper Mississippi River where the contributing watershed area increases to about 170,000 km 2. On the other hand, rates of historical vertical accretion during the period of agricultural dominance of the last 200 years average between 2 and 20 mm yr − 1 , with short episodes of even higher rates during times of particularly poor land conservation practices. Significant hydrologic effects of mining and agricultural started by the 1820s and became widespread in the study region by the mid-19th century. The hydrologic and geomorphic influences of mining were relatively minor compared to those related to agriculture. High resolution dating of floodplain vertical accretion deposits shows that large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper Mississippi River. The relative importance of large floods as contributors to floodplain vertical accretion is noteworthy because global atmospheric circulation models indicate that the main channel upper Mississippi River should experience increased frequencies of extreme hydrologic events, including large floods, with anticipated continued global warming. Instrumental and stratigraphic records show that, coincident with global warming, a shift to more frequent large floods occurred since 1950 on the upper Mississippi River, and these floods generally contributed high magnitudes of floodplain sedimentation.

Late Holocene fluvial chronology of Spain: The role of climatic variability and human impact

A database of published and unpublished radiocarbon dates from Late Holocene fluvial environments in Spain was critically analysed in order to obtain a fluvial chronology for this period. The principal types of depositional environment represented in the record were slackwater flood deposits (28 dates), alluvial overbank facies (12 dates) and alluvial channel facies (7 dates). The sedimentary context of each date was classified in order to identify those samples that specifically dated change in the fluvial environment. Analysis of the different depositional environment data-sets using summed probability plots identified phases of alluviation at 2800–2350 and 800–500 cal. BP. Increased frequency of large magnitude floods occurred at 2850–2500, 1000–800 and 520–250 cal. BP. The results indicate that in Mediterranean regions slackwater flood deposits are valuable archives of climatic variability even during periods of increased human impact.

The Holocene fluvial chronology of Spain: evidence from a newly compiled radiocarbon database

A critical analysis of 74 radiocarbon dates, selected from a total of 102 published and unpublished dates from Holocene fluvial environments in Spain, has identified a number of periods of increased fluvial activity in Spain, namely: 11,170–10,230; 9630–8785; 7980–6860; 5800–4800; 3880–3085; 2895–1820; 1300–0 cal BP, the latter period reflecting sustained fluvial response to increased human impact. The radiocarbon samples were classified according to type of depositional environment: (a) alluvial overbank, (b) flood basin, (c) alluvial channel gravels, (d) fluvio-torrential deposits and (e) slackwater flood deposits (palaeofloods). Dates from slackwater flood deposits indicated at least five phases of increased frequency of large magnitude floods during the Holocene: 10,855–10,230; 9530–8780; 2880–2430; 975–790; and 520–265 cal yr BP. Flood basin type deposition occurred from 7980 to 4830 cal yr BP. The main clusters of dates from alluvial floodplain deposits occurred at 2750–2150 and 930–520 cal yr BP. The record is discussed in relation to other palaeoenvironmental archives including pollen records of Holocene vegetation change and palaeoclimatic proxies such as the North Atlantic drift ice record. Comparison with the latter indicates that major Holocene flooding coincided with cold climate phases during 9530–9280, 2880–2430 and 520–265 cal yr BP; a cooling phase at 10,855–10,230; and warming phases at 9030–8780 and 975–790 cal yr BP.

Depositional history and evolution of the Paso del Indio site, Vega Baja, Puerto Rico

AbstractPotshards discovered during excavation of bridge pilasters for a major expressway over the Rio Indio floodplain, a stream incised within the karsts of north‐central Puerto Rico, required large‐scale archaeological excavation. Five‐meter‐deep bridge pilaster excavations in the alluvial valley provide a 4500‐year history of deposition. Stratigraphic analysis of the exposed pilaster walls in combination with textural and organic carbon analyses of sediment cores obtained over a much broader area suggest a fluvial system dominated by overbank deposition. Six sequences of alternating light and dark layers of sediment were identified. The darker layers are largely composed of silts and clays, whereas the lighter layers are rich in sand‐sized sediment. Archaeological evidence indicates the organic‐rich dark layers, believed to be buried A horizons, coincide with pre‐historic occupation by Cedrosan Saladoid, Elenan Ostionoid, and Chican Ostionoid, extending from A.D. 450 to A.D. 1500. Lighter layers below the dark soil horizons are interpreted as overbank deposits from large magnitude flood events. The floodplain aggraded discontinuously with rapid deposition of sand followed by gradual accumulation of silt, clay, and organic material. An approximately 1‐m‐thick layer of coarse sand and gravel halfway up the stratigraphic column represents an episode of more frequent and severe floods. Based on radiocarbon ages, this layer aggraded between A.D. 1000 and A.D. 1100, which is well within the Elenan Ostionoid era (A.D. 900–1200). Rates of sedimentation during this period were approximately 8 mm per year, ten times greater than the estimates of sedimentation rates before and after this flood sequence. The cause for the change in deposition is unknown. Nonetheless the Elenan Ostionoid would have had to endure frequent loss of habitation structures and crops during these events. © 2003 Wiley Periodicals, Inc.

Spatial and Temporal Variability in Floodplain Backwater Sedimentation Pool Ten, Upper Mississippi River

Sedimentation in Upper Mississippi River (UMR) aquatic habitats known as "backwaters" has been an issue of concern since the installation of navigation locks and dams in the late 1930s. The impounded river subjects previously protected aquatic habitat to increased durations of river flow and associated sedimentation. Four cores were extracted from isolated backwaters in Navigation Pool Ten near Bagley, Wisconsin, in summer 2001. Particle size, organic carbon, and carbonate analyses were used to determine sedimentation response to changes in flooding associated with the dams. UMR flooding heavily influences backwaters adjacent to the main channel, while backwaters more distant from the main channel have the potential to be influenced by small tributary streams. High spatial variability in sedimentation patterns of the Bagley Bottoms backwaters suggests the importance of relative site location when evaluating these habitats. In addition, recent large magnitude floods on the UMR seem to be a major source of backwater sediments. Therefore, potential changes in recurrence frequencies of large UMR floods and their driving forces should also be considered in evaluating the future of backwaters.

Rates of floodplain accretion in a tropical island river system impacted by cyclones and large floods

Fluvial processes, especially rates of floodplain accretion, are less well understood in the wet tropics than in other environments. In this study, the caesium-137 ( 137Cs) method was used to examine the recent historical sedimentation rate on the floodplain of the Wainimala River, in the basin of the Rewa River, the largest fluvial system in Fiji and the tropical South Pacific Islands. 137Cs activity in the floodplain stratigraphy showed a well-defined profile, with a clear peak at 115 cm depth. Our measured accretion rate of 3.2 cm year −1 over the last ca. 45 years exceeds rates recorded in humid regions elsewhere. This is explained by the high frequency of tropical cyclones near Fiji (40 since 1970) which can produce extreme rainfalls and large magnitude floods. Since the beginning of hydrological records, large overbank floods have occurred every 2 years on average at the study site. The biggest floods attained peak flows over 7000 m 3 s −1, or six times the bankfull discharge. Concentrations of suspended sediments are very high (max. 200–500 g l −1), delivered mainly by channel bank erosion. In the future, climatic change in the tropical South Pacific region may be associated with greater tropical cyclone intensities, which will probably increase the size of floods in the Rewa Basin and rates of floodplain sedimentation.

Recent large-magnitude floods and their impact on valley-floor environments of northeastern Yellowstone

The Lamar River watershed of northeastern Yellowstone contains some of the most diverse and important habitat in the national park. Broad glacial valley floors feature grassland winter range for ungulates, riparian vegetation that provides food and cover for a variety of species, and alluvial channels that are requisite habitat for native fish. Rapid Neogene uplift and Quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine–riparian ecosystems. Uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock. As a result, steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt. Recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley-floor meadows. Tree-ring dating identifies major floods in 1918, ca. 1873, and possibly ca. 1790. In 1996 and 1997, discharge during snowmelt runoff on Soda Butte Creek approached the 100-year flood estimated by regional techniques, with substantial local bank erosion and channel widening. Indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996, with similar duration and much greater flood plain impact. Nonetheless, 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper Yellowstone region. The ∼1873 and 1918 floods produced lasting impacts on the channel form and flood plain of Soda Butte Creek. Channels may still be locally enlarged from flood erosion, and net downcutting has occurred in some reaches, leaving the pre-1790 flood plain abandoned as a terrace. Gravelly overbank deposits raise flood-plain surfaces above levels of frequent inundation and are well drained, therefore flood-plain soils are drier. Noncohesive gravels also reduce bank stability and may have persistent effects on channel form. Overall, floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley-floor ecosystems in northeastern Yellowstone.

Overbank sand deposition in relation to transport volumes during large-magnitude floods in the Dutch sand-bed Rhine river system

Using aerial photographs and field measurements, sandy overbank deposits formed by the large-magnitude floods of 1993/94 and 1995 were quantified along two branches of the Dutch Rhine river system: the Waal (1993/94 and 1995) and the IJssel (1995). These deposits were laid down intermittently all along the length of these rivers on the top and landward slope of the natural levees, and covered about 4 per cent of the embanked floodplain on the Waal and about 1 per cent on the IJssel. The overbanks and transport mechanism is basically convective by nature. The spatial variability of overbank sedimentation points to the important role played by helicoidal currents in determining overbank deposition. The presence of embankments and training works appears to influence the sand transport to and morphological development of the floodplains along the Dutch Rhine river system. Overbank deposition volumes about equal present estimates of sand transport during a large-magnitude flood. It appears that studies on sand transport in the Dutch Rhine carried out so far underestimate sand transport during floods. © 1998 John Wiley & Sons, Ltd.

Flood Hydrology and Geomorphology of Monsoon-dominated Rivers: The Indian Peninsula

ABSTRACT The rivers of the southern part of the Indian Peninsula are monsoonal in nature and the hydrological characteristics of these rivers djffer markedly from those of Himalayan rivers. The rivers are subjected to severe floods during the monsoon season. In the last few decades, several large-magnitude floods (peak floods between 10,000 and 80,000 m3/s) have been recorded. In general, floods caused by precipitation from Bay of Bengal depressions include the peak flood of record. Analyses of the systematic, historical, and paleoflood records indicate that, in recent decades, the frequency of high-magnitude floods has increased significantly. Hydraulic estimates reveal that high flows are capable of bedrock erosion and transportation of coarse material. Such large floods are important, not only from the standpoint of geomorphic work, but also in terms of social impact. Floods in the peninsular region are responsible for colossal loss of human life, crops, and property. In recent years, various measures offlood control and management have been adopted. However, flooding in such large rivers profoundly challenges flood-hazard management, because of the inadequacy of conventional data and high spatio-temporal variability of floods. To understand flood hazards and environmental change it is imperative that engineers and hydrologists utilize historical and paleoflood records to improve risk analyses as well as to estimate probable maximum flood on rivers such as these in a highly flood-prone region.

Geomorphological response of a river regulated for hydropower: River Fortun, Norway

The River Fortun is situated in south-western Norway and is typical of the fiord landscape catchments with large areas above 1000 m a.s.l, steep valley walls and an alluvial river plain at the valley floor. It was regulated for hydropower in 1963 and is Norway's 10th largest regulation scheme. The hydropower scheme has resulted in a large reduction of the frequency of large magnitude flood events. This paper describes the long-term geomorphological response of a gravel-bed river channel to this regulation. The study was initiated after complaints from local farmers of an increase in flooding of farmland after regulation. Forty-seven cross-profiles of the river repeatedly measured in 1973, 1989 and 1995 along a 1600 m reach have been used to quantify aggradation and degradation in the reach. The profiles show a reduction in channel size and capacity since 1973, with a net aggradation of 12000 m3. The upper part of the reach has been subject to the greatest amount of aggradation, with the river bed being raised by up to 1.5 m, while the lower part has degraded, with lateral erosion cutting banks back by up to 30 m. Aerial photographs from 1964 and 1984 show that former gravel-bars and flood channels have been subject to vegetation encroachment. The hydraulic model HEC-2 was used to compute the water level in 1973 and 1989. The water level has been raised by 0.6 m at low flows in the upper parts of the studied reach. This corresponds to an increase in discharge of between 20–303/ s. Overbank flow therefore occurs at lower discharges today than prior to regulation. © 1997 John Wiley & Sons, Ltd.

Spatial patterns of historical overbank sedimentation and floodplain evolution, Blue river, Wisconsin

Increased soil erosion and overbank flooding associated with land use changes following European settlement have buried pre-settlement floodplain surfaces in many Driftless Area watersheds with as much as several meters of alluvium. This paper examines the processes influencing spatial variations in the storage of post-settlement vertical accretion alluvium and the impact on floodplain evolution in the Blue River watershed, Wisconsin. The cross-sectional area of historical vertical accretion sediments stored on floodplains was measured at 41 cross-valley transects in the 208 km 2 watershed. The storage of historical overbank sediments is influenced by valley width, stream power, and the development of meander belts. Post-settlement alluvial storage increases with valley width and decreases with cross-sectional stream power. In a multivariate regression model, valley width and cross-sectional stream power explain 67% of the variance in the amount of post-settlement alluvium. The development of meander belts, enlarged channels confined between alluvial terraces, by accelerated lateral channel migration influences historical floodplain storage by decreasing the influence of valley width and modifying stream power characteristics for flows, greater than bankfull. Meander belts are located in headwater and mid-basin valley reaches where high stream power produces rapid lateral channel migration, whereas low gradient channels in downstream valleys lack the power necessary to migrate laterally and develop meander belts. Meander belts influence the spatial pattern of historical sedimentation by: (1) containing large magnitude floods and preventing overbank flow, (2) increasing sediment-transport capacities, (3) initiating the erosion of previously deposited historical sediments, (4) reducing the attenuation of flood peaks, and (5) increasing the amount of sediment routed downstream to wide, low gradient valley bottoms lacking meander belts. Continued high rates of lateral channel migration will eventually be damped as widening of the meander belt decreases flow depths and stream power.

Deglacial Flood Origin of the Charleston Alluvial Fan, Lower Mississippi Alluvial Valley

Large-magnitude flooding of the Mississippi River from proglacial lakes Agassiz and Superior most likely occurred between 11,300 and 10,900 and 9900 and 9500 yr B.P. The Charleston alluvial fan, a depositional remnant of one of these floods, is located at the head of a wide alluvial plain near Charleston, Missouri. The fan is an elongate, convex-up sand body (16 × 24 km) composed of medium- and fine-grained sand at least 8 m thick. This sand contrasts with the older coarse-grained sand of the braided stream surface to the west and south and younger silty clay of the meandering stream level to the north and east. A weakly developed soil separates the underlying braided steam deposits from the alluvial fan. A bulk-soil radiocarbon date of 10,590 ± 200 yr B.P. from the contact between the fan and clays of the meandering stream system indicates that the Charleston fan was deposited near the end of the early interval of flooding from Lake Agassiz about 10,900 yr B.P. If the Charleston fan is the last remnant of deglacial flooding in the lower Mississippi Valley, then deposition of significant quantities of sediment from largemagnitude floods between 10,000 and 9500 yr B.P. did not extend into the lower Mississippi Valley through Thebes Gap.

Thresholds and the spatial variability of flood power during extreme floods

Analysis of the effects of extreme events on overall landscape form has shifted geomorphic attention away from a focus on the magnitude and frequency relationship of discharge to a focus on flood power. This paper analyzes the variation in flood power both at-a-station and downstream in a basin of differing lithologies, and discusses the geologic and geomorphic controls on flood power. Design floods ranging from the 2-year discharge tothe 500-year discharge are routed trough a drainage basin, and channel boundary shear stress and unit stream power are calculated using HEC-2 along 21 cross valley transects. Because of the strong lithologic control on valley width and channel slope, an irregular downstream pattern of flood power emerges which reflects primarily the pattern in valley width. Narrow valleys correspond to a greater percent of channel conveyance during large magnitude floods and increasing rates of depth increases - thus maximizing flood power in these zones. The opposite occurs in anomalously wide valleys where flood power is minimized. A downstream spatial pattern emerges reflecting both local and general controls. For a given probability event, flood power varies at least threefold within the basin with a maximum occuring in a specific downstream section. Holding flood power constant produces an extremely wide range in flow probabilities. In order to establish a link between critical flood power magnitude and flow frequency, minimum flood powers are developed from an analysis of previous work on “catastrophic” flooding for humid, alluvial channels. These minimum thresholds tentatively correspond to 100 N/m 2 or 300 W/m 2 for shear stress and unit stream power, respectively. Discharges required to attain these critical flood powers vary from events roughly twice the 100-year flood to a maximum of roughly 18 times the discharge of the 100 year flood.

Contemporary Squamish River sediment flux to Howe Sound, British Columbia

Squamish River drains 3600 km2 in the southern Coast Mountains of British Columbia to Howe Sound at Squamish, some 50 km north of Vancouver. This study uses differencing of digitized bathymetric surfaces based on the Canadian Hydrographic Services surveys of 1930, 1973, and 1984 at the head of Howe Sound to yield a long-term sedimentation rate for Squamish River delta. The sediment flux from Squamish River to Howe Sound is determined to be 1.29 × 106 m3 a−1 or 1.81 × 109 kg a−1, rates consistent with loads calculated from flow and sediment-concentration regimes measured in the estuary in 1973 – 1975 and 1987 – 1988. The latter data indicate that the modal discharge-effectiveness class is 600 – 700 m3 s−1, moving 13% of the annual suspended-sediment load. Discharges up to 1400 m3 s−1 constitute 99.8% of all flows and are responsible for transporting 81.5% of the annual load. A very significant 18.5% of the load is moved by large-magnitude floods (> 1400 m3 s−1), which occur less than 0.2% of the time.Squamish Delta is prograding downfjord at an average rate of 3.86 m a−1 although local extensions of the delta front in a given year may approach 20 m. Some of the geomorphic implications of extrapolating these contemporary rates of fjord infilling over the Holocene are discussed briefly.