DOCSLIB.ORG

Fan-Deltas and Braid Deltas: Varieties of Coarse-Grained Deltas

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Fan-Deltas and Braid Deltas: Varieties of Coarse-Grained Deltas Fan-deltas and braid deltas: Varieties of coarse-grained deltas John g. Mcpherson \ ,,,.,„ , , GANAPATHY SHANMUGAM > Mob" Research and Development Corporation, Dallas Research Laboratory, P.O. Box 819047 RICHARD J. MOIOLA J Dallas, Texas 75381 ABSTRACT delta sediments; they lack a muddy matrix; ening of the definition of fan-delta to the point of they display size grading and bar migration; confusion over its meaning and sedimentologic Two types of coarse-grained deltas are they commonly have a sheet geometry with implications. The term "fan-delta" is widely recognized: fan-deltas and braid deltas. Fan- high lateral continuity (tens to hundreds of used to describe depositional settings embracing deltas are gravel-rich deltas formed where an square kilometres); and they exhibit moderate a range of coarse-grained delta types other than alluvial fan is deposited directly into a stand- to high porosity and permeability. Valuable that of the fan-delta originally described by ing body of water from an adjacent highland. paleogeographic and tectonic information Holmes (1965). Many descriptions in the litera- They occupy a space between the highland concerning the proximity of highlands and ture of ancient fan-delta sequences contain no (usually a fault-bounded margin) and the major fault zones may be misinterpreted or evidence of the required presence of an alluvial standing body of water. In contrast, braid lost if these two coarse-grained deltaic sys- fan; instead they consist of deltaic deposits com- deltas (here introduced) are gravel-rich deltas tems are not differentiated. prising multilateral channel conglomerates and that form where a braided fluvial system pro- sandstones deposited from braided fluvial dis- grades into a standing body of water. Braid INTRODUCTION tributaries. It is important to clearly separate deltas have no necessary relationship with al- fan-deltas from other coarse-grained deltas in luvial fans, as exemplified by fluvioglacial The purpose of this paper is to describe the order to establish specific depositional and tec- braid deltas. Braid deltas have previously geomorphic and sedimentologic characteristics tonic settings which may be utilized to predict been classified as fan-deltas even though the of the principal types of coarse-grained deltas facies and facies sequences. geomorphic and sedimentologic settings of and, by doing so, illustrate and correct the cur- Coarse-grained deltas comprise two major the two systems can be vastly different. Braid rent problem with the terminology of coarse- types, namely those formed from the prograda- deltas are a common present-day geomorphic grained deltas. This has important implications tion of an alluvial fan into a standing body of feature and are abundant in the geological for facies modeling, tectonic assessment, and water (fan-deltas), and those deltas formed by record. hydrocarbon exploration and production. the progradation of a braided river into a stand- Fan-deltas and braid deltas can be distin- Coarse-grained (dominantly gravel and ing body of water (braid deltas). Coarse-grained guished in the rock record by distinctive sub- coarse sand) deltas (fan-deltas,* Holmes, 1965; deltas are distinguished from other deltas (the aerial components of these depositional short-headed deltas, Flores, 1975; or fjord- so-called "fine-grained deltas") principally by systems; the shoreline and subaqueous com- deltas, Kostaschuk, 1985) have been widely their coarse sediment size (mostly gravel and ponents of both are similar. Fan-delta se- quoted and invoked in the recent sedimentologic coarse sand). The independent variable of grain quences have a subaerial component that is literature. The current interest stems largely size is an important characteristic of deltas that an alluvial-fan facies comprising interbedded from their potential as hydrocarbon reservoirs, has not been accommodated within the gener- sheetflood, debris-flow, and braided-channel with examples in such high-interest exploration ally accepted classification schemes for deltas deposits. Fan-deltas produce small (a few targets as the North Sea (Harms and others, (see Fisher and others, 1969, and Galloway, tens of square kilometres), wedge-shaped 1981; Kessler and Moorhouse, 1984) and the 1975, for the classifications). bodies of sediment, commonly displaying North Slope of Alaska (Melvin and Knight, high variability in paleocurrent patterns and 1984; McGowen and Bloch, 1985). Recently, THE TERMINOLOGY PROBLEM abrupt changes in facies. The deposits are questions have been raised concerning usage of generally very coarse grained (with large out- the term "fan-delta," as well as recognition of Holmes (1965) defined a fan-delta as an al- sized clasts), very poorly sorted, matrix-rich, depositional facies and related processes of fan- luvial fan prograding directly into a standing polymictic, heterolithic, partially cemented by deltas and coarse-grained deltas in general body of water from an adjacent highland. De- penecontemporaneous carbonate, and have (Postma and Ori, 1984; McPherson and others, spite Holmes' clear geomorphic description of a low porosity and permeability. Braid-deltas, 1986). We believe that there has been a broad- fan-delta, it is apparent from a literature survey in contrast, have a subaerial component con- of the subject that his definition has been inter- sisting entirely of braided-river or braidplain preted in widely differing ways. The AGI Glos- facies. Their deposits display better sorting, sary of Geology (Bates and Jackson, 1980) roundness, and clast orientation than do fan- *In this paper, the term "fan-delta" is hyphenated. defines a fan-delta as "a gently sloping alluvial Geological Society of America Bulletin, v. 99, p. 331-340, 5 figs., 2 tables, September 1987. 331 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/3/331/3998307/i0016-7606-99-3-331.pdf by guest on 28 September 2021 332 MCPHERSON AND OTHERS deposit produced where a mountain stream Rust (1979), Hayward (1983), and Rust and typically deltaic features of the shoreline zone of flows out onto a lowland." Clearly this defini- Koster (1984) have suggested the term "coastal deltas, caused by reworking from waves and tion is both inadequate and misleading; it makes alluvial fan" as a substitute name for fan-delta longshore currents, characterize many fan-deltas no mention of the fundamental characteristic of on the basis of the fluvial dominance of fan-delta (and braid deltas) (Hayes and Michel, 1982; fan-deltas—that they comprise an alluvial fan systems. We suggest that the term "delta" is deposited into standing water. simply a reference to the outbuilding of a shore- Many workers have made the direct associa- line directly related to a sediment input, regard- FAN-DELTA tion of coarse-grained, bedload-dominated, del- less of the degree of basinal influences. The taic sedimentation with fan-deltas (McGowen, 1971; Erxleben, 1975; Galloway, 1976; Schumm, 1981; Vos, 1981; Hayes and Michel, Alluvial fan 1982; Galloway and Hobday, 1983; Postma and Roep, 1985). Galloway and Hobday (1983) de- Sea/Lake fined a fan-delta as forming "where braided riv- ers build into a standing body of water, and show a variable degree of peripheral modifica- tion." This definition removes any necessary as- sociation of a fan-delta with an alluvial fan and thus eliminates the very valuable diagnostic element of fan-deltas: that they are deposited immediately adjacent to a highland region (usu- ally a fault-bounded margin) and occupy a rela- BRAID DELTA tively narrow space between the highland and a Braided river standing body of water. We consider that this Figure 1. Schematic mod- broadening of the usage of the term "fan-delta" els to illustrate the deposi- to include all braided-river deposits in a deltaic tional settings of the two Sea/Lake setting is unnecessary and incorrect, and it is the coarse-grained delta types, principal source of confusion in the literature. including (A) a fan-delta; Nilsen (1985) suggests that the term "fan- (B) a braid delta that had its delta" is principally reserved for fan-shaped del- source in distant mountain- taic deposits, regardless of their source or ous uplands; (C) coalescing character. Nilsen (1985) stated: "fan deltas are braid deltas related to an ex- fan-shaped deltas built by rivers into standing tensive braidplain developed bodies of water without the presence of either an downslope of, but not neces- BRAID DELTAS adjacent highland or steep slopes." A fan-shaped sarily related to, mountain- delta, however, can be a product of many front alluvial fans; and Alluvial fans factors—for example, wave and current rework- (D) coalescing braid deltas Braidplain ing. In fact, the very term "delta" comes from a of fluvioglacial outwash plain comparison of the fan-shaped Nile delta with the (sandur) origin. Sea/Lake Greek character A. Fan-deltas are usually fan shaped, but not all fan-shaped deltas are fan- deltas. The plan-view shape of a delta should not be the sole criterion by which to define a fan-delta. Some of the esirly interpretations illustrated the general case of a fan-delta encompassing an alluvial fan with an associated, downdip BRAID DELTAS braided-river system (Brown and others, 1973, Glacier their Fig. 9; Brown and Fisher, 1977). This view Braidplain/Sandur of a fan-delta system has subsequently been widely employed. Sykes and Brand (1976) rec- ognized the potential dangers of such an inter- Sea/Lake pretation and stressed the importance of restrict-
Load more

Recommended publications
  • Alluvial Fans in the Death Valley Region California and Nevada
    Alluvial Fans in the Death Valley Region California and Nevada GEOLOGICAL SURVEY PROFESSIONAL PAPER 466 Alluvial Fans in the Death Valley Region California and Nevada By CHARLES S. DENNY GEOLOGICAL SURVEY PROFESSIONAL PAPER 466 A survey and interpretation of some aspects of desert geomorphology UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publications as follows: Denny, Charles Storrow, 1911- Alluvial fans in the Death Valley region, California and Nevada. Washington, U.S. Govt. Print. Off., 1964. iv, 61 p. illus., maps (5 fold. col. in pocket) diagrs., profiles, tables. 30 cm. (U.S. Geological Survey. Professional Paper 466) Bibliography: p. 59. 1. Physical geography California Death Valley region. 2. Physi­ cal geography Nevada Death Valley region. 3. Sedimentation and deposition. 4. Alluvium. I. Title. II. Title: Death Valley region. (Series) For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS Page Page Abstract.. _ ________________ 1 Shadow Mountain fan Continued Introduction. ______________ 2 Origin of the Shadow Mountain fan. 21 Method of study________ 2 Fan east of Alkali Flat- ___-__---.__-_- 25 Definitions and symbols. 6 Fans surrounding hills near Devils Hole_ 25 Geography _________________ 6 Bat Mountain fan___-____-___--___-__ 25 Shadow Mountain fan..______ 7 Fans east of Greenwater Range___ ______ 30 Geology.______________ 9 Fans in Greenwater Valley..-----_____. 32 Death Valley fans.__________--___-__- 32 Geomorpholo gy ______ 9 Characteristics of fans.._______-___-__- 38 Modern washes____.
    [Show full text]
  • Geomorphic Classification of Rivers
    9.36 Geomorphic Classification of Rivers JM Buffington, U.S. Forest Service, Boise, ID, USA DR Montgomery, University of Washington, Seattle, WA, USA Published by Elsevier Inc. 9.36.1 Introduction 730 9.36.2 Purpose of Classification 730 9.36.3 Types of Channel Classification 731 9.36.3.1 Stream Order 731 9.36.3.2 Process Domains 732 9.36.3.3 Channel Pattern 732 9.36.3.4 Channel–Floodplain Interactions 735 9.36.3.5 Bed Material and Mobility 737 9.36.3.6 Channel Units 739 9.36.3.7 Hierarchical Classifications 739 9.36.3.8 Statistical Classifications 745 9.36.4 Use and Compatibility of Channel Classifications 745 9.36.5 The Rise and Fall of Classifications: Why Are Some Channel Classifications More Used Than Others? 747 9.36.6 Future Needs and Directions 753 9.36.6.1 Standardization and Sample Size 753 9.36.6.2 Remote Sensing 754 9.36.7 Conclusion 755 Acknowledgements 756 References 756 Appendix 762 9.36.1 Introduction 9.36.2 Purpose of Classification Over the last several decades, environmental legislation and a A basic tenet in geomorphology is that ‘form implies process.’As growing awareness of historical human disturbance to rivers such, numerous geomorphic classifications have been de- worldwide (Schumm, 1977; Collins et al., 2003; Surian and veloped for landscapes (Davis, 1899), hillslopes (Varnes, 1958), Rinaldi, 2003; Nilsson et al., 2005; Chin, 2006; Walter and and rivers (Section 9.36.3). The form–process paradigm is a Merritts, 2008) have fostered unprecedented collaboration potentially powerful tool for conducting quantitative geo- among scientists, land managers, and stakeholders to better morphic investigations.
    [Show full text]
  • Hydrogeomorphic Processes and Torrent Control Works on a Large Alluvial Fan in the Eastern Italian Alps
    Nat. Hazards Earth Syst. Sci., 10, 547–558, 2010 www.nat-hazards-earth-syst-sci.net/10/547/2010/ Natural Hazards © Author(s) 2010. This work is distributed under and Earth the Creative Commons Attribution 3.0 License. System Sciences Hydrogeomorphic processes and torrent control works on a large alluvial fan in the eastern Italian Alps L. Marchi1, M. Cavalli1, and V. D’Agostino2 1Consiglio Nazionale delle Ricerche – Istituto di Ricerca per la Protezione Idrogeologica (CNR IRPI), Padova, Italy 2Dipartimento Territorio e Sistemi Agroforestali, Universita` di Padova, Legnaro (Padova), Italy Received: 17 November 2009 – Revised: 26 February 2010 – Accepted: 8 March 2010 – Published: 23 March 2010 Abstract. Alluvial fans are often present at the outlet of 1 Introduction small drainage basins in alpine valleys; their formation is due to sediment transport associated with flash floods and Alluvial fans at the outlet of small, high-gradient drainage debris flows. Alluvial fans are preferred sites for human set- basins are a common feature of alpine valleys. Although de- tlements and are frequently crossed by transport routes. In bris flows commonly dominate the formation and develop- order to reduce the risk for economic activities located on or ment of these alluvial fans, also bedload and hyperconcen- near the fan and prevent loss of lives due to floods and de- trated flows contribute to the transfer of sediment from the bris flows, torrent control works have been extensively car- drainage basin to the alluvial fan and its distribution on the ried out on many alpine alluvial fans. Hazard management fan surface. These flow processes result in major risk when on alluvial fans in alpine regions is dependent upon reliable they encroach settlements and transport routes, which are of- procedures to evaluate variations in the frequency and sever- ten present on the alluvial fans of the European Alps.
    [Show full text]
  • Moving Water Shapes Land
    KEY CONCEPT Moving water shapes land. BEFORE, you learned NOW, you will learn •Erosion is the movement of • How moving water shapes rock and soil Earth’s surface • Gravity causes mass movements • How water moving under- of rock and soil ground forms caves and other features VOCABULARY EXPLORE Divides drainage basin p. 579 How do divides work? divide p. 579 floodplain p. 580 PROCEDURE MATERIALS alluvial fan p. 581 •sheet of paper 1 Fold the sheet of paper in thirds and tape delta p. 581 • tape it as shown to make a “ridge.” sinkhole p. 583 •paper clips 2 Drop the paper clips one at a time directly on top of the ridge from a height of about 30 cm. Observe what happens and record your observations. WHAT DO YOU THINK? How might the paper clips be similar to water falling on a ridge? Streams shape Earth’s surface. If you look at a river or stream, you may be able to notice something about the land around it. The land is higher than the river. If a river is running through a steep valley, you can easily see that the river is the low point. But even in very flat places, the land is sloping down to the river, which is itself running downhill in a low path through the land. NOTE-TAKING STRATEGY Running water is the major force shaping the landscape over most A main idea and detail of Earth. From the broad, flat land around the lower Mississippi River notes chart would be a good strategy to use for to the steep mountain valleys of the Himalayas, water running downhill taking notes about streams changes the land.
    [Show full text]
  • Alluvial Fans
    GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111 Lecture Note Series Sedimentary Environments 1: Alluvial Fans Lecture Goals A) Depositional/Sedimentary Environments B) Alluvial fan depositional environments C) Sediment and rocks that form on alluvial fans Reference: Press et al., 2004, Chapter 7; Grotzinger et al., 2007, Chapter 18, p 449 GY 111 Lab manual Chapter 3 Note: At this point in the course, my version of GY 111 starts to diverge a bit from my colleagues. I tend to focus a bit more on sedimentary processes then they do mostly because we live in an area that is dominated by sedimentation and I figure that you should be as familiar as possible with the subject. As it turns out, the web notes are also more comprehensive than what you'll find in your text book A) Depositional/sedimentary environments Last time we met, we discussed how sediment moved from one place to another. Remember that sediment is produced in a lot of different locations, but it seldom stays where it is produced. The action of water, wind and ice transport it from the sediment source to the sediment sink. The variety of sediment sinks that exist on the planet is truly amazing. There are river basins, lakes, deserts, lagoons, swamps, deltas, beaches, barrier islands, reefs, continental shelves, the abyssal plains (very deep!), trenches (even deeper!) etc. As we discussed last time, these places are called depositional environments (also known as sedimentary environments). Each depositional environment may also have several subdivisions. For example, there are open beaches, sheltered beaches, shingle beaches, sand beaches, strandline beaches, even mud beaches.
    [Show full text]
  • Flooding Dynamics in a Large Low-Gradient Alluvial Fan, the Okavango Delta, Botswana, from Analysis and Interpretation of a 30-Year Hydrometric Record
    Hydrology and Earth System Sciences, 10, 127–137, 2006 www.copernicus.org/EGU/hess/hess/10/127/ Hydrology and SRef-ID: 1607-7938/hess/2006-10-127 Earth System European Geosciences Union Sciences Flooding dynamics in a large low-gradient alluvial fan, the Okavango Delta, Botswana, from analysis and interpretation of a 30-year hydrometric record P. Wolski and M. Murray-Hudson Harry Oppenheimer Okavango Research Centre, Maun, Botswana Received: 11 August 2005 – Published in Hydrology and Earth System Sciences Discussions: 6 September 2005 Revised: 29 November 2005 – Accepted: 3 January 2006 – Published: 21 February 2006 Abstract. The Okavango Delta is a flood-pulsed wetland, tem, and the flood frequency distribution itself can change, which supports a large tourism industry and the subsistence compared to that at upstream locations (Wolff and Burges, of the local population through the provision of ecosystem 1994). The ecological role of the channel-floodplain interac- services. In order to obtain insight into the influence of var- tion is expressed by the flood pulse concept (Junk et al., 1989; ious environmental factors on flood propagation and distri- Middleton, 1999). According to this concept, floodplain wet- bution in this system, an analysis was undertaken of a 30- lands and riparian ecosystems adjust to, and are maintained, year record of hydrometric data (discharges and water lev- by the pulsing of water, sediment and nutrients that occurs els) from one of the Delta distributaries. The analysis re- during over-bank flow conditions. Odum (1994) addition- vealed that water levels and discharges at any given channel ally identifies flood pulsing at various spatial and temporal site in this distributary are influenced by a complex interplay scales as an energy subsidy to wetland ecosystems, explain- of flood wave and local rainfall inputs, modified by channel- ing in part the high ecological productivity associated with floodplain interactions, in-channel sedimentation and techni- such systems.
    [Show full text]
  • Alluvial Fan Hazards & Design Issues for Design
    Presentation Program Outline Overview Countermeasures / Planning Process Levees / Dikes / Diversions Channelization / Conveyance Grade Control Structures Detention Basin / Debris Basin Case Study – Localized Subdivision Protection (THOUSAND PALMS, CA) Case Study – Whole Fan Facilities (INDIAN WELLS, CA) Structural Countermeasures Overview / Planning Process & Design Considerations Alluvial Fan Hazards & Design Issues for Design • Uncertainty of flow depths (R&U analysis) • Inundation extents / flow direction / impingement • Sediment deposition • Scouring and undermining • Impact forces • Channel avulsions and entrenchments • Hydrostatic and buoyant forces • High velocities • Unpredictable flow path (R&U analysis) • Flooding from both debris and water flows “Riverine” vs. “Alluvial Fan” - Structural Countermeasure Design Issues / Considerations • Flow uncertainty • Velocity • Flow duration • Sediment deposition • Seepage control • Impingement • Flow direction and path uncertainty Alluvial Fan Riverine “Whole Fan” Solutions vs. Localized Protection – Structural Countermeasures “Whole Fan” Solutions vs. Localized Protection – Structural Countermeasures Structural Countermeasures for Alluvial Fans – Basic Building Blocks Collection Channels Conveyance Channels Dispersion Channels Structural Countermeasures for Alluvial Fans – Basic Building Blocks - Example Standard Alluvial Fan Structural Countermeasures Effectiveness of Alluvial Fan Structural Countermeasures for Different Hazards Structural Countermeasure – General Design Considerations
    [Show full text]
  • Flooding Dynamics in a Large Low-Gradient Alluvial Fan, the Okavango Delta, Botswana, from Analysis and Interpretation of a 30-Year Hydrometric Record
    Hydrol. Earth Sys. Sci. Discuss., 2, 1865–1892, 2005 Hydrology and www.copernicus.org/EGU/hess/hessd/2/1865/ Earth System SRef-ID: 1812-2116/hessd/2005-2-1865 Sciences European Geosciences Union Discussions Papers published in Hydrology and Earth System Sciences Discussions are under open-access review for the journal Hydrology and Earth System Sciences Flooding dynamics in a large low-gradient alluvial fan, the Okavango Delta, Botswana, from analysis and interpretation of a 30-year hydrometric record P. Wolski and M. Murray-Hudson Harry Oppenheimer Okavango Research Centre, Maun, Botswana Received: 11 August 2005 – Accepted: 22 August 2005 – Published: 6 September 2005 Correspondence to: P. Wolski ([email protected]) © 2005 Author(s). This work is licensed under a Creative Commons License. 1865 Abstract The Okavango Delta is a flood-pulsed wetland, the resources of which support a large tourism industry and subsistence of the local population. In order to obtain an insight into the influence of various environmental factors on flood propagation and distribution 5 in this system, an analysis was undertaken of a 30-year record of hydrometric data (dis- charges and water levels) from one of the Delta distributaries. The analysis revealed that water levels and discharges at any given channel site in the analysed distributary are influenced by a complex interplay of flood wave and local rainfall input modified by channel-floodplain interactions, in-channel sedimentation and technical interventions, 10 both at the given site and upstream. Additionally, cyclical variation of channel margin vegetation due to nutrients recycling might play a role. It was shown that data from channels do not adequately represent flood dynamics and its change at the distribu- tary level.
    [Show full text]
  • Geomorphology, Hydrology, and Ecology of Great Basin Meadow
    Chapter 2: Controls on Meadow Distribution and Characteristics Dru Germanoski, Jerry R. Miller, and Mark L. Lord Introduction precipitation values are recorded in the central and southern Toiyabe and southern Toquima Mountains (fig. 1.4) and, as eadow complexes are located in distinct geomorphic expected, meadow complexes are abundant in those areas. Mand hydrologic settings that allow groundwater to be In contrast, basins located in sections of mountain ranges at or near the ground surface during at least part of the year. that do not have significant area at high elevation (greater Meadows are manifestations of the subsurface flow system, than approximately 2500 m) and that are characterized by and their distribution is controlled by factors that cause lo- limited annual precipitation contain relatively few meadows. calized zones of groundwater discharge. Knowledge of the For example, much of the Shoshone Range and the north- factors that serve as controls on groundwater discharge and ern and southernmost portions of the Toiyabe, Toquima, and formation of meadow complexes is necessary to understand Monitor Ranges are lower in elevation and receive less pre- why meadows occur where they do and how anthropogenic cipitation. Therefore, meadow complexes are uncommon in activities might affect their persistence and ecological condi- these regions. Analyses of color-enhanced satellite images tion. In this chapter, we examine physical factors that lead to of the region reveal that meadow complexes also are uncom- the formation of meadow complexes in upland watersheds mon in low-elevation mountain ranges in the surrounding of the central Great Basin. We then describe variations in area, including the Pancake Range, Antelope Range, the meadow characteristics across the region and parameters Park Range, and Simpson Park Mountains.
    [Show full text]
  • Fact Sheet 2: What Is an Alluvial
    FEMA Risk MAP Style Guide What Is An Alluvial Fan? FOR NEFF’S CREEK MAY 2016 WHAT IS AN ALLUVIAL FAN? An alluvial fan is a fan-shaped area where silt, sand, gravel, boulders, and woody debris are deposited by rivers and streams over a long period of time. Alluvial fans are created as flowing water interacts with mountains, hills, or steep canyon walls. Sediment and debris can be deposited over time by powerful rivers or small creeks. The top, or narrow point, of the alluvial fan is called the “apex,” and the wider portion is called the “apron” or “cone.” Alluvial fans can be small or large, depending on the historical water flows. Alluvial fans formed on a steep slope, like the one at Neff’s Creek, are narrow, and thus more cone than fan-shaped. 8 Canyon Apex Cone Alluvial Fan (Potential Flow Paths) COMMON IN THE WESTERN UNITED STATES In the United States, alluvial fans are typically found along the base of mountain fronts in Utah, California, Arizona, Nevada, Colorado, Idaho, New Mexico, Wyoming, Montana, Oregon, and Washington. Here, infrequent but intense storms typical of arid climates, plus abrupt changes in topography, create the necessary conditions for alluvial fan formation. FEMA Risk MAP Style Guide HOW ALLUVIAL FANS ARE IDENTIFIED Alluvial fans can be difficult to identify by the untrained eye. Many homeowners don’t realize the panoramic views of the valley below are due to their elevated location on an alluvial fan. They may also be unaware that the large boulders which dot their landscape were once carried downhill by powerful floodwaters.
    [Show full text]
  • Effects of Post-Wildfire Erosion on Channel Environments, Boise River, Idaho
    Effects of Post-Wildfire Erosion on Channel Environments, Boise River, Idaho Lee Benda1, Daniel Miller2, Paul Bigelow1, Kevin Andras1 Manuscript accepted to Forest Ecology and Management, to be cited as “in press” Prepublication copy subject to some correction or change. 1 Earth Systems Institute, California Office 310 N Mt Shasta Blvd, Suite 6, Mt Shasta, CA 96067, USA 530-926-1066 phone [email protected] 2 Earth Systems Institute, Washington Office 3040 NW 57th St., Seattle, WA 98107, USA 1 Abstract What is the geological or ecological context that earth scientists, biologists, and resource managers use to understand large-scale watershed disturbances, such as fires, mass wasting, and floods? We address this question using a field study of post-fire channel changes in the Boise River basin in central Idaho based on surveys of over 27 km of channels. Intense rill and gully erosion from the Rabbit Creek fire (1995) greatly increased sediment supply to numerous third- through sixth-order valley floors. We concentrated our field study where recently aggraded and enlarged alluvial fans impinged on channels in drainage areas of 100 - 350 km2. Alluvial fans that had enlarged because of post-fire sedimentation triggered a number of morphological changes in channels and valley floors. Alluvial fans created nick points in receiving channels that caused an increase in channel gradient immediately downstream of fans and a decrease in channel gradients upstream of fans for distances up to 4 km. Wide floodplains, side channels, and the beginning of terrace construction were associated with increased sediment storage in proximity to aggraded fans.
    [Show full text]
  • Alluvial Fan
    R E S O U R C E L I B R A R Y E N C Y C L O P E D I C E N T RY Alluvial fan An alluvial fan is a triangle-shaped deposit of gravel, sand, and smaller materials called alluvium. G R A D E S 4 - 12+ S U B J E C T S Earth Science, Geology, Geography, Physical Geography C O N T E N T S 5 Images For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/alluvial-fan/ An alluvial fan is a triangle-shaped deposit of gravel, sand, and even smaller pieces of sediment, such as silt. This sediment is called alluvium. Alluvial fans are usually created as flowing water interacts with mountains, hills, or the steep walls of canyons. Streams carrying alluvium can be trickles of rainwater, a fast-moving creek, a powerful river, or even runoff from agriculture or industry. As a stream flows down a hill, it picks up sand and other particles—alluvium. The rushing water carries alluvium to a flat plain, where the stream leaves its channel to spread out. Alluvium is deposited as the stream fans out, creating the familiar triangle-shaped feature. The narrow point of the alluvial fan is called its apex, while the wide triangle is the fan's apron. Alluvial fans can be tiny, with an apron of just a few centimeters spreading out from the trickle of a drainpipe. They can also be enormous. Over time, water flowing down the Koshi River in Nepal, for example, has built up an alluvial fan more than 15,000 square kilometers (almost 5,800 square miles) wide.
    [Show full text]