Slackwater Deposits Research Articles

Palaeohydrology and sedimentary impacts of jökulhlaups from Kverkfjöll, Iceland

Jökulhlaups (glacial outburst floods) occur frequently within Iceland and within most glaciated regions of the world. The largest jökulhlaups known to have occurred within Iceland drained from the northern margin of the Vatnajökull and along the Jökulsá á Fjöllum during the Holocene. However, little is known about the number, age and flow characteristics of the Jökulsá á Fjöllum jökulhlaups. One source of meltwater into the Jökulsá á Fjöllum is Kverkfjöll, a glaciated stratovolcano. This paper provides detailed sedimentological evidence demonstrating that jökulhlaups have routed through Kverkfjallarani and hence from Kverkfjöll. Sedimentological evidence of jökulhlaups includes valley-fill deposits and slack water deposits. Lithofacies, which are indicative of high-magnitude fluvial sedimentation, show that these deposits cannot be the result of nonjökulhlaup processes. The situation and nature of the sediments permit palaeoflow reconstructions. Fine-grained deposits within slack water deposits mark a break in jökulhlaup deposition and suggest that at least three jökulhlaups have drained through Hraundalur, the predominant valley within Kverkfjallarani. Evidence of lava overrunning ‘wet’ jökulhlaup deposits indicates that jökulhlaups occurred in close association with volcanic eruptions in the Biskupsfell fissure. The largest jökulhlaup was initially hyperconcentrated and subsequently became more fluid. Slope–area reconstructions indicate that the largest jökulhlaup had a probable average peak discharge of 45,000–50,000 m 3 s −1; however, the peak discharge attenuated by 25–30% in just 25 km. These observations quantify the number, rheology, hydraulics and chronology of jökulhlaups from Kverkfjöll and hence within the Jökulsá á Fjöllum. This study presents a model of jökulhlaup impacts and characteristics from glaciated volcanoes and/or within volcanic rifting zones.

Pool‐fills: a window to palaeoflood history and response in bedrock‐confined rivers

AbstractChannel‐scale sedimentary units associated with bedrock‐controlled riffle‐pool morphology are examined in detail along Sandy Creek gorge, an ephemeral stream in arid south‐eastern central Australia. Pool‐fills comprise cut‐and‐fill assemblages of poorly sorted sediments ranging in texture from muds to boulders. Five unit types are defined based on particle size, sedimentary structures, geometry and bounding surface character: (1) coarse‐grained bar platform; (2) fine‐grained bar supraplatform; (3) fine‐grained pool‐fill; (4) fine‐grained bench; and (5) modern pool‐fill. The last coarse‐grained unit currently lining the pools suggests an altered sedimentation style over the post‐settlement period (post‐ad 1860s). Situated at bedrock valley constrictions, pool‐fills are compared with other sedimentary units associated with recirculating currents: eddy bars and slackwater deposits. But only the fine‐grained bench units reflect eddy recirculation; the pool‐fills are principally forced‐bars associated with bedrock‐controlled or ‘forced’ riffle‐pool morphology. A late Holocene palaeoflood history is proposed based on radiocarbon ages from the pool‐fills: multiple phases of cut‐and‐fill activity were preceded by a superflood 3400–1900 years ago that eroded the pool‐fills to bedrock. The resilience of the pool‐fills was illustrated by the passage of a 1‐in‐100‐year flood in 1992, which caused only minor erosion. The presence of pool‐fills may provide a window to past phases of river activity that cannot be extracted from either historical records/observations or palaeoflood slackwater sediment analyses. The formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences, but the occurrence of high‐magnitude floods exerts the dominant control.

Geomorphological evidence for jökulhlaups from Kverkfjöll volcano, Iceland

Jökulhlaups (glacial outburst floods) are known to have drained along the Jökulsá á Fjöllum river in Iceland during the Holocene. However, little is known about their number, age, source, and flow characteristics. This paper provides detailed geomorphological evidence for jökulhlaups that have routed from the Kverkfjöll ice margin and hence into the Jökulsá á Fjöllum. Erosional evidence of jökulhlaups from Kverkfjöll includes gorges, cataracts, spillways, subaerial lava steps, and valley-wide scoured surfaces. Depositional evidence includes wash limits, boulder bars, cataract-fill mounds, terraces, slackwater deposits, and outwash fans. Some of these landforms have been documented previously in association with jökulhlaups. However, subaerial lava surfaces that have been scoured of the upper clinker, gorges within pillow–hyaloclastite ridges, gorges between pillow–hyaloclastite ridges and subaerial lava flows, subaerial lava lobe steps, cataract-fill mounds, and boulder run-ups are previously undocumented in the literature. These landforms may therefore be diagnostic of jökulhlaups within an active volcanic rifting landscape. The nature and spatial distribution of these landforms and their stratigraphic association with other landforms suggest that there have been at least two jökulhlaups through Kverkfjallarani. The Biskupsfell eruption occurred between these two jökulhlaups. Kverkfjallarani jökulhlaups were very strongly influenced by topography, geology, and interevent processes that together determined the quantity and nature of sediment availability. Such controls have resulted in jökulhlaups that were probably fluidal, turbulent, and supercritical over large areas of the anastomosing channel bed. Kverkfjallarani jökulhlaups would have had highly variable hydraulic properties, both spatially and temporally. The knowledge of flow characteristics that can be gained from jökulhlaup impacts has implications for recognising jökulhlaups in the rock record and for hazard analysis and mitigation within similar landscapes and upon other glaciated volcanoes.

Climate change in southern Illinois, USA, based on the age and δ 13C of organic matter in cave sediments

Matrix-supported diamicton and uniform to laminated, silty, fine-grained sediment deposited from about 42,500 to 27,600 cal yr B.P. under slackwater conditions nearly filled two caves in southwestern Illinois. At some point, most of the sediment was flushed from the caves and from about 22,700 to 4000 cal yr B.P., floods deposited a drape of sandy and silty sediment on remnant slackwater successions, cobbly alluvium, and bedrock (especially from 7700 to 4000 cal yr B.P.). Clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly Petersburg Silt, a smectite- and illite-rich proglacial lacustrine unit present in the overlying Illinois Episode glacial succession. Today, remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and, in at least two locations, cover a cobble-mantled strath terrace 1.3 to 1.5 m above active stream channels. Slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits. Slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes (in addition to sinkhole formation) and enter caves; however, the finer organics, some of them comminuted during transport into the caves, become part of the cave alluvium. This finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods. Twenty-four 14C ages on humified organic matter provide chronologic control. The δ 13C values of the organic matter reflect the proportion of C 4-type to C 3-type vegetation growing in and around swallets and sinkholes at the time of redeposition. Drought-tolerant C 4-type vegetation was more prevalent relative to C 3-type vegetation from 42,500 to 31,200 cal yr B.P. compared to conditions from 28,800 cal yr B.P. to the present. The δ 13C values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols.

Palaeoflood record of the Tagus River (Central Spain) during the Late Pleistocene and Holocene

Palaeoflood hydrology of the Tagus River (Central Spain) was reconstructed from slackwater deposits and palaeostage indicators in two canyon reaches located at El Puente del Arzobispo in the central part of the catchment ( 35,000 km 2 in drainage area) and in Alcántara, at the lower part of the catchment ( 52,000 km 2 in drainage area) near the Portuguese border. The palaeoflood record, with more than 80 flood events, shows clusters of floods at specific periods from 9440 to 9210 14 C yr BP (8540–8110 BC), 8500 to 8000 14 C yr BP (7500–7000 BC), ∼6750 14 C yr BP (∼5000 BC), 1200 to 950 14 C yr BP (AD 785–1205), ∼410 14 C yr BP (AD 1450–1500), 170 to 80 14 C yr BP (AD 1670–1950). The largest flood(s) occurred during the periods 9440 to 9210 14 C yr BP, ∼6750 14 C yr BP and 1200 to 950 14 C yr BP reaching minimum discharge estimates of between 4000 and 4100 m 3 s −1 in the El Puente del Arzobispo reach and 13,700– 15,000 m 3 s −1 in the Alcántara reach. These periods with increased flood magnitude and/or frequency in the Tagus River are strongly related to increased moisture influx and winter precipitation in the Iberian Peninsula, especially in the western zone. Proxy records sensitive to winter precipitation such as lake levels and vegetation changes (indicated by pollen records) are in good agreement with the clusters of floods found in the Tagus River. This flood variability seems to correspond to changes in the prevailing atmospheric circulation pattern affecting the Iberian Peninsula.

Preservation potential for Late Quaternary river alluvium

AbstractValley sequences of Late Quaternary alluvial units reflect alluvial preservation as well as alluvial production factors. Effects of lateral channel migration, incision, aggradation and channel stability on preservation potential are explored and then considered in the light of 14 available data sets: cartographically dated and model data based on lateral channel migration; well‐mapped and dated Late Quaternary valley unit surveys; and composite age–frequency plots for dated alluvial units and flood sediments.Despite much expectable variation between sites, and the complex effects of river‐activity combinations, a common characteristic of the data sets examined is the significance of preservation factors. Lateral migration tends to eliminate older units as it creates new alluvial deposits, whereas incision may lead to the preferential preservation of older units beyond the incision slot. Aggradational environments are likely to preserve more complete records, although simultaneous lateral migration may eliminate, possibly repeatedly, the upper parts of alluvial units. The common pattern of inset and incised streams within Pleistocene and early Holocene fills or bedrock gives finite extent to later units within narrowing valleys so that development of new valley‐floor units is necessarily at the expense of reprocessing earlier ones. Floods associated with both slack water deposits and berms are also responsible for the removal of accessible earlier materials, thus limiting the preserved record of earlier events.In light of these censoring effects of river activities, the sequence of preserved Late Quaternary units within UK sequences is reconsidered. It is concluded that preservation potential factors have led to spatial and temporal bias in the alluvial record, and that both here and elsewhere preservation potential needs to be considered systematically alongside variable sedimentation resulting from allogenic environmental factors when interpreting the alluvial archive. Copyright © 2003 John Wiley & Sons, Ltd.

A paleohydrologic reinterpretation of the Homeb Silts, Kuiseb River, central Namib Desert (Namibia) and paleoclimatic implications

Slack water deposits are usually fine-grained (fine sand and coarse silt) flood sediments deposited in areas of the floodplain that are sheltered from high-velocity flood flows. Slack water deposits have been recognized by many investigators in a wide variety of physiographic and climatic settings. Late Quaternary slack water deposits have not been described from the extremely arid Namib Desert of southwestern Africa. Fine-grained flood sediments in the Kuiseb Valley near Homeb, accumulated between 23,000 and 19,000 14C-years BP, have been interpreted by previous workers as (i) sediments deposited behind dune dams, (ii) river endpoint accumulations, and (iii) flood deposits of an aggrading river controlled either by a base level change in the lower reaches or a change in the hydrological regime in the catchment area. Furthermore, the so-called Homeb Silts could document more arid climatic conditions in the Kuiseb catchment area during the last glacial maximum (LGM). Our research shows that the Homeb Silts are slack water deposits and that in many valleys of the Namib Desert, slack water sediments are observed most commonly at the junctions of major rivers and minor tributaries in bedrock canyon settings. Here we present evidence that these slack water deposits accumulated as a result of more humid conditions and/or more intense precipitation events in the upper reaches of the river. They do not document climatic changes in the desert itself as postulated by many researchers.

Late Holocene slackwater deposits on the Nakagawa River, Tochigi Prefecture, Japan

A continuous succession of slackwater deposits was investigated on the Nakagawa River, Karasuyama Town in Tochigi Prefecture, Japan. Forty-one and 32 flood events were estimated from two sedimentary cores. Minimum sedimentation rate is estimated at 4.6 mm/year and the calculated maximum flood recurrence interval is 14 years. The sedimentation rates are high compared to other published data and are thought to be the key reason as to why these deposits were preserved in Japan, previously considered too humid. High sediment accumulation rates are thought to compensate for any bioturbation activity that would disturb and homogenise the slackwater deposits. This study is important because it presents a new arena for palaeoflood analysis, suggesting that there is potential for slackwater preservation in similar humid settings.

Quantitative sourcing of slackwater deposits at Boila rockshelter: A record of lateglacial flooding and Paleolithic settlement in the Pindus Mountains, Northwest Greece

AbstractEvidence for large floods in the Pindus Mountains of Northwest Greece during the Lateglacial period is provided by a sequence of fine‐grained slackwater sediments preserved beneath the Late Upper Paleolithic deposits at Boila rockshelter in the Voidomatis River basin. We have applied a quantitative sediment fingerprinting approach to estimate the relative contributions from different catchment sediment sources to these flood sediments. Our approach utilizes recent developments in hydrology and fluvial geomorphology where researchers have used sediment properties to establish the source of suspended sediments transported in rivers during present‐day flood events. The data set used to create the composite sediment source fingerprint comprised nine tracer properties including eight trace elements (determined by X‐ray fluorescence techniques) and magnetic susceptibility. The Boila sequence has been dated by conventional and AMS radiocarbon techniques, and the central and upper part of the flood sediments were deposited between ca. 14,310 ± 200 and 13,960 ± 260 radiocarbon yr B.P. These flood events took place during the global cooling associated with Heinrich event 1, although the sediment fingerprinting data indicate that glacial activity in the Voidomatis basin was no longer supplying large volumes of sediment to the fluvial system at this time. The slackwater deposits are mainly derived from flysch sediments and soils from nonglaciated parts of the Voidomatis River basin. The results of this approach compare well with lithological estimates based on micromorphological analyses and are in good agreement with existing data on the Late Pleistocene and Holocene behavior of this river system. The slackwater sediments offer important insights into the nature of the river environment and the wider catchment system during the Lateglacial period when the Klithi rockshelter (ca. 2 km upstream of Boila) was the focus of Paleolithic activity in the Voidomatis Gorge. In appropriate depositional contexts, quantitative sediment fingerprinting constitutes a valuable geoarchaeological tool for the investigation of rockshelter and cave sediment records that can improve our understanding of site formation processes and their broader environmental context. This approach is a valuable complement to the micromorphological investigation of rockshelter sediment records. © 2001 John Wiley & Sons, Inc.

Sedimentary records of large Holocene floods from the middle reaches of the Yellow River, China

Slackwater deposits have been used as evidence for the magnitude and frequency of palaeofloods which occurred prior to gauged and historical data sets, e.g., Holocene extreme flood reconstruction in North America and Australia. The slackwater deposits from Xiaolangdi Reach of the Yellow River have been identified and related to floods according to both eyewitness accounts and the analysis of sediments. The technique successfully delineated the peak flood level and estimated a discharge of the 1994 AD flood in the Yellow River. The discrepancy of the estimated discharge was less than 5% compared with the instrumental records. When we applied the technique to palaeoflood analysis, the highest slackwater deposits and flood indicators from the tributary mouths, the low river terraces, and the loess platforms and loess alcoves, together with radiocarbon dating, provided evidence of at least six floods with peak discharges greater than 27,300 m 3 s −1 at Xiaolangdi Section over the last 8500 yr. The largest flood in ca. 7362 yr B.P. had an estimated discharge of 42,900 m 3 s −1, which is the maximum flood event known in the Yellow River. The results provide a basis for (a) determining the floods which were comparable to the most cataclysmic flood during the last 2360 yr in 1843 AD and (b) estimating the flood magnitude for a 10,000 yr time-scale relevant for building Xiaolangdi Dam, one of the largest dams under construction in the world. The reconstruction of palaeofloods also provides evidence for a phase between ca. 6000–8000 yr B.P., which was much wetter than today, as a result of increased summer precipitation and enhanced Pacific monsoon conditions in eastern China.

Late Pleistocene evolution of the lower Mississippi River valley, southern Missouri to Arkansas

The Mississippi River served as the principal conduit for meltwaters discharging from the southern margins of North American ice sheets. The northern half of the Mississippi River alluvial valley in Missouri and Arkansas can be subdivided into two distinct channel courses, the Western and Eastern Lowlands, which are separated by Crowleys Ridge. The Eastern Lowlands contain two entry points, the Bell City–Oran and Thebes Gaps; the latter is a narrow bedrock gorge that contains the modern river. This paper summarizes a model for evolution of channel courses in the Western and Eastern Lowlands during the Illinoian and Wisconsin glacial stages. For the Western Lowlands, stratigraphic relations between loess units and subjacent fluvial deposits show that major continuous terrace surfaces, or valley trains, are Illinoian or late Wisconsin in age. Investigations of tributary valley fills further show the Western Lowlands were deeply incised during the early to middle Wisconsin stage, then aggraded during the late Wisconsin stage to such an extent that tributary valleys were backfilled with slackwater deposits. Moreover, analysis of loess thinning and fining trends shows that early to middle Wisconsin Roxana Silt and the late Wisconsin Peoria Loess covers on Western Lowlands valley trains were derived from deflation of valley trains in the Eastern Lowlands. We conclude that the Western Lowlands were the primary Mississippi River course during Illinoian deglaciation but were abandoned shortly thereafter, then episodically reoccupied during the late Wisconsin stage. For the Eastern Lowlands course, valley trains of early to middle Wisconsin age are not preserved, with the exception of a small remnant where the Mississippi River broke through a gap in the southern margins of Crowleys Ridge and reentered the Western Lowlands. All valley trains in the Eastern Lowlands are late Wisconsin age. A series of discontinuous terrace surfaces in the Bell City–Oran Gap represent dissected remnants of the primary late Wisconsin full glacial and early late glacial course of the Mississippi River or combined Mississippi and Ohio Rivers, whereas younger continuous braided stream surfaces in the Bell City–Oran and Thebes Gap courses represent valley trains of latest Wisconsin and earliest Holocene age. Hence, the Bell City–Oran Gap was the primary course during the entire Wisconsin stage, and the Thebes Gap was permanently occupied in earliest Holocene time. As much as 5 m difference in elevation between successive late Wisconsin terrace surfaces of the Eastern Lowlands indicates that valley trains were rapidly constructed, incised, and abandoned in response to fluctuations in the delivery of meltwater and outwash from the former ice margin. Moreover, a variety of data indicates that deglacial meltwater floods were commonly of sufficient magnitude to overflow from the primary course in the Bell City–Oran Gap. Throughout the late Wisconsin substage, floodwaters overflowed into the Western Lowlands, constructed a series of secondary valley trains, and filled tributary valleys with slackwater deposits. During the latest Wisconsin stage, perhaps in association with the drainage of Glacial Lake Agassiz, floodwaters overflowed into the Western Lowlands and broke through the Thebes Gap. The Charleston fan represents the initial deposits of the Thebes Gap course. Following deposition of a large crevasse splay at the Western Lowlands entrance, that course was no longer active, but flow was routed through the Thebes Gap during periods of high-magnitude flooding until the Bell City–Oran Gap was abandoned and the Thebes Gap course was permanently occupied during earliest Holocene time.

Late Pleistocene evolution of the lower Mississippi River valley, southern Missouri to Arkansas

Late Pleistocene evolution of the lower Mississippi River valley, southern Missouri to Arkansas

The palaeoflood record of a hyperarid catchment, Nahal Zin, Negev Desert, Israel

The palaeohydrology of Nahal Zin, a 1400 km2 catchment in the hyperarid Negev Desert, is inferred from slackwater deposits and palaeostage indicators in a canyon near its lower end. The palaeoflood record, augmented by the instrumental and historical records of the last decade, includes 28 floods ranging from 200 to 1500 m3s−1 over the last 2000 years. This helps to reanalyse the frequency of floods in this drainage system. The clusters of floods around 1000 years BP and again during the last 60 years are characterized by high flow magnitudes. Periods with many floods correspond well to periods with high Dead Sea levels and are probably relatively wet periods, while periods with few floods correspond well to low Dead Sea levels indicating a drier climate. Fluctuations in the frequency of floods are typical of periods of transition from one climate regime to another. Copyright © 2000 John Wiley & Sons, Ltd.

Extreme hydrological events, palaeo-information and climate change

Extreme flood events have been and continue to be one of the most important natural hazards responsible for deaths and economic losses. Extreme floods result in direct destructive effects during the time of the event, and they also may be followed by a related chain of indirect calamities such as famines and epidemics that produce additional damages and suffering. Extreme hydrological events that have occurred in the historical past may also occur in the future. Knowledge about magnitudes and recurrence frequencies of past extreme hydrological events in most regions are too short to adequately evaluate potential magnitudes and recurrence frequencies of extreme hydrological events. Stationary climate in which the mean and variance do not change over time is a basic underlying assumption of standard methodological procedures for estimating recurrence probabilities of extreme hydrological events. Palaeo-archives contained in river and lake sediments, fossil plant and animal matter, ice layers, and other natural archives show that the assumption of stationary climate is not valid when the time scale is extended beyond centuries and millennia. Records of past extreme floods that occurred long before the period of instrumentation can be reconstructed from the distribution of slackwater flood deposits or from derivation of water depths competent to transport the largest rocks found in flood deposited sediment. Palaeoflood records reconstructed from the Upper Mississippi and Lower Colorado River systems in the United States confirm nonstationary behaviour of the mean and variance in hydrological time series. These stratigraphic records have shown that even very modest climatic changes have resulted in very important changes in the magnitudes and recurrence frequencies of extreme floods. A close relationship was found between the palaeo-flood record of extreme floods in the Upper Mississippi River system and a palaeo-record of stable isotopes of oxygen and carbon preserved in speleothem calcite from a local cave. The relationship suggests that isotopic records elsewhere might be calibrated to provide insight about how future potential climate changes might impact extreme flood magnitudes and recurrence frequencies there. Atmospheric global circulation models (GCMs) mainly simulate average climatic conditions and are presently inadequate sources of information about how future climate changes might be represented at the extreme event scale. Palaeo-flood archives, however, provide basic information about how magnitudes and recurrence frequencies of extreme hydrological events responded to past climate changes and they also provide a reference base against which GCM simulations can be calibrated regionally and be better interpreted to decipher hydrological information at the extreme event scale.

Response of extreme floods in the southwestern United States to climatic variations in the late Holocene

A regional synthesis of paleoflood chronologies on rivers in Arizona and southern Utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations. The flood chronologies were constructed using fine-grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events. High-magnitude floods were frequent on rivers throughout the region from 5000 to 3600 14C yrs BP (dendrocalibrated age = 3800-2200 BC) and increased again after 2200 BP (400 BC), with particularly prominent peaks in magnitude and frequency around 1100-900 BP (AD 900–1100) and after 500 yrs BP (AD 1400). In contrast, the periods 3600-2200 BP (2200-400 BC) and 800-600 yrs BP (1200–1400 AD) are marked by sharp decreases in the occurrence of large floods on these rivers. In the modern record, storms that generate large floods (≥ 10-year) in the region fall into three categories: (1) winter North Pacific frontal storms; (2) late-summer and fall storms that draw in moisture from recurved Pacific tropical cyclones; and (3) summer storms, mainly convective thunderstorms. Winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study, and are the most probable causes of the paleofloods over the last 5000 years. Floods from both winter storms and tropical cyclones occur when deep mid-latitude troughs steer storm systems into the region. Composite anomaly maps of daily 700-mbar heights indicate that these floods are associated with a low-pressure anomaly off the California coast and a high-pressure anomaly over the Aleutians or Gulf of Alaska. A strong connection exists between the negative phase of the Southern Oscillation Index (often associated with El Nin˜o conditions) and the large floods associated with winter storms and tropical cyclones. The paleoflood records confirm the existence of centennial-scale variations in the conditions conducive to the occurrence of extreme floods and flood-generating storms in this region. The episodes with an increased frequency of high-magnitude floods coincide with periods of cool, wet climate in the western U.S., whereas warm intervals, such as the Medieval Warm Period, are times of dramatic decreases in the number of large floods. A positive relationship between the paleofloods and long-term variations in the frequency of El Nin˜o events is evident over the last 1000 years. This relationship continues over at least the last 3000 years with warm coastal sea-surface temperatures indicative of El Nin˜o-like conditions.

River-derived slackwater sediments in caves along Cheat River, West Virginia

The November 1985 Cheat River flood produced overbank, slackwater deposits in caves of the Cheat River canyon. The deposits include loamy silt, loamy sand to very fine sand, and flotsam. Sediments deposited by the November 1985 flood lie within 1 m of the established high water mark and are good indicators of peak stage. Prehistoric overbank, slackwater deposits are present in one cave. These sediments are inferred to be ≥400,000 years old and are, therefore, unrelated to the modern Cheat River. Overbank slackwater deposits are poorly preserved in the examined caves, because cave streams actively remove sediments and the warm, moist environment of the caves fosters decay of woody flotsam. Deposits of the 1985 flood are projected to survive at most a few centuries. In contrast, slackwater sediments inferred to be ≥400,000 years old are preserved within a cave shielded from significant runoff and biogenic activity.

Debris-Fan Formation and Rapid Modification at Warm Springs Rapid, Yampa River, Colorado

In 1965 a debris flow from Warm Springs Draw, Colorado, deposited approximately 15,000 metric tons of sediment in the channel of the Yampa River, constricting the river against the opposite cliff and forming Warm Springs Rapid. Radiocarbon ages associated with several older debris-flow deposits bisected by the 1965 flow indicate a time interval of approximately 1000 years between large, channel-constricting debris flows from Warm Springs Draw. The older debris-flow deposits probably constricted the Yampa River in a manner analogous to that of the 1965 flow. The configuration of these tributary debris-fan rapids, which occur along canyon rivers throughout the western U.S., is a function of the sediment characteristics of the debris flow and subsequent hydraulic conditions within the channel constriction. Erosion of the debris-flow deposits and transport of the boulders through the rapid during different discharges was estimated with step-backwater modeling. Shear stress and critical flow best indicate the hydraulic conditions needed for entrainment and transport of the large boulders in Warm Springs Rapid. Results show that the present configuration of Warm Springs Rapid was formed during several high flows since 1965, principally the high flow of $$1200 m^{3}/s$$ in 1984. Model results also suggest that flow duration may be as important as magnitude in modifying debris-fan constrictions. After modeling discharge magnitudes obtained from stream-gage records and slackwater deposits, we suggest that the geomorphic response time of the Yampa River to a disturbance of the magnitude of the 1965 event is less than 1000 years.

Transport and deposition of plutonium-contaminated sediments by fluvial processes, Los Alamos Canyon, New Mexico

Between 1945 and 1952 the development of nuclear weapons at Los Alamos National Laboratory, New Mexico, resulted in the disposal of plutonium into the alluvium of nearby Acid and (to a lesser degree) DP Canyons. Previous research has identified some of this material in the Rio Grande and defined its distribution in the regional river system. The purpose of this paper is to explore the connection between the disposal sites and the main river, a 20 km link formed by the fluvial system of Acid, Pueblo, DP, and Los Alamos Canyons. Empirical data from 15 yr of annual sediment sampling throughout the canyon system has produced 458 observations of plutonium concentration in fluvial sediments. These data show that, overall, mean plutonium concentrations in fluvial sediment decline from 10 000 fCi/g near the disposal area to 100 fCi/g at the confluence of the canyon system and the Rio Grande. In finer detail, the concentrations fluctuate with downstream distance depending on the trap efficiency of various reaches, as controlled by hydraulic conditions. Temporal data from sites repeatedly sampled show the passage of waves of contaminated sediment through the canyon system. Field mapping identified 108 deposits of sediment, including active bed load, flood plains, bars, channel fills, and slack-water deposits. Volumes of sediment in these deposits (calculated from field measurements of the dimensions of the features), combined with the mean concentration values, produced a first approximation of the amount of plutonium in each deposit. The geographic distribution of deposits and plutonium is clustered: of the 1000 mCi of plutonium in the canyon system, 78% is in lower Pueblo Canyon, 18% is in lower Los Alamos Canyon, and the remainder is in the upper reaches of the system. Simulations using a computer model for water, sediment, and plutonium routing in the canyon system show that discharges as large as the 25 yr event would fail to develop enough transport capacity to completely remove the contaminated sediments from Pueblo Canyon. Lesser flows would move some materials to the Rio Grande by remobilization of stored sediments. The simulations also show that the deposits and their contaminants have a predictable geography because they occur where stream power is low, hydraulic resistance is high, and the geologic and/or geomorphic conditions provide enough space for storage.

Relief peels in the study of palaeoflood slack-water sediments

The use of slack-water sediments as palaeostage indicators in palaeoflood hydrological analysis requires detailed sedimentological description. Conventional descriptions have relied mainly on field descriptions of slack-water profiles. However, this method often fails to reveal important detail. By employing the well-established technique of relief peels, modified for use in slack-water sediment studies, the problem of insufficient data recovery from slack-water deposits was successfully overcome during a palaeoflood hydrological investigation of South African rivers. The use of relief peels showed that this technique is a useful, and in some cases, indispensable aid in the description and interpretation of palaeoflood deposits. For example, relief peels have shown that slack-water sediments are for the most part not massive exhibiting mainly flat lamination. This indicates that slack-water sedimentation was characterised by moderate rates of deposition rather than sudden or rapid rates. Relief peels have also shown that the base of tributary back-flooded slack-water sediments often contain intraformational rip-up clasts. This indicates that reworking of the existing slack-water sediments is a common process during tributary back-flooding. The technique was particularly effective in enhancing sedimentary structure and in differentiating between interpalaeoflood, non-palaeoflood and intraflood pulses. Relief peels can also be stored permanently as a surface sample of the slack-water sequence to be retrieved for later study.

A 30,000 year record of extreme floods in tropical Australia from relict plunge‐pool deposits: Implications for future climate change

Relict plunge‐pool sedimentary sequences provide much longer paleoflood records than normally provided by slackwater deposits and other previously reported paleoflood sedimentary signatures. The only two relict plunge‐pool sedimentary sequences so far reported lie 300 km apart in tropical northern Australia and provide a record of extreme floods for the last 30,000 years. Each of these sequences identify the early to mid‐Holocene and the period immediately prior to the Last Glacial Maximum as the two periods of greatest flood magnitudes of the late Quaternary. Flood discharges at these times were up to five times greater than any floods experienced over the last 4,000 years. Most climate models predict that the magnitude and frequency of storms and rainfall events will increase under a warmer ‘greenhouse’ climate. The plunge‐pool sedimentary sequences, however, show that periods of greatly enhanced discharge compared to present can be associated with both warm, wet and cool, wet phases of climate.