Channel incision by headcut migration: Reconnection of the Colorado River to its estuary and the Gulf of California during the floods of 1979–1988     

Steven M. Nelson  Eloise Kendy  Karl W. Flessa  J. Eliana Rodríguez-Burgueño  Jorge Ramírez-Hernández  Tomás E. Rivas-Salcedo 《水文研究》2020,34(22):4156-4174

The macrotidal Colorado River Delta at the northern end of the Gulf of California in Mexico is hydrologically complex. We review historical accounts, data, field notes and photographs to evaluate the hydrological processes active on the delta prior to the advent of upstream dams. We also employ satellite imagery as well as recent LIDAR data to illustrate the critical role played by headcut erosion in restoring the river's fluvial/tidewater connection during the 1979–1988 floods. Prior to human manipulation, the river's contribution of fresh water to the Gulf was periodically interrupted by natural overflowing, avulsing, and flooding into the sub-sea level Salton Sink on the north slope of the delta plain. River flow south towards the Gulf was also subject to occasional overflow into Laguna Salada, another sub-sea level basin. In the mid-20th century, the Delta was disconnected from its fluvial supply following installation of upstream dams and reservoirs. A tidal sediment obstruction developed in the estuary channel, forming a final barrier to fluvial connectivity. Release of Colorado River floodwaters into Mexico between 1979 and 1988 provided a natural experiment on the hydrological response of a long-disconnected macrotidal delta to restoration of fluvial supply.  相似文献   

Conceptualization of the hydrogeological system of some sedimentary aquifers in Savelugu–Nanton and surrounding areas,Northern Ghana     

Nelson Attandoh  Sandow Mark Yidana  Aliou Abdul‐Samed  Patrick Asamoah Sakyi  Bruce Banoeng‐Yakubo  Prosper M. Nude 《水文研究》2013,27(11):1664-1676

Groundwater resources play a pivotal role in the rural water delivery system in Ghana. The hydrogeological system of Middle Voltaian terrain was simulated using available data on hydraulic heads and boundary conditions. The objective was to characterize the general groundwater flow pattern and provide local estimates of the distribution of hydraulic conductivity and recharge fields. The results suggest a predominant NE–SW flow direction, which ties in with the general regional structural trend and indicates that the hydrogeological conditions of the rocks are controlled by structural entities created in the wake of fracturing and/or weathering of the rocks whose primary permeabilities are considerably reduced because of high compaction and low‐grade metamorphism. Calibrated hydraulic conductivities range between 1.90 and 10.81 m/d. The spatial distribution appears to reflect the intensity of fracturing and/or weathering of the rock and the proportion of the clay fraction of the weathered zone. Vertical groundwater recharge has been estimated to range between 0.3% and 4.1% of the annual rainfall. This recharge rate is quite low and reflects the imperviousness of the thick overburden because of high clay content in some places and high compaction in others. Despite this apparently low recharge rate, groundwater resources potential in the area appear to be high, and increased abstraction from existing abstraction wells by up to 50% does not appear to register significant effects on groundwater budgets at the simulated recharge rates. This suggests that the well yields are much lower than the potential of the aquifer system. The apparently low yields might be associated with poor well development and the choice of inappropriate well completion materials. This study recommends a monitoring system to be developed for a much more regional groundwater flow simulation under transient conditions. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

The 1974 spill of the Bouchard 65 oil barge: petroleum hydrocarbons persist in Winsor Cove salt marsh sediments     

Peacock EE  Hampson GR  Nelson RK  Xu L  Frysinger GS  Gaines RB  Farrington JW  Tripp BW  Reddy CM 《Marine pollution bulletin》2007,54(2):214-225

Petroleum hydrocarbons persist in salt marsh sediments in Winsor Cove (Buzzards Bay, Massachusetts) impacted from the 1974 spill of No. 2 fuel oil by the barge Bouchard 65. Intertidal sediment cores were collected from 2001 to 2005 and analyzed for total petroleum hydrocarbons (TPHs). TPHs content was greatest (as high as 8.7 mg g(-1) dry weight) in the surface sediments and decreased with distance landward. Select samples were analyzed for polycyclic aromatic hydrocarbons (PAHs) with values as high as 16.7 microg g(-1) for total naphthalenes and phenanthrenes/anthracenes. These remaining PAHs are mainly C(4)-naphthalenes and C(1)-, C(2)-, and C(3)-phenanthrenes/anthracenes revealing preferential loss of almost all of the naphthalenes and the parent compound phenanthrene. Inspection of the data indicates that biodegradation, water-washing and evaporation were major removal processes for many of the petroleum hydrocarbons in the marsh sediments. In addition, historical data and photographs combined with their recent counterparts indicate that erosion has physically removed these contaminants from this site.  相似文献   

Electron-Temperature Maps of the Low Solar Corona: ISCORE Results from the Total Solar Eclipse of 29 March 2006 in Libya     

Nelson L. Reginald  O. C. St. Cyr  Joseph M. Davila  Douglas M. Rabin  Madhulika Guhathakurta  Donald M. Hassler 《Solar physics》2009,260(2):347-361

We conducted an experiment in conjunction with the total solar eclipse of 29 March 2006 in Libya that measured the coronal intensity through two filters centered at 3850 Å and 4100 Å with bandwidths of ≈?40 Å. The purpose of these measurements was to obtain the intensity ratio through these two filters to determine the electron temperature. The instrument, Imaging Spectrograph of Coronal Electrons (ISCORE), consisted of an eight inch, f/10 Schmidt Cassegrain telescope with a thermoelectrically-cooled CCD camera at the focal plane. Results show electron temperatures of 10⁵ K close to the limb to 3×10⁶ K at 1.3R _⊙. We describe this novel technique, and we compare our results to other relevant measurements. This technique could be easily implemented on a space-based platform using a coronagraph to produce global maps of the electron temperature of the solar corona.  相似文献   

Comparison of AnnAGNPS and SWAT model simulation results in USDA‐CEAP agricultural watersheds in south‐central Kansas     

Prem B. Parajuli  Nathan O. Nelson  Lyle D. Frees  Kyle R. Mankin 《水文研究》2009,23(5):748-763

This study was conducted under the USDA‐Conservation Effects Assessment Project (CEAP) in the Cheney Lake watershed in south‐central Kansas. The Cheney Lake watershed has been identified as ‘impaired waters’ under Section 303(d) of the Federal Clean Water Act for sediments and total phosphorus. The USDA‐CEAP seeks to quantify environmental benefits of conservation programmes on water quality by monitoring and modelling. Two of the most widely used USDA watershed‐scale models are Annualized AGricultural Non‐Point Source (AnnAGNPS) and Soil and Water Assessment Tool (SWAT). The objectives of this study were to compare hydrology, sediment, and total phosphorus simulation results from AnnAGNPS and SWAT in separate calibration and validation watersheds. Models were calibrated in Red Rock Creek watershed and validated in Goose Creek watershed, both sub‐watersheds of the Cheney Lake watershed. Forty‐five months (January 1997 to September 2000) of monthly measured flow and water quality data were used to evaluate the two models. Both models generally provided from fair to very good correlation and model efficiency for simulating surface runoff and sediment yield during calibration and validation (correlation coefficient; R², from 0·50 to 0·89, Nash Sutcliffe efficiency index, E, from 0·47 to 0·73, root mean square error, RMSE, from 0·25 to 0·45 m³ s^?1 for flow, from 158 to 312 Mg for sediment yield). Total phosphorus predictions from calibration and validation of SWAT indicated good correlation and model efficiency (R² from 0·60 to 0·70, E from 0·63 to 0·68) while total phosphorus predictions from validation of AnnAGNPS were from unsatisfactory to very good (R² from 0·60 to 0·77, E from ? 2·38 to 0·32). The root mean square error–observations standard deviation ratio (RSR) was estimated as excellent (from 0·08 to 0·25) for the all model simulated parameters during the calibration and validation study. The percentage bias (PBIAS) of the model simulated parameters varied from unsatisfactory to excellent (from 128 to 3). This study determined SWAT to be the most appropriate model for this watershed based on calibration and validation results. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Clast Fabric in Relict Periglacial Colluvium, Salamanca Re-Entrant, Southwestern New York, USA     

Susan W.S. Millar  & Frederick E. Nelson 《Geografiska Annaler: Series A, Physical Geography》2001,83(3):145-156

Samples of macrofabric data obtained from colluvial deposits near the late-Wisconsinan glacial margin display widely divergent characteristics. Those from highly weathered and jointed sandstone plot as girdle distributions. Samples from red colluvium of early-Wisconsinan age form moderately strong clusters. Fabrics from shallow depth in medium-textured brown colluvium of late-Wisconsinan age generally have modes aligned with the local slope but are weak, and appear to have been modified by frost heaving. Clasts in the basal layers of an overlying loess unit were emplaced by frost heaving from the brown colluvium and have very weak or uniform fabrics. Particle shape exerts an influence on fabric strength in the brown colluvium, with samples composed of more elongated clasts displaying stronger fabrics. Fabrics from near-surface deposits show generally weaker shape and strength characteristics and higher inclinations than those at depth. Calculated seasonal frost penetration is consistent with the depth to which weaker, more steeply inclined fabrics are found. Sampling in relict periglacial deposits should be performed only below the level of disturbance by seasonal frost.  相似文献   

ROBUST: The ROle of BUffering capacities in STabilising coastal lagoon ecosystems     

Rutger de Wit  Lucas J. Stal  Bente Aa. Lomstein  Rodney A. Herbert  Hans van Gemerden  Pierluigi Viaroli  Victor-Ugo Cecherelli  Francisco Rodríguez-Valera  Marco Bartoli  Gianmarco Giordani  Roberta Azzoni  Bart Schaub  David T. Welsh  Andrew Donnelly  Ana Cifuentes  Josefa Antn  Kai Finster  Lise B. Nielsen  Anne-Grethe Underlien Pedersen  Anne Turi Neubauer  Marina A. Colangelo  Sander K. Heijs 《Continental Shelf Research》2001,21(18-19)

“Buffer capacities” has been defined in ecology as a holistic concept (e.g., Integration of Ecosystem Theories: A Pattern, second ed. Kluwer, Dordrecht, 1997, 388pp), but we show that it can also be worked out in mechanistic studies. Our mechanistic approach highlights that “buffering capacities” can be depleted progressively, and, therefore, we make a distinction between current and potential “buffering capacities”. We have applied this concept to understand the limited “local stability” in seagrass ecosystems and their vulnerability towards structural changes into macro-algal dominated communities. We explored the following processes and studied how they confer buffering capacities to the seagrass ecosystem: (i) net autotrophy is persistent in Zostera noltii meadows where plant assimilation acts as a sink for nutrients, this contrasted with the Ulva system that shifted back and forth between net autotrophy and net heterotrophy; (ii) the Z. noltii ecosystem possesses a certain albeit rather limited capacity to modify the balance between nitrogen fixation and denitrification, i.e., it was found that in situ nitrogen fixation always exceeded denitrification; (iii) the nitrogen demand of organoheterotrophic bacteria in the sediment results in nitrogen retention of N in the sediment and hence a buffer against release of nitrogen compounds from sediments, (iv) habitat diversification in seagrass meadows provides shelter for meiofauna and hence buffering against adverse conditions, (v) sedimentary iron provides a buffer against noxious sulfide (note: bacterial sulfide production is enhanced in anoxic sediment niches by increased organic matter loading). On the other hand, in the coastal system we studied, sedimentary iron appears less important as a redox-coupled buffer system against phosphate loading. This is because most inorganic phosphate is bound to calcium rather than to iron. In addition, our studies have highlighted the importance of plant–microbe interactions in the seagrass meadows.  相似文献   

Transition metal ions in silicate melts—I. Manganese in sodium silicate melts     

Carolynn Nelson  William B White 《Geochimica et cosmochimica acta》1980,44(6):887-893

Optical absorption spectra obtained on glasses quenched from sodium silicate melts show Mn³⁺ to be the dominant species for melts heated in air and Mn²⁺ to be the dominant species for melts heated at Po₂ = 10^?17 bar. The absorption spectrum of Mn³⁺ consists of an intense band at 20,000cm^?1 with a 15,000cm^?1 satellite possibly arising from the Jahn-Teller effect. The independence of the spectrum from melt composition and the high band intensity is offered as evidence for a distinct Mn³⁺ complex in the melt. The spectrum of Mn²⁺ is weak and many expected bands are not observed. A two-band luminescence spectrum from Mn²⁺ has been tentatively interpreted as due to Mn²⁺ in interstitial sites in the network and Mn²⁺ coordinated by non-bridging oxygens.  相似文献   

Minor and trace element partitioning between immiscible ocelli-matrix pairs from lamprophyre dikes and sills,Monteregian Hills petrographic province,Quebec     

G. Nelson Eby 《Contributions to Mineralogy and Petrology》1980,75(3):269-278

Many lamprophyre dike and sill rocks in the Monteregian Hills petrographic province of southwestern Quebec contain felsic segregations (ocelli) which have been interpreted as globules of immiscible liquid (Philpotts 1976). Ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements. The major element data, when plotted on a Greig diagram, outline a field of possible silicate-liquid immiscibility at higher alumina+alkali content than that previously mapped in iron-rich experimental systems. The trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high-charge density cations are preferentially concentrated in the matrix (mafic) material, a result which is consistent with theoretical and experimental studies.The distribution of minor and trace elements between ocelli and matrix indicates that several factors control the partitioning of these elements between immiscible felsic and mafic liquids. These factors include the difference in relative polymerization (as measured by the Si∶O ratio) of the two liquids, with an increase in this difference favoring partitioning of the high-charge density cations into the mafic liquid; the concentration of P₂O₅ in the mafic liquid which favors the partitioning of high-charge density cations into this liquid; the presence of a CO₂ vapor (?) phase which favors the partitioning of high-charge density cations into the CO₂ enriched phase; and the presence of solid phases at the onset of immiscibility. These observations indicate that the chemical compositions of two possibly immiscible melts should be known if minor and trace element data are to be used as evidence for silicate-liquid immiscibility.  相似文献   

Comment on late Wisconsinan glaciation models of northern Maine and adjacent Canada     

Nelson R. Gadd 《Quaternary Research》1982,17(3):410-412

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