fluvial landforms floodplains, terraces, deltas, and alluvial fans rio terraba, costa rica. foto:...

Fluvial LandformsFloodplains, Terraces, Deltas, and Alluvial Fans

Rio Terraba, Costa Rica. Foto: Lachniet (2004)

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

Natural Levees

From Hamblin, 1989. The Earth’s Dynamic Systems.

Natural Levee


Floodplains

• II) Lateral Accretion–Meander migration • Bank erosion • Point bar deposition

Point Bar Deposits

Overbank Deposits

From Ritter et al., 2002. Process Geomorphology, Fourth Edition

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

Meander scroll topography


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

TerraceMultiple channels (“braids”)

Splay

Braid bars

Chute

Copper River, at Chitina, Alaska (Lachniet, 2009)

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


Terrace Formation I

Terrace Formation II


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

Stream terraces in Furnace Creek Wash, Death Valley National Park

With Alex Roy, photo by Lachniet, 2007

Note former stream bed of graded channelNotch cut into bedrock lowered base levelIncision into stream bed resulted in terracesFlooding to Furnace Creek Fan was alleviated

Copyright © Matthias Jakob 2002

Ancient fluvial terraces in Mustang, Nepal.

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

Stripped Structural Surface

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

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)

Barbados sea level

• Sea Level reached near modern level by ca. 8000 to 5000 yr BP

• ALL major deltas visible on the planet are thus young

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

Lobate Delta Landsat Image

http://www.landsat.org/landsat_gallery/P79R16D100200.html

Path: 79Row: 16Date: October 2, 2000 Location: Bering Straight, Alaska

Mississippi River delta Landsat image

http://www.landsat.org/landsat_gallery/P22R39D122200.html

Delta Beds and Morphology

From Easterbrook, 1999. Surface Processes and Landforms, second edition.

Delta Plain

Pro DeltaDelta 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

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

Figures 7-38 and 7-39

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

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

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

Type I Alluvial Fan

Black Mountains, near Badwater, Death Valley. Foto: Lachniet (2004)

Debris Flow morphology

• Fig. 7.24 portions to show morphology of debris flow deposits on fans

From Ritter et al., 2002. Process Geomorphology, Fourth Edition

Debris flow levees, Death Valley

Debris flow fan in Death Valley

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

Table 7-3

Bajada Coalesced alluvial fans forming an apron

Bajada on E slope of Panamint Mountains, Death Valley, CA. Foto: Lachniet (2003)

Alluvial Fan Morphology

• Apex• Feeder Channel• Fanhead Trench• Incised channel• Intersection point• Active depositional lobe

• Fig. 7.20 A

Feeder Channel

Apex

Incised Channel

Intersection point:Where active lobe elevation=inactive lobe elevation

• Fig. 7-20 B

Humid-type alluvial fan

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

Miniature Alluvial Fan

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”

Tributary Drainage – Black Mountains front, Death Valley CA

Inactive lobe

Active lobe

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

Distributary Drainage

Panamint Mountains

BajadaDeath Valley, CA

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

fluvial landforms floodplains, terraces, deltas, and alluvial fans rio terraba, costa rica. foto:...

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