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Floodplains, Terraces, Deltas, and Alluvial Fans

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Presentation on theme: "Floodplains, Terraces, Deltas, and Alluvial Fans"— Presentation transcript:

1 Floodplains, Terraces, Deltas, and Alluvial Fans
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, 1989. The Earth’s Dynamic Systems.

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

5 Floodplains II) Lateral Accretion Meander migration Bank erosion
Point bar deposition Overbank Deposits Point Bar 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 Multiple channels (“braids”) Terrace Active channel Splay Braid Island 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 Terrace Formation I From Hamblin, The Earth’s Dynamic Systems.

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

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

13 Stream terraces in Furnace Creek Wash, Death Valley National Park
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 With Alex Roy, photo by Lachniet, 2007

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

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 Ritter

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
Delta Plain Pro Delta Delta Slope From Easterbrook, Surface Processes and Landforms, second edition. 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 Easterbrook

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 15o 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
Stephen Hlojwski, 2004 Death Valley Field Trip

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 8o slopes Further from mountain front Braided/ephemeral streams primary depositional process

33 Table 7-3

34 Warm and dry environment
Type II alluvial fan: Warm and dry environment Two views of Badwater alluvial fan (exiting from Bad Canyon).  Main oblique photo taken by Ron Dorn.  The inset view shows a smaller-scaled view image (source: U.S. Geological Survey Open File Report 01-51). Note active faulting at the fan head, an important reason why deposition occurs all over the surface of the fan.  Also note grabens on the lower left side of the imagery.  The parking lot for Badwater, the lowest subaerial elevation in the western hemisphere, is in the upper left hand part of the image. Copyright © Ron Dorn 2002

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

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. 7.20 A Feeder Channel Apex Incised Channel Intersection point:
Where active lobe elevation =inactive lobe elevation

39 Humid-type alluvial fan
Fig B

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

41 Lobes Active Inactive Distributary drainage Tributary 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 Fan Evolution GE: Warm Springs Canyon Fan, Death Valley N.P.
Warm Springs Canyon Fan: Exiting the eastern side of the titled Panamint Range, this fan illustrates  several classic aspects of alluvial-fan morphogenesis. First, the upper sections of the fan have been faulted, resulting in "telescoping" of the younger fan deposits. The faulting is most easily seen in the mid-left side of the fan as a series of multiple fault scarps.  Second, the older (upper) fan deposits have gone from depositional landforms to erosional landforms -- developing the "ballena" topography of a series of rounded interfluves.  Note how the drainage developed on those interfluves is tributary, in contrast to the distributary drainage on the younger (lower) section.  Third, the color of fan surfaces can be seen going from light to dark to salt and pepper.  Light (most recent deposits in the active channel) to dark (late Pleistocene, well-varnished desert pavements , seen in the lower left and mid-right) to salt and pepper (mid-Pleistocene, lighter, eroded fan units where the eroding calcrete provides bright spots that intermingle with remnants of desert pavements). Fourth, alluvial-fan hazards in the southwestern United States are concentrated where the fan has "telescoped" and active deposition occurs in the lowest portions of the fan complex. GE: Warm Springs Canyon Fan, Death Valley N.P. Copyright © Ron Dorn 2002

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