Rare Flood Events Research Articles

The Sources of Belief in Personal Capability: Antecedents of Self-Efficacy in Private Adaptation to Flood Risk.

Self-efficacy is one of the strongest and most consistent drivers of private flood mitigation behavior; however, the factors influencing self-efficacy in the context of flooding remain unclear. The present study examines three potential antecedents of self-efficacy: personal and vicarious experiences of floods or building-related events, social norms for private flood preparedness, and personal competencies such as technical abilities and social skills. While controlling for other drivers in a protection motivation theory (PMT) framework, these antecedents are tested as precursors of self-efficacy and intentions to improve flood resilience. Structural equation modeling is applied to conduct mediation analyses with survey data of 381 flood-prone households in Austria. Contrary to theoretical expectations, personal and vicarious experiences do not predict self-efficacy, presumably because rare flood events and changing hazard characteristics do not facilitate generalizable performance accomplishments. Social norms strongly and consistently influence self-efficacy, especially for actions observable by others, and also directly influence protective responses. Personal competencies increase self-efficacy and support protective action, particularly with regard to preventive and structural measures. The strength and direction of the antecedents of self-efficacy as well as of other PMT determinants vary between general and specific protective responses. This study provides important insights for risk managers, suggesting that interventions involving social norms and personal competencies can be effective in stimulating self-efficacy and, in turn, private flood mitigation. Interventions and research should clearly differentiate between general intention and the implementation of specific measures, and should address cumulative, synergistic, or tradeoff interrelations between multiple measures.

High resilience of aquatic community to a 100-year flood in a gravel-bed river

Our understanding of ecosystem responses to exceedingly large rare flood events is currently limited. We report the resilience of aquatic community to a 100-year record-high flood, and how it varies depending on levels of water pollution, in a fourth-order gravel-bed river in northern Japan. We used data on riparian landscape structure, channel morphology, and community structure of aquatic fauna, which were collected in sites with and without effluent before (1 month–3 years) and after (10 months) the flood. Carbon and nitrogen stable isotope ratios of consumers and basal resources were measured only before (1 year) the flood. We observed aquatic food web with introduced rainbow trout (Oncorhynchus mykiss) as the top predator, with variable relative contributions of basal resources and their pathways to the rainbow trout, under the effects of water pollution. Biofilm-originating dietary carbon became the more dominant resource, with a slightly shorter food-chain length in the polluted sites. The flood led to a loss of riparian forest and a substantial increase in the proportion of exposed gravel bars (5–24%). While the average river-bed elevation changed a little, the localized scours of river bed down to > 2 m were observed with lateral shifts of channel thalweg. Despite the landscape-level physical and structural changes of ecosystem, aquatic community showed a remarkably high resilience exhibiting negligible changes in abundance, except in the polluted site where only fish abundance showed a slight decrease. This study suggests that the abundance of aquatic organisms in gravel-bed rivers is resilient to a flood of unprecedented magnitude in recent history.

Dry Dams in the Desert: Chasing Land Subsidence and Earth Fissures with Geophysics

Homogenous embankment dam flood retention structures (FRSs) were built throughout central Arizona desert basins in the mid-twentieth century to protect mostly agricultural lands and military installations from rare flood events. Many of these FRSs eventually suffered transverse cracking due to desiccation of the embankment soils in the hot, dry climate. While this dam safety hazard has been recognized and mitigated, another hazard was developing under several dams. Differential land subsidence was gradually tilting some FRSs by several feet; ground cracks known as earth fissures have developed near two FRSs. These earth fissures are large (typically) tension cracks in basin alluvium with lengths of hundreds of feet to several miles and depths up to perhaps hundreds of feet. They can become conduits for uncontrolled erosion and earthen structure failure. Compounding the impacts of these geohazards, continuous metropolitan growth was changing the land use that these FRSs protected so the consequences of dam failure were increasing dramatically.

Learning from and for rare floods in Dresden – how public officials interpret damage simulation results at the building type level

Public officials in Dresden are concerned about learning from and for rare flood events like the Elbe river flood in August 2002. This is interesting because research on individual as well as organizational learning from rare events indicates that this kind of learning faces significant difficulties (e.g., overestimation of rare events for decision-making based on “emotionalized event experience”). Up to now, only little is known what and how public officials in Dresden specifically learn from and for rare floods. Therefore, the paper follows an exploratory purpose in line with principles of qualitative social research. Firstly, the paper explores dealing with rare floods with reference to a conceptual framework that highlights relations between regulative, normative, and cognitive institutions on the one hand and learning of public officials on the other. Secondly, it adopts a single case study design in Dresden with embedded sub-cases that are defined with reference to organizations of FRM. The case study shows, among others, that regulations like the Floods Directive are important for justifying FRM with regard to rare flood events which is less obvious than it sounds. However, public officials display different interpretations of the term “rare flood event”, for instance, in the context of analysing the consequences of floods on the building stock. Furthermore, the case study findings indicate that public officials may follow alternative approaches to sustain commitment in the context of rare flood events (systematic versus pragmatic approach).

Hydrological conditions explain variation in wood density in riparian plants of south‐eastern Australia

Summary Wood density is a key plant functional trait which integrates the trade‐offs characteristic to riparian plant ecological strategies. Although high‐density wood is costly to construct, it confers mechanical stiffness to stems, increasing a plant's capacity to withstand flooding, and also enables increased tolerance to water stress. For riparian plants, fluctuations in soil moisture driven by surface hydrology should therefore be an important driver of variation in wood density. We asked the following questions in the study: (1) Does wood density increase with increasing frequency and magnitude of flood disturbance? (2) Does wood density increase with increasing unpredictability of water availability in the riparian zone? (3) Does dispersion of wood density peak at intermediate levels of hydrological disturbance? We surveyed wood density of dominant species at 15 riparian sites along flow‐gauged rivers across south‐eastern Australia. Due to the broad range of hydrological variability associated with Australian river systems, this set of sites functions as a useful model for assessing the response of riparian plants to changing hydrological conditions. We found wood density varied strongly along a single axis of hydrological variability. This axis integrates flood intensity and frequency with metrics of hydrological unpredictability and can be conceptualized as a gradient of environmental harshness, with higher wood density associated with harsher conditions. Synthesis. Our study highlights the importance of hydrological conditions, particularly disturbance and environmental unpredictability, as determinants of ecological strategy in riparian plants. Large, rare flood events in particular appear to favour higher wood density strategies. This is likely to have significant ecological consequences for riparian plant communities in a south‐east Australian context, as well as in other regions where increasing climatic variability and frequency of extreme events are hallmarks of climate change predictions.

Microinsurance Demand After a Rare Flood Event: Evidence From a Field Experiment in Pakistan

This paper examines the characteristics that determine demand for microinsurance when individuals have personal or observed experience with a rare weather event: the severe 2010 flooding in Pakistan. Using a sample of 384 individuals (192 in flood-affected and 192 non-affected villages matched using pre-flood propensity data), we combine post-flood survey data with behavioural experiments to test the impact of prior loss experience on willingness to purchase insurance. In the framed experiment setting, we allow participants to choose insurance payments through many rounds of random flood losses, testing whether experiment behaviour is significantly related to real-world experience or observation and whether individuals change insurance demand after experiencing losses. We find that 2010 flood-affected individuals demand significantly more insurance than non-affected individuals, and that both personal losses and observations of others’ losses are significant determinants of demand, when controlling for location-specific flood propensity, pre-flood mitigation, information sources, post-flood assistance, exogenous changes in assets, potential migrant attrition and other household characteristics. Having prior experience with less severe floods before the 2010 event also increases insurance demand, although the effect disappears when controlling for 2010 flood. Contrary to expectation, household beliefs that insurance is non-Islamic are not found to be a significant barrier to take-up.

Morphed block-crack preferential sedimentation in a reservoir bed: a smart design and evolution in nature

A pedological study of the reservoir bed of Al-Khoud Dam, Oman, revealed an unusual sedimentation pattern which evolved into an intricate composition of silt blocks surrounded by vertical cracks and horizontal layers filled with a “proppant” sand. The discovered soil morphology reflects the complex topology of water motion (infiltration–seepage–evaporation) through the sand-filled cracks/layers and blocks during both the rare flood events and ensuing periods of ponding, and the long, intervening dry periods. These naturally formed soils demonstrate an ability to preserve a large quantity of water inside the silty blocks at depths of 0.5 to 1.5 m, despite the high temperature and dryness of the topsoil. The hydrological optimality and “smartness” of these soils is attributed to the unique block-crack system. Natural, lush vegetation was found in adjacent zones of the reservoir bed, and acted as a footprint of the shallow “fractured perched aquifer”. Planted “ivy” (Convolvulaceae) in the vertical face of one pedon showed intensive growth without irrigation. Soil moisture content data confirmed the hydrological immobility of water in the blocks if not depleted by transpiration. The novel phenomena reported unveil the possible alteration of soil heterogeneity for optimization of the soil–water system in arid zone soils.Editor D. Koutsoyiannis; Associate editor F.F. HattermannCitation Al-Ismaily, S.S., Al-Maktoumi, A.K., Kacimov, A.R., Al-Saqri, S.M., Al-Busaidi, H.A., and Al-Haddabi, M.H., 2013. Morphed block-crack preferential sedimentation in a reservoir bed: a smart design and evolution in nature. Hydrological Sciences Journal, 58 (8), 1779–1788.

Palaeoflood estimates of Pleistocene coarse grained river terrace landforms (Río Almanzora, SE Spain)

A series of palaeoflood estimation techniques are applied to an inset sequence of Middle to Late Pleistocene river terrace landforms associated with the Río Almanzora, SE Spain. The study area is a 7km-long transverse reach, where 4 terrace levels (Level 1=highest and oldest, Level 4=lowest and youngest) document some 200m of incision across an uplifted basement block during the Pleistocene. For the broader region, river terrace aggradation is attributed to increased sediment supply during glacial to interglacial transitions (Level 1=Marine oxygen isotope stages [MOIS] 12/11, L2=10/9, L3=8/7 and L4=6/5). Within the transverse reach, terrace Levels 2, 3 and 4 are characterised by coarse boulder-rich gravels (Dmax=2.5m) organised into 3–5m-high cross-beds. Level 4 is characterised by a series of km-scale abandoned meander loops, with Levels 2 and 3 showing evidence for similar degraded high sinuosity forms. The fluvial setting is interpreted as a braided river environment, with lateral and longitudinal gravel barforms. Using field-derived sedimentary (boulder size etc) and morphological (width, slope etc) features, flow competence- and regime-based methods were applied. Palaeoflood estimates varied from 40 to 2859m3/s, with mean maximum discharges showing an increase through time (L2=278, L3=413, L4=1361m3/s). Palaeoflood estimates were compared to flood events associated with the ephemeral modern Río Almanzora (catchment=2500km2) to establish whether palaeoflood values were reasonable estimations. A 46-year (1963–2009) modern flood gauge record shows infrequent flood events with discharges typically <40m3/s. A rare flood event in 1973 (5680m3/s) provides an upper value for comparison with the palaeoflood estimates. Although estimates appear reasonable they should be considered as minimum values due to 1) some disparities between flow depths derived from the palaeoflood equations (0.7–4m) and field-based evidence (3–5m-high cross-beds); 2) issues related to field sampling of boulders from sections, and 3) uncertainties as to the relationship between flood hydrology and transportation of boulders, formation of 3–5m-high cross-beds and the position of the terrace remnants in the river valley. The palaeoflood estimates suggest that relatively large discharges occurred during valley floor aggradation in the glacial/interglacial transitions, periods characterised by low vegetation cover and high sediment yield. Discharge increases through time can be explained by the long-term drainage evolution of the Río Almanzora, characterised by an expanding catchment area linked to river capture, driven by regional uplift patterns. The 3.5-times larger discharge estimates recorded in terrace Level 4 are coeval with large and rapid changes in precipitation or elevated seasonal discharge linked to the MOIS 6/5 glacial interglacial climate transition. Overall, within tectonically active regions affected by river capture, fluvial systems could be responding to loss or expansion of catchment areas. Long-term palaeoflood records from inset flights of river terraces could store such signals and may mask/complicate any climate–palaeoflood relationships.

URBAS: forecasting and management of flash floods in urban areas

Recent studies on climate change indicate a growing frequency and intensity of flash flood events in western Europe. Flash floods are caused by extreme local precipitation. The URBAS project is concentrating on the urban occurrence of flash floods in Germany. URBAS has as a main objective to increase the preparedness and the range of possible actions of urban actors (e.g. communities, public enterprises) before and during rare small-scale flood events. Within URBAS the meteorological parameters, runoff and damage of flash floods are investigated. Innovative and feasible actions and precautionary measures at a reasonable cost-value-ratio are to be developed.

Portrait of a Gondwanan ecosystem: A new late Permian fossil locality from KwaZulu-Natal, South Africa

The Clouston Farm locality, assigned to the Lopingian Epoch and occurring within the Normandien Formation of the northeastern Karoo Basin, provides evidence for a community of diverse vascular plants occupying riparian woodland. The depositional environment is interpreted as an abandoned trunk channel that preserved a megaflora in slack-water phases punctuated by overbank deposits from rare flood events. Of 9772 plant specimens tabulated from an unbiased census of all fragments greater than ~ 1 cm 2, there are 51 distinct organ morphotypes, including glossopterids, sphenopsids, and ferns, collectively represented as foliage, axes, fructifications, and dispersed seeds. Of the 11 most abundant morphotypes 10 are glossopterid morphotypes or variant subtypes, in addition to a sphenopsid. Glossopterid morphotype dominance also is reflected in the palynoflora. Palynological data indicate a Wuchiapingian age for the locality. A specimen of the dicynodont ‘ Oudenodon,’ found in a nearby stratigraphically equivalent outcrop, is attributable to the Dicynodon Assemblage Zone, assigned a younger Changhsingian age. A rich record of plant–insect associations demonstrates an elevated frequency of external foliage feeding by mandibulate insects and lower incidence of oviposition by palaeodictyopteroid and odonatopteroid taxa. Evidence for piercing-and-sucking and galling is rare. The most abundant plant taxon (glossopterid Morphotype C2a) is the most intensively herbivorized, overwhelmingly by external feeding and ovipositing insects. Insect damage on this host is beyond that predicted by floristic abundance alone. This specificity, and high herbivory levels on other glossopterid taxa, demonstrates extension of the Euramerican pattern toward the preferential targeting of pteridosperms. The Clouston Farm site provides a glimpse into a late Permian ecosystem of primary producers, herbivores, and insectivores—a prelude to the crisis that engulfed life at the end of the period.

Erosion rates on different timescales derived from cosmogenic 10Be and river loads: implications for landscape evolution in the Rhenish Massif, Germany

We determined erosion rates on timescales of 101–104 years for two catchments in the northeastern Rhenish Massif, in order to unravel the Quaternary landscape evolution in a Variscan mountain range typical of central Europe. Spatially averaged erosion rates derived from in situ produced 10Be concentrations in stream sediment of the Aabach and Mohne watersheds range from 47 ± 6 to 65 ± 14 mm/ka and integrate over the last 9–13 ka. These erosion rates are similar to local rates of river incision and rock uplift in the Quaternary and to average denudation rates since the Mesozoic derived from fission track data. This suggests that rock uplift is balanced by denudation, i.e., the landscape is in a steady state. Short-term erosion rates were derived from suspended and dissolved river loads subsequent to (1) correcting for atmospheric and anthropogenic inputs, (2) establishing calibration curves that relate the amount of suspended load to discharge, and (3) estimating the amount of bedload. The resulting solid mass fluxes (suspended and bedload) agree with those derived from the sediment volume trapped in three reservoirs. However, resulting geogenic short-term erosion rates range from 9 to 25 mm/ka and are only about one-third of the rates derived from 10Be. Model simulations in combination with published sediment yield data suggest that this discrepancy is caused by at least three factors: (1) phases with higher precipitation and/or lower evapotranspiration, (2) rare flood events not captured in the short-term records, and (3) prolonged periods of climatic deterioration with increased erosion and sediment transport on hillslopes.

The arid land invasive weed Nicotiana glauca R. Graham (Solanaceae): Population and soil seed bank dynamics, seed germination patterns and seedling response to flood and drought

Disturbances in plant communities provide opportunities for weed germination, propagation, spread, and invasion. When the population density of a weed increases, fast-tracked and appropriate control management strategies are required. The objectives of this study were to: (i) examine the population and soil seed bank dynamics of Nicotiana glauca; (ii) compare the germination patterns of invasive N. glauca seeds collected from two states in Australia, and (iii) investigate the impact of a flood in September 1997 and subsequent drought on N. glauca seedlings. The density of N. glauca followed a steep positive increment during the sampling time (September 1999 to October 2004). The increment pattern was similar in flooded fenced and unfenced plots. Plant density increased over much of the observation period, but had declined to 80 and 432 stems ha −1, respectively, by October 2004. Stem density recorded in October 2004 along two transects radiating from the central point of the newly created lake demonstrated that a significant number of stems appeared to be dead. A soil seed bank study revealed that seed density varied significantly ( p=0.0001) between flooded fenced (598.75±71) and flooded unfenced (327.5±66) plots. In contrast, no N. glauca seedlings were recruited from the soil collected from the control plots. Germination trials were undertaken on N. glauca seed collected from New South Wales. There was no significance difference detected between treatments light and temperature. Similarly, no interaction was found between light and temperature. A comparative study on seed germination patterns of N. glauca seeds collected from Ivanhoe, New South Wales, and the Flinders Ranges, South Australia, showed that temperature had a significant effect on N. glauca seed germination. The effect varied significantly with main variables (state, length and time) and also with different interactions, except state×light ( p=0.3840). N. glauca seedlings exposed to flood were found to withstand partial flooding for at least 58 days. Under waterlogged conditions, the seedlings showed stem hypertrophy and produced adventitious roots. Only one seedling was found dead in the drought treatment. In conclusion, it is clear that N. glauca invaded the area after a rare flood event and began to function as a casual weed. Established seedlings in the field can withstand extreme ecological events such as flood and drought. Understanding the plants’ ecological characteristics through a study such as this at an early rather than late stage in the invasion will help us to take appropriate control measures for this species.

MODERN ANALOGUES FOR DRYLAND SANDY FLUVIAL-LACUSTRINE DELTAS AND TERMINAL SPLAY RESERVOIRS

Ephemeral sandy fluvial-lacustrine deltas and terminal splays associated with dryland depositional environments are important reservoirs in many basins around the world, in both pericratonic and intracratonic settings (Triassic of Algeria; Triassic of the North Sea; and Pliocene of the Caspian Sea). Research on modern depositional analogues from dryland basins offers insights into these types of reservoirs. Australia’s modern Lake Eyre Basin, an arid to hyper-arid, low-accommodation intracratonic basin in central Australia, provides an ideal natural laboratory.This paper highlights field observations of modern, sand-prone, reservoir analogues from the Neales River and Umbum Creek, on the western fringe of Lake Eyre, including unique aerial observations of sedimentation from a rare flood event in an ephemeral fluvial system. These rivers flow irregularly in a dryland setting, but are prone to flash flooding and highly variable discharge that moves large volumes of sediment over a few hours or days. Although there are variations in sediment type and discharge, similarities exist with the key reservoir elements common to most modern and ancient dryland fluvial-lacustrine systems.Distinctive elements include fluvial point bar and associated overbank deposits, distributive avulsion channels and down-dip terminal splays, either on the floodplain or onto the playa lake fringe. The terminal splays are formed, where there is not a pre-existing standing body of water, during rapidly decelerating flows with high-flow regime, transitional to low-flow regime conditions. Typical structures include parallel lamination, convex-upward parallel lamination, climbing ripples and small-scale 2D and 3D dunes. Flow interference with in-channel and floodplain vegetation is an important sediment-trapping mechanism with reservoir quality implications. Aeolian deflation is also significant as it causes the removal of fine-grained sediments during dry periods. The main controls on sediment preservation include the overall low-accommodation setting and rare major lake-filling events controlled by flooding out-of-phase with flows down the western rivers. Depositional products are either high-net-to gross fluvial- terminal splay sheet sands or lower net-to-gross fluvial- terminal splay-lacustrine delta sand sheets or stringers.

Hydrological impacts of climate change on inflows to Perth, Australia

The effects of climate change due to increasing atmospheric CO2 onthe major tributaries to the Swan River (Perth, Western Australia) have been investigated. The climate scenarios are based on results from General Circulation Models (GCMs) and 1000 year time series are produced using a stochastic weather generator. The hydrological implications of these scenarios are then examined using a conceptual rainfall-runoff model, CMD-IHACRES, to model the response of six catchments, which combine to represent almost 90% of the total flow entering the upper Swan River,and hence the Perth city urban area. The changes in streamflow varies considerably between catchments, exhibiting a strong dependence on the physical attributes of the catchment in question. The increase in the magnitudes of rare flood events despite significant decreases in mean streamflow levels found in some catchments emphasizes the importance of estimating changes in the nature of the precipitation (variance, length of storm and interstorm periods), along with changes in the mean, in climate change scenarios.

HYDROTREND: A CLIMATE-DRIVEN HYDROLOGIC-TRANSPORT MODEL FOR PREDICTING DISCHARGE AND SEDIMENT LOAD TO LAKES OR OCEANS

HYDROTREND is an ANSI-standard FORTRAN-77 numerical model that creates synthetic river discharge time series as a function of climate trends. As a drainage basin simulator, the model provides simulation of daily discharge hydraulics at a river mouth, including the sediment load properties. Model outputs may provide direct input to lake or shelf circulation and sedimentation models. HYDROTREND accepts direct input from meteorological station data or global circulation model output (monthly statistics of temperature, precipitation and evaporation and their interannual variations), and input of basin characteristics (attitudinal temperature gradients, topography, glacier equilibrium line altitudes, and sediment transport and erosion coefficients). HYDROTREND applied to the temperate Eel River drainage basin provides a test of the model's handling of rare flood events brought on decadal, centennial or longer climatic events. The USGS observations on the Eel River include the decadal storms and the 400 y-return 1964 flood event. HYDROTREND captures the range in magnitude and return interval of peak discharge events and their sediment loads.

Ecological impact of a fresh-water ?reef kill? in Kaneohe Bay, Oahu, Hawaii

Storm floods on the night of December 31, 1987 reduced salinity to 15‰ in the surface waters of Kaneohe Bay, resulting in massive mortality of coral reef organisms in shallow water. A spectacular phytoplankton bloom occurred in the following weeks. Phytoplankton growth was stimulated by high concentrations of plant nutrients derived partially from dissolved material transported into the bay by flood runoff and partially by decomposition of marine organisms killed by the flood. Within two weeks of the storm, chlorophyll a concentrations reached 40 mg m-3, one of the highest values ever reported. The extremely rapid growth rate of phytoplankton depleted dissolved plant nutrients, leading to a dramatic decline or “crash” of the phytoplankton population. Water quality parameters returned to values approaching the long-term average within 2 to 3 months. Corals, echinoderms, crustaceans and other creatures suffered extremely high rates of mortality in shallow water. Virtually all coral was killed to depths of 1–2m in the western and southern portions of the bay. Elimination of coral species intolerant to lowered salinity during these rare flood events leads to dominance by the coral Porites compressa. After a reef kill, this species can eventually regenerate new colonies from undifferentiated tissues within the “dead” perforate skeleton. Catastrophic flood disturbances in Kaneohe Bay are infrequent, probably occurring once every 20 to 50 years, but play an important role in determination of coral community structure. The last major fresh water reef kill occurred in 1965 when sewage was being discharged into Kaneohe Bay. Coral communities did not recover until after sewage abatement in 1979. Comparison between recovery rate after the two flood events suggests that coral reefs can recover quickly from natural disturbances, but not under polluted conditions.

River channel changes in the lower mbokodweni river: A case study of the 1987 flood

This article deals with river channel conditions before and after a rare flood event. The characteristics of five cross‐sections before the flood are described and compared with post‐flood conditions. Hydraulic and flow data for the pre‐ and post‐flood channel are presented. It is maintained that the land adjoining the new channel can be developed if certain precautionary measures are taken.

Flood geomorphology of the Katherine Gorge, Northern Territory, Australia

The Katherine Gorge is a narrow, deep canyon system developed in resistant sandstone. The tropical monsoonal climate results in rare flood events of very large magnitude. These floods flush sand-sized sediment as washload through the gorge, but local accumulations are preserved as slack-water deposits at tributary mouths. Paleoflood hydrologic analysis of the deposits, employing step-backwater flow modeling, allows quantitative estimates to be made of geomorphically significant flows. For an annual exceedence probability of 10 −2 , the gorge may experience discharges of 6,000 m 3 /s, mean velocities of 7.5 m/s, flow depths of 15 to 45 m, stream powers per unit area as great as 1 × 10 4 watts/m 2 , and bed shear stresses of 1.5 × 10 3 N/m 2 . Such flows are required to transport boulders 3 m or more in intermediate diameter that occur at large-scale riffles. At maximum stage, pools reach depths of 45 m and are preferentially developed at vertical joint intersections in the rock, probably through intense hydraulic action. Pool-and-riffle development thus reflects the channel boundary characteristics for extreme flow conditions. Additional indicators of especially intense flow phenomena include potholes, flutes, abraded facets on rock bar surfaces, and scabland development on upland bedrock surfaces.

RECONSTRUCTING PALEOFLOOD HYDROLOGY WITH SLACKWATER DEPOSITS: VERDE RIVER, ARIZONA

Fine-grained slackwater flood deposits provide a key to the paleoflood history of the Verde River in central Arizona. During large flows, the slackwater deposits accumulate in areas of sharply reduced flow velocity at channel expansions and contractions or in back-flooded tributaries. The upper surfaces of the highest deposits approximate the peak stages of the associated floods. Stratigraphic analysis, radiocarbon and archaeological dating, and correlation between slack-water sites along the study reach revealed multiple floods, including two paleo-floods that pre-date the 80-year observational record of flows on the Verde River. Discharges calculated with the HEC-2 computer flow model extended the record of rare flood events by 1000 years. The largest flow in that time period measured only 5000-5400 cms, which supports the contention of an upper limit on the maximum flood that can be reasonably expected. Revised recurrence intervals based on the extended paleoflood record were used as a basis to compare...