Presentation on theme: "Floodplains, Terraces, Deltas, and Alluvial Fans"— Presentation transcript:
1Floodplains, Terraces, Deltas, and Alluvial Fans Fluvial LandformsFloodplains, Terraces, Deltas, and Alluvial FansRio Terraba, Costa Rica. Foto: Lachniet (2004)
2Floodplains I) Vertical Accretion via overbank flow 1) Flood stage 2) water velocity decreases3) Sediment settles outCoarsest near river, finer farther away, creates natural levees
3Natural Levees From Hamblin, 1989. The Earth’s Dynamic Systems.
4Natural LeveeHamblinFrom Hamblin, The Earth’s Dynamic Systems.
5Floodplains II) Lateral Accretion Meander migration Bank erosion Point bar depositionOverbank DepositsPoint Bar DepositsFrom Ritter et al., Process Geomorphology, Fourth Edition
6Lateral Accretion Landforms Meander scrolls:old meander topography (aka “bar and swale”) now dryMeander cutoffsOld meander channels no longer carrying main flow, but still filled with river waterOxbow lakes:Old meander channels now isolated from channel and containing standing water; contains fine sediments and clay plugs
7Meander scroll topography From Hamblin, The Earth’s Dynamic Systems.
8Braided River landforms “Braids” = multiple channels formed in weak non-cohesive sedimentBraid bars and islands = zones of deposition, formed during high flow; may be stabilized by vegetation if they are oldSplays and chutes: ‘shortcuts’ across a bar or Island; chutes are largerTerrace: former river levels formed prior to river incisionMultiple channels (“braids”)TerraceActive channelSplayBraid IslandBraid barsChuteCopper River, at Chitina, Alaska (Lachniet, 2009)
9Cyclic Stream Terraces Terraces are abandoned floodplainsMark older relative high water levelForm due to1) uplift2) base-level lowering3) climatic changeErosional or depositional
10Terrace Formation IFrom Hamblin, The Earth’s Dynamic Systems.
11Terrace Formation IIFrom Hamblin, The Earth’s Dynamic Systems.
12Paired and unpairedPaired = terraces on each side of valley at the same altitude and formed at the same timeUnpaired = Not the aboveFigure 7-14
13Stream terraces in Furnace Creek Wash, Death Valley National Park Note former stream bed of graded channelNotch cut into bedrock lowered base levelIncision into stream bed resulted in terracesFlooding to Furnace Creek Fan was alleviatedWith Alex Roy, photo by Lachniet, 2007
22Delta Beds and Morphology Delta PlainPro DeltaDelta SlopeFrom Easterbrook, Surface Processes and Landforms, second edition.Upper delta plain – entirely fluvialLower delta plain – modified by tidesTidal flats, mangroves, marshesDelta slope – deposition of fluvial sedimentPro delta – deposition of marine or lacustrine sedimentEasterbrook
23Delta Evolution Controlled by base level changes Avulsion Channel abandonment to take a shorter route to the oceanBIG problem with the Mississippi RiverAtchafalya River would avulse and capture the main Mississippi River flow if not controlled by humans
25PiedmontsSloping surface that connects mountains to intervening flat plainsUsually consist of planar eroded bedrock surfaces called pedimentsAnd aggradational alluvial fansFrom Bloom. Geomorphology, 2nd Edition
26Alluvial Fans Most common in arid to semi-arid environments Also found in humid glacial, humid tropical, and humid temperate environmentsCharacterized by fan (or cone) shape radiating outward from a central pointDeposits reflects net aggradation as channel gradient decreases upon leaving mountain
27Type I: Debris Flow Alluvial Fans Form in areas with a low water/sediment ratio (w/s)Debris flow dominantFlow within channels, and leave well-defined margins with distinct ridgesIntermittent flow and movement on the fan, with recurrence intervals of 1-50 yr5 to 15o slopesMost common in arid environments
28Type I Alluvial FanBlack Mountains, near Badwater, Death Valley. Foto: Lachniet (2004)
29Debris Flow morphology Fig portions to show morphology of debris flow deposits on fansFrom Ritter et al., Process Geomorphology, Fourth Edition
30Debris flow levees, Death Valley Stephen Hlojwski, 2004 Death Valley Field Trip
32Type II: Sheetflood Alluvial Fans Common in humid areas with high w/s ratiosE.g., glaciated landscapes in Alaska, or other humid areasFluvial flow and sheetfloods dominant processConstant to seasonal recurrence intervals2 to 8o slopesFurther from mountain frontBraided/ephemeral streams primary depositional process
40Miniature Alluvial Fan Formed in eroding dune sand, beach along Lake Michigan. Foto: Lachniet (1994)
41Lobes Active Inactive Distributary drainage Tributary drainage Single channel diverges into multiple channelsInactiveTributary drainageClassic dendritic drainageGullies formed by rainfall that don’t head in the mountains above the fanOften separated from mountain front: “beheaded fan”
42Tributary Drainage – Black Mountains front, Death Valley CA Inactive lobeActive lobe
43Tributary Drainage – the Big Dip, Death Valley National Park, CA
44Distributary Drainage Panamint Mountains Bajada Death Valley, CA
45Fan Evolution Climate change is dominant control on fan evolution Tectonics is secondaryMost fan surfaces have inactive lobesAnd fans can undergo net aggradation or incision depending on climate changeWet = aggradation via increased debris flowDry = incision due to decreased sediment delivery