Alluvial Floodplain Research Articles

Spatial Variability Analysis of Soil Physical Properties of Alluvial Soils

Analysis and interpretation of spatial variability of soils is a keystone in site‐specific farming. Soil survey maps may have up to 0.41‐ha inclusions of dissimilar soils within a mapping unit. The objectives of this study were to determine the degree of spatial variability of soil physical properties and variance structure, and to model the sampling interval of alluvial floodplain soils. Soil profiles (n= 209) from 18 parallel transects were sampled with a mean separation distance of 79.4 m. Each profile was classified into surface, subsurface, and deep horizons. Structural analysis of soil bulk density (ρb), sand, clay, saturated hydraulic conductivity (Ks), volumetric water content (θv) at seven pressure potentials (Ψa) (−1, −10, −33, −67, −100, −500, and −1500 kPa) were modeled for the three horizons. Variance of soil physical properties varied from as low as 0.01% (ρb) to as high as 1542% (Ks). The LSD test indicated significant (P< 0.05) differences in sand, clay, ρb,Ks, and θvat various ΨaGeostatistical analyses illustrated that the spatially dependent stochastic component was predominant over the nugget effect. Structured semivariogram functions of each variable were used in generating fine‐scale kriged contour maps. Overall autocorrelation, Moran'sI, indicated a 400‐m sampling range would be adequate for detection of spatial structure of sand, silt, clay, and a 100‐m sampling range for soil hydraulic properties and ρbThe magnitude and spatial patterns soil physical property variability have implications for variable rate applications and design of soil sampling strategies in alluvial floodplain soils.

What is the amount of mercury accumulated in the Idrijca River overbank sediments?

Part of mine tailings and roasted ore remains of the Idrija mercury mine was deposited in the bed of the Idrijca River that transported the load downstream at high waters. It was deposited in alluvial sediments of the Idrijca and Soča Rivers, and in the Trieste Bay, essentially contributing to mercury pollution of environment. The goal of the reported research was to estimate the mercury amount accumulated in the Idrijca alluvial deposits. This was attained by multiplying the mercury contents in sediments by their weight. For calculation of this amount the Idrijca Holocene sediments in the terrace systems from Idrija to confluence of Bača river into it were mapped. For assessing the spatial distribution of mercury contents in sediments the alluvial floodplains were sampled according to an analysis-of-variance sampling design. This was important for establishing the amounts of mercury also in alluvial deposits that were not sampled. The analysis of variance indicated the mercury contents in alluvium to be the most dependent upon the position of the material within the alluvial plain, i.e. either on the most polluted floodplain, on the first or on some higher terrace. By considering this indication and the mapping results we estimated the amount of mercury accumulated in the Idrijca river alluvial sediments from Idrija to the confluence of Bača at 2029 tons.

The current status, threats and protection way of Sanjiang Plain wetland, Northeast China

The Sanjiang Plain is a vast area of alluvial floodplains and low hills in northeast Heilongjiang Province. It is the largest tract of wetlands in China and East Asia outside of Siberia. 28 wetland nature reserves exist on the Sanjiang Plain, and three of them have been listed as wetlands of international importance to water bird conservation by the Ramsar Convention Bureau. The wetlands of Sanjiang Plain are noteworthy for its rich biodiversity, but they continue to decline in area and deteriorate in quality currently. The main threats or constrains, immediate cause, root cause and required response are analyzed in this paper, and the four aspects opinion such as improvement of watershed management, enhancement of protection and restoration of habitats and biodiversity, alternative livelihoods of rural residents living in and near natural wetland reserve, and reinforcement of capacity building of natural reserves are brought forward as the effective measures for the Sanjiang Plain wetland protection.

The late Quaternary evolution of the Negro River, Amazon, Brazil: Implications for island and floodplain formation in large anabranching tropical systems

The Rio Negro has responded significantly in the Late Pleistocene and Holocene to lagged environmental changes largely associated with activity during the last glacial in the Amazon basin. On the basis of geological structure, the Rio Negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles. No deposits of the Upper Pleniglacial were recognized in the field. The oldest recognizable Late Pleistocene alluvial unit is the Upper Terrace of Middle Pleniglacial age (ca. 65–25 ka) (reach I), tentatively correlated with the oldest terrace identified on the left bank of reach III. At that time, the river was mainly an aggradational bed load system carrying abundant quartz sand, a product of more seasonal conditions in the upper catchment. The late glacial (14–10 ka) is represented by a lower finer-grained terrace along the upper basin (reach I), which was recognized in the Tiquié, Curicuriarí, and Vaupes rivers. At that time, the river carried abundant suspended load as a response to climatic changes associated with deglaciation. Since about 14 ka, the river has behaved as a progradational system, infilling in downstream series a sequence of structurally controlled sedimentary basins or ‘compartments,’ creating alluvial floodplains and associated anabranching channel systems. Reach II was the first to be filled, then reach III, both accumulating mainly sand. Fine deposits increase downstream in reach III and become predominant in some anabranch islands of the distal reach. The lowermost reaches of the Negro (V and VI) have been greatly affected by a rising base level and associated backwater effect from aggradation of the Amazon during late glacial and recent times. Reach V has acted almost entirely as a fine sediment trap. The remarkable Anavilhanas archipelago is the product of Holocene deposition in the upper part of this sedimentary basin; however, suspended sediment load declined about 1.5 ka, prior to the lower part of this basin becoming infilled. The progradational behavior of the Rio Negro, filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones, illustrates how a large river system responses to profound changes in Late Quaternary base level and sediment supply. The most stable equilibrium conditions have been achieved in the Holocene in reaches IIb and IIIa, where an anabranching channel and erosional–relictual island system relatively efficiently convey water and sediment downstream. Reaches IIIb and V never achieved equilibrium conditions during the Holocene, characterised as they are today with incomplete floodplains and open water.

Influence of management practices on soil organic matter changes in the Northern China plain and Northeastern China

Soil organic matter (SOM) is strongly related to soil type and management practices. Changes in government policy have brought drastic changes in farm management practices in the last two decades in rural China. This study investigates changes in SOM in two different soils: Ustepts and Udolls. Ustepts, in the North China Plain where the climate is warm and sub-humid, developed from an alluvial flood plain with organic matter <10 g kg −1. Udolls, in Northeastern China where the climate is cool and sub-humid, developed from loess-like materials with organic matter >20 g kg −1. Two locations for Ustepts and three locations for Udolls were used to collect 567 soil samples in 1980–1982 and again in 2000 for SOM analysis. Soil organic matter increased for Ustepts and decreased for Udolls soils over the sampling period, resulting from differences in fertilizer rates and crop residue input to soil. Higher fertilizer input and crop intensity and initially very low SOM content in Ustepts all contributed to greater OM input than oxidation release. In contrast, lower fertilizer input and crop intensity, and initially high SOM content in the Udolls, led to lower OM input than oxidation release. Increasing SOM content through higher mineral fertilizer input is a valuable option for sustainable agriculture production in areas where SOM is low and there is a shortage or potential shortage of food supply.

Increased inbreeding and inter-species gene flow in remnant populations of the rare Eucalyptus benthamii

Eucalyptus benthamii Maiden & Cambage is a forest tree of interest for conservation and plantation forestry. It is vulnerable to extinction, occurring on the alluvial floodplains of the Nepean River and its tributaries, south-west of Sydney, Australia. These floodplains were largely cleared of native vegetation for agriculture by the mid-1800s. Flooding of the Cox Valley for Sydney’s water supply further decreased the species distribution. The species is now confined to one population of approximately 6500 trees in the Kedumba valley and three remnant populations on the Nepean River at Bents Basin (about 300 trees), Wallacia (nine trees) and Camden (about 30 trees). Genetic analysis of the four populations using microsatellite markers revealed significant divergence among all populations, despite the Bents Basin, Wallacia and Camden remnants being separated by distances of only a few kilometres. Trees in these populations have been estimated to range from 35 to 200 years old, suggesting genetic divergence among populations occurred prior to land clearing. To investigate the impact of fragmentation on the next generation, outcrossing rates were estimated from 41 families. While no direct relationship was found between population size and outcrossing rates, fragmentation and the isolation of trees appears to have resulted in higher levels of selfing and biparental inbreeding in seed collected from the Camden and Wallacia remnants. There was also evidence from seedling morphology that inter-species gene flow increased with fragmentation since 20% of the progeny from Camden and 30% of the progeny from Wallacia were hybrids. Seed viability and germination rates were significantly lower in the remnant populations, reducing their value as seed sources for regeneration and plantation forestry. To maintain the genetic integrity of the remnant populations, germplasm should be sourced from the local area. Outcrossed, non-hybrid seed could be produced by controlled pollination in ex-situ conservation stands or by using seedling morphology and microsatellites to screen seedlings from the remnant populations.

Late Quaternary landscape evolution in the Keep River region, northwestern Australia

This paper evaluates the Late Quaternary chronostratigraphic context of archaeological sites in the Keep River region, Northern Territory, Australia. Cosmogenic dating, luminescence dating and sediment characterisation reveal sedimentary processes commencing from erosion of the escarpment and plateaux source through temporary storage in sand sheets, to final deposition in alluvial floodplains. Erosion of the sandstone plateaux (∼5 mm ka −1) and escarpment faces (probably ∼50–100 mm ka −1) provide the main sediment source for the adjacent sand sheets which have evolved over the past 100,000 years as the product of ongoing cycles of accumulation and denudation. The rate of sediment accumulation is lowest near the escarpments on the low-energy sediment-limited sand sheets (<100 mm ka −1) and greatest near the main streams (>400 mm ka −1) that have more numerous sediment sources. Collectively, luminescence ages indicate an apparent increase in sediment accumulation rate in the sand sheets from ∼100 mm ka −1 in the late Pleistocene to over 200 mm ka −1 in the Holocene. This most likely reflects enhanced monsoonal activity following postglacial marine transgression. Palaeosol horizons in the creek profile distinguished by sediment mottling mark potentially significant palaeoenvironmental and palaeoclimatic changes during the Quaternary.

Plant response strategies to stress and disturbance: the case of aquatic plants

The environmental factors controlling the establishment and development of plants in different ecosystems are of two types, stress and disturbance. The effects of stress or disturbance on aquatic systems are discussed in relation to the following questions: Can we predict the state and rate of recolonization after a disturbance? What are the strategies of recolonization developed by plants? How high is the resilience of a disturbed system? Two theories, the intermediate disturbance hypothesis, and the patch dynamics concept proposed to predict the composition, structure and dynamics of plants due to physical-chemical factors, were tested on two scales, that of communities and that of species, within two alluvial floodplains (the Rhine and the Rhone systems in France). With regard to the change of community on a larger scale (i.e. the whole network of the cut-off channels in the floodplain), large gradients of connection and disturbance induce high diversities within communities. Moreover, the highest flood disturbance induces a higher species richness and the occurrence of a particular species. The change in species is analysed using biological traits (morphological, reproductive or physiological). In the floodplain of the river Rhone, the response of plants corresponds well to theory, i.e. that habitats with an intermediate disturbance are richer than more or less disturbed habitats. So we can predict, through the biological traits, the functioning of a habitat. The last remaining question is that of the resilience of the system, which can be discussed in terms of species competition and the risk of biological invasion after an opening of habitat.

Organic carbon in floodplain alluvium: Signature of historic variations in erosion processes associated with deforestation, Waipaoa River basin, New Zealand

We use C‐org and δ13C to trace the origin of the alluvium deposited on the Waipaoa River floodplain at McPhail's Bend between 1853 and 2002. The overbank deposits exhibit a more positive range of δ13C values (−26.3 ± 0.6) than contemporary suspended sediment in transport at intermediate flows (−27.8 ± 0.2) when gully erosion releases most material to stream channels, reflecting the greater contribution made by shallow landsliding in the headwaters during large precipitation events. Overbank sediment associated with recent floods contains a higher (∼50%) amount of organic carbon than alluvium deposited by pre‐1927 floods, which was derived from outcrops of weathered bedrock on steep, riparian hillslopes that were the primary sediment source before the soil‐mantled hillslopes elsewhere in the headwaters of the basin were destabilized by deforestation. The recent alluvium resembles the Bw horizon of soils present on these hillslopes. Sediment deposited during floods generated by localized storms and an overbank event that did not feature widespread shallow landsliding also has a distinctive signature that provides an indication of its provenance. The organic carbon associated with the alluvium appears to be old (4031 ± 40 B.P., in the case of the carbon associated with sediment deposited in 2002) and to be derived from weathered Cretaceous and Tertiary sedimentary rocks.

A patch hierarchy approach to modeling surface and subsurface hydrology in complex flood‐plain environments

AbstractGeomorphology interacts with surface‐ and ground‐water hydrology across multiple spatial scales. Nonetheless, hydrologic and hydrogeologic models are most commonly implemented at a single spatial scale. Using an existing hydrogeologic computer model, we implemented a simple hierarchical approach to modeling surface‐ and ground‐water hydrology in a complex geomorphic setting. We parameterized the model to simulate ground‐ and surface‐water flow patterns through a hierarchical, three‐dimensional, quantitative representation of an anabranched montane alluvial flood plain (the Nyack Flood Plain, Middle Fork Flathead River, Montana, USA). Comparison of model results to field data showed that the model provided reasonable representations of spatial patterns of aquifer recharge and discharge, temporal patterns of flood‐water storage on the flood plain, and rates of ground‐water movement from the main river channel into a large lateral spring channel on the flood plain, and water table elevation in the alluvial aquifer. These results suggest that a hierarchical approach to modeling ground‐ and surface‐water hydrology can reproduce realistic patterns of surface‐ and ground‐water flux on alluvial flood plains, and therefore should provide an excellent ‘quantitative laboratory’ for studying complex interactions between geomorphology and hydrology at and across multiple spatial scales. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Irrigation as a disturbance for interstitial crustacean communities in a French Pyrenean alluvial aquifer

While the impacts of groundwater contamination on interstitial invertebrate communities are well studied, very few work have examined other potential disturbance resulting from agricultural practices. In the present study, the interstitial crustacean fauna was studied in an alluvial aquifer characterized by intensive cereal irrigation, to test the influence of groundwater extraction on the assemblage characteristics (abundance, diversity, persistence, species distribution patterns). Micro- and macrocrustaceans were sampled at 4 seasons in 15 wells with contrasted groundwater use (cereal irrigation / domestic use / no groundwater use) scattered over a 250 km 2 alluvial floodplain. The aquifer harbored diverse assemblages dominated numerically and specifically by cyclopoids. Total abundance, diversity expressed using species richness or Shannon index, overall persistence in species composition did not vary between stations whether micro- or macrocrustaceans were considered. Yet, marked differences in the cyclopoids assemblages composition were found in relation to groundwater utilization. Disturbed irrigation wells were dominated by epigean species, while hypogean species were mainly found in less impacted domestic wells. In contrast, amphipods/isopods did not exhibit particular assemblages composition between irrigation, domestic or abandoned wells. The differential responses between micro- / macrocrustaceans, epigean / hypogean species emphasized that levels of disturbance are strongly taxondependent. At the scale of the aquifer, these results suggest that groundwater extraction resulting from intensive irrigation has to be considered as a potential disturbance for the interstitial communities.

A multivariate taxonomic analysis of the Late Carboniferous vertebrate ichnofauna of Alveley, southern Shropshire, England

A diverse Late Carboniferous (Westphalian D; Moscovian) vertebrate ichnofaunal assemblage from the Alveley Member (Salop Formation, Warwickshire Group) of Alveley, southern Shropshire, UK, is a significant example of an early, transitional tetrapod community. Positive tetrapod footprint casts (convex hyporeliefs) within fine‐ and medium‐grained sandstone red‐beds were formed subaqueously in an alluvial floodplain setting. A statistical analysis of this material, the first using multivariate techniques on Late Palaeozoic trackways, has been undertaken to determine the ichnospecific diversity and morphological variation within the ichno‐assemblage. Nine ichnotaxa have been identified, following a study of more than 200 trackways. These include the amniote ichnogenera Dimetropus Romer and Price, 1940, Ichniotherium Pohlig, 1885 and Hyloidichnus Gilmore, 1927, but there is a dominance of trackways of the ichnogenus Limnopus Marsh, 1894, which represent stem‐lissamphibian ‘temnospondyls’. Following statistical analysis, the ichnogenera Limnopus and Batrachichnus are subsumed as ichnosubgenera under the senior available name Limnopus. The Alveley ichnofaunal assemblage provides significant range extensions for a number of amniote and stem‐lissamphibian trackmakers from the latest Carboniferous or Permian down into the mid‐Late Carboniferous. It also marks a key transitional stage in the evolution of tetrapod communities, from the ‘amphibian’ grade assemblages of the Carboniferous to the more terrestrial, amniote‐dominated communities of the Early Permian.

A discontinuous ca. 80 ka record of Late Quaternary environmental change from Lake Omapere, Northland, New Zealand

We present an integrated record of environmental change from Lake Omapere, Northland, New Zealand, based on palaeolimnological analysis of a 7-m-long core and eight adjunct cores spanning part of the last 80 calendar (cal.) ka. The chronology was developed using tephrochronology, palaeomagnetism and radiocarbon dating together with climato- and palyno-stratigraphy. Two of 14 tephra layers in the cores provide markers for correlating the record with other New Zealand climato-stratigraphic sequences, deep-sea cores and the global marine isotope record. Pollen, diatom, palaeomagnetic, sedimentologic and pigment analyses show that Lake Omapere, currently ∼2 m deep, has had a discontinuous history. Occupying a shallow basin perched partly within old basalt lavas, the initial (alkaline) lake formed towards the end of MIS 5a at ca. 80 cal. ka, presumably because of blockage of local drainage of a swampy alluvial floodplain, inundating peat deposits and forest trees, including kauri ( Agathis australis). The lake filled rapidly to a level ∼1–2 m above that at present. Such filling was probably unrelated to climate but a subsequent phase of variable but generally falling lake levels and increasing dystrophy may have been climatically controlled, commencing during MIS 4 and culminating in periods early in MIS 3 when the lake became swampy or dry. Conditions of non-deposition (or non-preservation) obtained for most of the period after ca. 55 cal. ka (Rotoehu Ash) until formation of the modern lake approximately 600–700 cal. years ago or soon after, as indicated by the presence of Tephra-1, identified in part as Kaharoa Tephra (AD 1314), near the top of the core. The modern lake originated possibly through damming of the western outlet as a consequence of accelerated erosion accompanying earliest Polynesian deforestation, an interpretation supported by Maori oral tradition. The pollen record indicates that beech Nothofagus (presumably N. truncata) was much more common in Northland during the Last Glacial, and that for this region the relative abundance of Nothofagus vs. Agathis pollen serves as a better indicator of cooler versus warmer intervals during the Quaternary than the ratio of tree to non-tree pollen. However, it seems likely that moisture balance was a more critical factor than temperature in vegetation composition and distribution, particularly during the LGM, and the long periods of hiatus may also be linked to a drier climate than present. Correlation coefficients between the pollen curves confirm that several tree species ( Halocarpus bidwillii, H. biformis and Phyllocladus alpinus), previously palynologically concealed within generic taxonomic groups, occurred about two degrees latitude further north and at a much lower altitude than their current limits during cooler or drier phases of the Last Glacial. A temperature depression in Northland of 4 °C at various times during the Last Glacial is inferred from these range expansions. Nevertheless, the persistence of widespread forest cover suggests that the late Pleistocene climate of Northland was less severe than for most of the rest of New Zealand and strengthens the argument for a heightened temperature gradient across northern New Zealand during the Last Glacial.

SCOURING AROUND SPUR-DIKES IN ALLUVIAL FLOODPLAIN RIVERS

Characteristics of bank shape along approach channels at upstream reaches of spur-dikes in alluvial floodplain rivers of Bangladesh are investigated. The shape of river bank at approach channel sections are found to be non-rectangular that can be approximated as nearly trapezoidal. Available methods for the prediction of maximum local scour depth around spur-dikes are not capable to handle such problems where cross sectional shapes in approach flows are different from rectangular. Therefore, an analytical model is developed incorporating the variation of cross sectional shape of approach channels and applied to some of the spur-dikes along the alluvial floodplain rivers.

Statistical analysis of soil variability in a humid forest landscape of Central Cameroon

The quantitative evaluation of key factors of soil variability together with the spatial pattern of underlying variables are important steps in understanding the functioning of soilscapes. Fifteen physical and chemical variables describing 24 pedons were used to elucidate processes of soil differentiation in a humid forest environment in Southern Cameroon. Landscape positions delineated on the basis of remote-sensed imageries included flat convex summits (FCS), linear valley slopes (LVS), colluvial inland valleys (CIV), and alluvial floodplain. Ordination by principal component analysis (PCA) resulted in a three-factor model, which efficiently summarized the dataset explaining 81.4% of total soil variance (TSV). The pedogenetic factor of ferralitization emerged as the main factor accounting for 43.7% of TSV. Identified underlying processes included basic cation leaching, solum acidification, and in situ clay build up. Cation exchange properties as influenced by organic constituents and soil available P were the second and third most important factors associated with 26.7 and 11.4% of TSV respectively. Organic constituents and related biological processes had strong control on nutrient availability and cation-exchange properties, while soil available phosphorus (AP) showed a strong dependence on potential acidity and organic matter mineralization status. The analysis of variance highlighted significant differences for most soil properties across landscape position and soil depth except for organic carbon (OC), total nitrogen (TN), and total phosphorus. All the soils, however, have low pH, low basic cation saturation, and correlatively high exchangeable Al irrespective of their position in the landscape.

Lateral connectivity of a fragmented large river system: assessment on a macroscale by dragonfly surveys (Insecta: Odonata)

Abstract The ecological status of floodplain areas along the Austrian section of the Danube was assessed by an approach based on dragonfly surveys. Although this river section contains a relatively high portion of the river‐type‐specific alluvial floodplain areas, most of them are influenced by river regulation and damming. A key element of the assessment procedure, which is oriented towards the new EC Water Framework Directive, is the Odonate Habitat Index. Classification of ecological status is based on the comparison between the status quo and reference conditions derived from a historical situation minimally influenced by human activities. Data from 408 sites from 14 investigation areas were used. Ten of the 14 areas were ranked in class II (‘good ecological status’) within the five‐tiered system, which is the level of ecological status targeted in the directive. One area corresponds to the reference condition (class I, ‘high ecological status’), and three areas do not meet the quality objective (class III, ‘moderate ecological status’). A total of 49 species were found along the whole section, which represents 82% of the river‐type‐specific reference list. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Assessment of the quantitative and qualitative buffer function of an alluvial wetland: hydrological modelling of a large floodplain (Garonne River, France)

AbstractFor 2 years, water flow‐patterns in the Garonne floodplain of south‐western France were studied in the field and through hydrodynamic modelling (MARTHE Hydrodynamic Software developed by BRGM). Water flow‐paths and the transport of dissolved elements between river and aquifer have been investigated and modelled. In order to quantify the buffer function of the alluvial floodplain, we focused our work on the effect of a major flood on the water flow‐direction, and on nitrate transport. Thus, we showed that the effect of a large flood in the river was rapidly lost with increasing distance from the river. During the observation period, a hydrologically active strip only 300 m wide on either side of the riverbed played a buffering role in absorbing the flood crest. It was also found that meanders favour the exchange between river and alluvial aquifer, shown by the creation of bypasses between the upstream and downstream parts of meanders. This, in turn, contributes to a dilution of nitrates in the phreatic aquifer, which here has higher nitrate content than the surface water; such dilution may result in an overestimation of the denitrification process in the wooded riverbanks. The coupling of chemical measurements—especially of chlorides and nitrate—with modelling of the dissolved‐element transport allows us to establish the water balance for the riparian wetland, and to separate the effect of dilution and denitrification on nitrate concentration. This indicated the existence of areas in the riparian wetlands where denitrification is particularly strong, leading to reductions in nitrate concentrations of 10 to 30 mg/l NO3− during the flood. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Impacts of the Columbia River Hydroelectric System on Main-Stem Habitats of Fall Chinook Salmon

Salmonid habitats in main-stem reaches of the Columbia and Snake rivers have changed dramatically during the past 60 years because of hydroelectric development and operation. Only about 13% and 58% of riverine habitats in the Columbia and Snake rivers, respectively, remain. Most riverine habitat is found in the upper Snake River; however, it is upstream of Hells Canyon Dam and not accessible to anadromous salmonids. We determined that approximately 661 and 805 km of the Columbia and Snake rivers, respectively, were once used by fall chinook salmon Oncorhynchus tshawytscha for spawning. Fall chinook salmon currently use only about 85 km of the main-stem Columbia River and 163 km of the main-stem Snake River for spawning. We used a geomorphic model to identify three river reaches downstream of present migration barriers with high potential for restoration of riverine processes: the Columbia River upstream of John Day Dam, the Columbia-Snake-Yakima River confluence, and the lower Snake River upstream of Little Goose Dam. Our analysis substantiated the assertion that historic spawning areas for fall chinook salmon occurred primarily within wide alluvial floodplains, which were once common in the main-stem Columbia and Snake rivers. These areas possessed more unconsolidated sediment and more bars and islands and had lower water surface slopes than did less extensively used areas. Because flows in the main stem are now highly regulated, the predevelopment alluvial river ecosystem is not expected to be restored simply by operational modification of one or more dams. Establishing more normative flow regimes—specifically, sustained peak flows for scouring—is essential to restoring the functional characteristics of existing, altered habitats. Restoring production of fall chinook salmon to any of these reaches also requires that population genetics and viability of potential seed populations (i.e., from tributaries, tailrace spawning areas, and hatcheries) be considered.

DIFFERENCES IN COTTONWOOD GROWTH BETWEEN A LOSING AND A GAINING REACH OF AN ALLUVIAL FLOODPLAIN

Interstitial flow of river (hyporheic) water influences algal productivity, benthic assemblages, and locations of fish spawning. However, little is known of the effects of hyporheic flow on the growth of riparian vegetation. By increasing water availability and nutrient delivery, regional upwelling of hyporheic water may increase the growth of terrestrial vegetation. We tested and accepted the hypothesis that cottonwood trees (Populus trichocarpa) in a gaining reach of an alluvial floodplain grow faster than trees in a losing reach by comparing basal areas and ages on an expansive floodplain in western Montana (USA). Trees in the gaining reach had basal areas twice the size of the trees in the losing reach, after correcting for tree age. In addition, the carbon-to-nitrogen ratios in leaves were 16% lower in the gaining reach. Lower cottonwood stem densities, deeper layers of fine sediments, and a higher water table occurred in the gaining compared to the losing reach. Each of these variables was significa...

Multiple pathways for woody plant establishment on floodplains at local to regional scales

Summary The structure and functioning of riverine ecosystems is dependent upon regional setting and the interplay of hydrologic regime and geomorphologic processes. We used a retrospective analysis to study recruitment along broad, alluvial valley segments (parks) and canyon segments of the unregulated Yampa River and the regulated Green River in the upper Colorado River basin, USA. We precisely aged 811 individuals of Populus deltoides ssp. wislizenii (native) and Tamarix ramosissima (exotic) from 182 wooded patches and determined the elevation and character of the germination surface for each. We used logistic regression to relate recruitment events (presence or absence of cohort) to five flow and two weather parameters. Woody plant establishment occurred via multiple pathways at patch, reach and segment scales. Recruitment occurred through establishment on (1) vertically accreting bars in the unregulated alluvial valley, (2) high alluvial floodplain surfaces during rare large flood events, (3) vertically accreting channel margin deposits in canyon pools and eddies, (4) vertically accreting intermittent/abandoned channels, (5) low elevation gravel bars and debris fans in canyons during multi‐year droughts, and (6) bars and channels formed prior to flow regulation on the dammed river during controlled flood events. The Yampa River's peak flow was rarely included in models estimating the likelihood that recruitment would occur in any year. Flow variability and the interannual pattern of flows, rather than individual large floods, control most establishment. Regulation of the Green River flow since 1962 has had different effects on woody vegetation recruitment in canyons and valleys. The current regime mimics drought in a canyon setting, accelerating Tamarix invasion whereas in valleys the ongoing geomorphic adjustment of the channel, combined with reduced flow variability, has nearly eliminated Populus establishment. A single year's flow or a particular pattern of flows over a sequence of years, whether natural or man‐made, produces different recruitment opportunities in alluvial and canyon reaches, in diverse landforms within a particular river reach, and for Populus and Tamarix. The design of flows to restore riparian ecosystems must consider these multiple pathways and adjust the seasonal timing, magnitude and interannual frequency of flows to match the desired outcome.