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Fluvial Landforms Floodplains, Terraces, Deltas, and Alluvial Fans Rio Terraba, Costa Rica. Foto: Lachniet (2004)

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Presentation on theme: "Fluvial Landforms Floodplains, Terraces, Deltas, and Alluvial Fans Rio Terraba, Costa Rica. Foto: Lachniet (2004)"— Presentation transcript:

1 Fluvial Landforms Floodplains, Terraces, Deltas, and Alluvial Fans Rio Terraba, Costa Rica. Foto: Lachniet (2004)

2 Floodplains I) Vertical Accretion via overbank flow – 1) Flood stage – 2) water velocity decreases – 3) Sediment settles out Coarsest near river, finer farther away, creates natural levees

3 Natural Levees From Hamblin, The Earth’s Dynamic Systems.

4 Natural Levee From Hamblin, The Earth’s Dynamic Systems.

5 Floodplains II) Lateral Accretion – Meander migration Bank erosion Point bar deposition Point Bar Deposits Overbank Deposits From Ritter et al., Process Geomorphology, Fourth Edition

6 Lateral Accretion Landforms – Meander scrolls: old meander topography (aka “bar and swale”) now dry – Meander cutoffs Old meander channels no longer carrying main flow, but still filled with river water – Oxbow lakes: Old meander channels now isolated from channel and containing standing water; contains fine sediments and clay plugs

7 Meander scroll topography From Hamblin, The Earth’s Dynamic Systems.

8 Braided River landforms “Braids” = multiple channels formed in weak non-cohesive sediment Braid bars and islands = zones of deposition, formed during high flow; may be stabilized by vegetation if they are old Splays and chutes: ‘shortcuts’ across a bar or Island; chutes are larger Terrace: former river levels formed prior to river incision Braid Island Active channel Terrace Multiple channels (“braids”) Splay Braid bars Chute Copper River, at Chitina, Alaska (Lachniet, 2009)

9 Cyclic Stream Terraces Terraces are abandoned floodplains Mark older relative high water level Form due to – 1) uplift – 2) base-level lowering – 3) climatic change Erosional or depositional

10 From Hamblin, The Earth’s Dynamic Systems. Terrace Formation I

11 Terrace Formation II From Hamblin, The Earth’s Dynamic Systems.

12 Figure 7-14 Paired and unpaired Paired = terraces on each side of valley at the same altitude and formed at the same time Unpaired = Not the above

13 Stream terraces in Furnace Creek Wash, Death Valley National Park With Alex Roy, photo by Lachniet, 2007 Note former stream bed of graded channel Notch cut into bedrock lowered base level Incision into stream bed resulted in terraces Flooding to Furnace Creek Fan was alleviated

14 Copyright © Matthias Jakob 2002 Ancient fluvial terraces in Mustang, Nepal.

15 Stripped Structural Surfaces Selective stripping of weak rocks from resistant rocks NOT TO BE CONFUSED WITH TERRACES Profile of surfaces unrelated to river profile AKA “Cliff and Bench” topography

16 Stripped Structural Surface Not terraces even though the may look like it! Surfaces defined by bedrock orientation, does not slope like the stream

17 Deltas Deposition occurs as velocity decreases where water leaves confined channel Upper delta surface = water level Classified based on morphology and process (net deposition or degradation)

18 Barbados sea level Sea Level reached near modern level by ca to 5000 yr BP ALL major deltas visible on the planet are thus young

19 Constructional Deltas  Fluvial Activity dominant process  Lobate: Classic delta shape Numerous distributaries Nile River  Elongate or ‘birds foot’ Fewer distributaries Finer grained Modern Mississippi Delta

20 Lobate Delta Landsat Image Path: 79Row: 16Date: October 2, 2000 Location: Bering Straight, Alaska

21 Mississippi River delta Landsat image

22 Delta Beds and Morphology From Easterbrook, Surface Processes and Landforms, second edition. Delta Plain Pro Delta Delta Slope  Upper delta plain – entirely fluvial  Lower delta plain – modified by tides Tidal flats, mangroves, marshes  Delta slope – deposition of fluvial sediment  Pro delta – deposition of marine or lacustrine sediment

23 Delta Evolution Controlled by base level changes Avulsion – Channel abandonment to take a shorter route to the ocean BIG problem with the Mississippi River – Atchafalya River would avulse and capture the main Mississippi River flow if not controlled by humans

24 Figures 7-38 and 7-39

25 Piedmonts Sloping surface that connects mountains to intervening flat plains Usually consist of planar eroded bedrock surfaces called pediments And aggradational alluvial fans From Bloom. Geomorphology, 2nd Edition

26 Alluvial Fans Most common in arid to semi-arid environments Also found in humid glacial, humid tropical, and humid temperate environments Characterized by fan (or cone) shape radiating outward from a central point Deposits reflects net aggradation as channel gradient decreases upon leaving mountain

27 Type I: Debris Flow Alluvial Fans Form in areas with a low water/sediment ratio (w/s) Debris flow dominant – Flow within channels, and leave well-defined margins with distinct ridges Intermittent flow and movement on the fan, with recurrence intervals of 1-50 yr 5 to 15 o slopes Most common in arid environments

28 Type I Alluvial Fan Black Mountains, near Badwater, Death Valley. Foto: Lachniet (2004)

29 Debris Flow morphology Fig portions to show morphology of debris flow deposits on fans From Ritter et al., Process Geomorphology, Fourth Edition

30 Debris flow levees, Death Valley

31 Debris flow fan in Death Valley

32 Type II: Sheetflood Alluvial Fans Common in humid areas with high w/s ratios – E.g., glaciated landscapes in Alaska, or other humid areas Fluvial flow and sheetfloods dominant process Constant to seasonal recurrence intervals 2 to 8 o slopes Further from mountain front Braided/ephemeral streams primary depositional process

33 Table 7-3

34 Copyright © Ron Dorn 2002 Type II alluvial fan: Warm and dry environment

35 Copyright © Norm Catto 2002 Type II Alluvial Fan: Cold and Humid environment Alluvial Fan - Snake River, Yukon, August 1982.

36 Bajada Coalesced alluvial fans forming an apron Bajada on E slope of Panamint Mountains, Death Valley, CA. Foto: Lachniet (2003)

37 Alluvial Fan Morphology Apex Feeder Channel Fanhead Trench Incised channel Intersection point Active depositional lobe

38 Fig A Feeder Channel Apex Incised Channel Intersection point: Where active lobe elevation =inactive lobe elevation

39 Fig B Humid-type alluvial fan

40 Formed in eroding dune sand, beach along Lake Michigan. Foto: Lachniet (1994) Miniature Alluvial Fan

41 Lobes Active – Distributary drainage Single channel diverges into multiple channels Inactive – Tributary drainage Classic dendritic drainage – Gullies formed by rainfall that don’t head in the mountains above the fan – Often separated from mountain front: “beheaded fan”

42 Tributary Drainage – Black Mountains front, Death Valley CA Inactive lobe Active lobe

43 Tributary Drainage – the Big Dip, Death Valley National Park, CA

44 Distributary Drainage Panamint Mountains Bajada Death Valley, CA

45 Fan Evolution Climate change is dominant control on fan evolution Tectonics is secondary Most fan surfaces have inactive lobes And fans can undergo net aggradation or incision depending on climate change – Wet = aggradation via increased debris flow – Dry = incision due to decreased sediment delivery

46 Copyright © Ron Dorn 2002 Fan Evolution GE: Warm Springs Canyon Fan, Death Valley N.P.


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