PART V: FLUVIAL LANDFORMS. Missouri River at Decision Point, MT

Transcription

1 PART V: FLUVIAL LANDFORMS Missouri River at Decision Point, MT

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3 Floodplains relatively flat surfaces occupying a portion of the valley bottom; normally they are underlain by unconsolidated alluvium, & covered every 1-2 yrs by flood waters alluvial veneer tends to fine upward, & away from the channel floodplains are formed by a combination of within-channel & overbank deposition, accumulating through lateral & vertical accretion

4 Kowanyama River, n Australia (Jeff Shellberg)

5 within-channel deposits occur on point bars chutes sloughs sand ridges channel fill overbank deposits occur as splays levees backswamps colluvium vertical & lateral accretion occurs through natural levees direction deposition in extra-channel areas development of channel islands that attach to bank

6 Sand Silt-loamy sand Silty clay Point bar Terrace Ridge Swale Sand bar Chute Splay Channel fill Levee Overbank deposit Lateral accretion surface

7 Kowanyama River, n Australia (Jeff Shellberg)

8 scroll topography oxbow lake meandering rivers on coastal plain near Kotzebue, Alaska

9 Wulik River, Alaska coastal plain near Kotzebue

10 the relative importance of lateral & vertical accretion varies vertical tends to be more important where there is frequent flooding & a lot of fine sediments, but the vertical thickness depends on the rate of lateral migration East Inlet Creek, w CO

11 Wolman & Leopold (1957) described floodplains along rivers of a variety of sizes in Maryland and North Carolina concluded that floodplains are composed primarily of point-bar (lateral-accretion deposits), with a thin veneer of overbank (vertical accretion) resulted in restrictive model of floodplain formation such that rivers along which floodplains did not form by gradual lateral accretion were regarded as exceptions to the rule took more than 20 yrs to establish that depositional styles & floodplain stratigraphy vary greatly, depending on the environment

12 Nanson (1986) high energy, laterally stable (confined by bedrock) channels in southeastern Australia floodplains formed by gradual overbank deposition punctuated by flood-stripping in contrast to low-energy streams in the same area which did not undergo periodic stripping stripping is intermittent along each river, as well as among rivers these processes affect the frequency of bankfull flow this type of situation also occurs in arid environments the crucial factor is the relationship between the erosional force in the channel at times of maximum stress, & the ability of the boundary to resist this force continuum of interacting floodplain form & fluvial process resulting from the balance between erosional energy & alluvial resistance

13 Implications of Nanson s work include: 1) when zoning for floods, it is not sufficient to simply use recorded discharges & model/route the highest flow through the existing channel you have to consider the possibility of the entire system changing 2) terrace formation, & use of terraces as paleoenvironmental indicators? ( stripping intermittent along each river, as well as between rivers) 3) equilibrium?

14 Miller (1990) evolution of channels & bottomlands in mountain valleys of the central Appalachians strongly influenced by sediment supplied from mass wasting during extreme storms type of change during storm varies with basin scale i) small, steep drainages scour ii) drainages of 1-65 km 2 have mixed erosion & deposition, with continuous reworking of the valley floor iii) drainages larger than 100 km 2 have localized, discontinuous valley-floor reworking sequential occurrence of extreme storms with different hydrologic characteristics creates a bottomland mosaic of surfaces with varying elevations & textures

15 Tributary inputs of water (Mertes, 2000) and sediment (Florsheim, 2004) can also form important contributions to some floodplains Q in river input Q out river output Q tr major tributary Q l local tributary Q p precipitation Q e evaporation Q l Q in Q rf Q fr Q tr Q p Q e Q rf river to floodplain Q fr floodplain to river Mertes (2000) Inundation hydrology Q gw groundwater river water local water Q out Q gw

16 Floodplain during dry season, northern Australia

17 Rio Amazonas in flood

18 Mertes et al. (1996), Dunne et al. (1998) Amazon transports 1240 Mt of suspended sediment from Andean erosion Mt of reworked floodplain sediments to delta annually upstream reaches have erosion in main channel & deposition in floodplain channels that are an order of magnitude smaller in Q than main channel downstream reaches have channels restricted by stabilizing levees & floodplain construction dominated by overbank deposition estimate that modern floodplain of Brazilian Amazon could be recycled in < 5000 yr; is recycled more rapidly in upstream than in downstream reaches exchanges between channel & floodplain each direction exceed annual flux of sediment out of river at Obidos (1200 Mt/yr) exchanges via bank erosion, bar deposition, settling from diffuse overbank flow, & sedimentation in floodplain channels

19 Dunne et al. (1998)

20 upper downstream middle

21 Meandering Channel Forest chronosequence Channel migration Abandoned channel/ oxbow Anastomosing In-channel wood jam Mature forest patches Buried wood jam hard point Secondary channel Avulsion around hardpoints Migration between hardpoints Braided Ephemeral patches of pioneer vegetation Collins et al., 2012 Frequently-shifting channels

22 (narrow, discontinuous) wide continuous moderate continuous narrow discontinuous wide continuous Longitudinal & lateral extent of floodplain

23 Recent approaches use of shallow geophysical techniques (GPR) to map extent, thickness, & stratigraphy of floodplains (Froese et al., 2005) coupled hydrology-hydraulics-sediment transport numerical models of floodplain inundation & sedimentation (Nicholas et al., 2006) use of multivariate statistics to differentiate spatial patterns of floodplain sedimentation at channel (10 3 km), floodplain process zone (10 km), and geomorphic unit (10 2 m) spatial scales (Thoms et al., 2007) even where system is near long-term mass balance equilibrium, almost all sand & much of silt & clay in transit through system spends some time stored in floodplain (Lauer and Parker, 2008)

24 Terraces tread scarp/riser tread classifications: erosional/depositional strath (bedrock)/fill (alluvial) paired/unpaired tectonic/climatic erosional depositional

25 paired unpaired depositional erosional Both terraces are alluvial. Upper is depositional because it represents the level of the valley fill. Lower is erosional because formed by lateral erosion across fill.

26 Plomosa Mountains, Arizona Rio Mira, Ecuador

27 Usual interpretation of erosional & depositional terraces 1) For erosional terraces, lateral erosion is the dominant process constructing the tread, with flood scouring as the channel migrates laterally. The capping alluvium is the same age as the surface exposure of the underlying bedrock, in contrast to depositional terraces. 2) For depositional terraces, prolonged aggradation triggered by glacial outwash, climatic change, base level or source area rise, or land-use changes, followed by incision complex as a result of prior conditions, non-synchronous response of tributaries & master stream, and proximity to thresholds terraces serve as indicators of external perturbation

28 Interpretations of the Cody Terrace near Cody, Wyoming observed by Mackin, 1937 erosional or strath terrace seismic profiles across terrace by Moss & Bonini, 1961 depositional terrace

29 Big Creek, CA fill terraces Annapurna region, Nepal southern Israel

30 Schick (1974) terraces in Nahal Yael, a small (1 km 2 ) watershed in Israel terraces result mainly from a quasi-continuous valley bottom aggradation from ordinary floods, interrupted by much less frequent, randomly distributed major floods which clear out most sediment super floods leave terraces entrenched in accumulated sediment important features are limited areal extent of morphologically significant floods rapid downstream loss of flows by infiltration existence of valley-deepening mechanism through weathering of alluvium-bedrock interface obliteration of terrace remnants by lateral processes after 2-3 super floods

31 Bed elevation at a cross section inherent areal inconsistency of terraces & terrace sequences along desert streams terraces in deserts, excepting special cases, are not attributable to climatic changes, unless these include a change in the magnitude or frequency of super floods 5m super flood super flood Time 1000 yrs

32 Nahal Yael, Israel

33 Temporary fill terraces, Buffalo Creek, CO

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36 Boll et al. (1988) computer simulation of river terrace formation by climatic change & varying uplift patterns main inputs sediment production based on climate uplift sediment transport equilibrium longitudinal profile equilibrium headwaters terrace sector

37 changed input of climate & tectonics continuously, but the output of the simulation was discontinuous in the formation of terrace steps interpreted this to mean that the formation of terraces depends on the operation of thresholds in a combination of factors (rather than in a single factor) increasing uplift = narrowing valleys, & vice-versa longer climate cycles = larger terraces

38 Mattole River, California Madison River, Montana

39 northern California

40 Green River, Utah Narrows Picnic Area, Poudre River, CO

41 Alluvial Fans depositional landforms where channel slope or geometry changes, generally at the mountain front fans resemble a segment of a cone radiating away from a single point source, although adjacent fans may merge to form bajadas, alluvial aprons, or alluvial slopes generally flatten toward the toe, with the steepest slopes associated with coarse-grained loads, low water discharge, high sediment discharge, & other types of sediment transport, such as debris flows upper fan gradient is similar to that of the stream mouth, so that deposition on the fan is not caused by slope decrease, but by the sudden increase in channel width

42 the longitudinal profiles of fans tend to be segmented, while the cross-profiles are convex fan area is primarily related to basin area, although climate, lithology & tectonics also exert some control deposition on fans occurs through water flows, hyperconcentrated flows, & debris flows type of flow depends on lithology & climate (weathering & precipitation) deposition shifts across the fan surface with time

43 incised fanhead trenches often occur at the fan apex, but towards the toe of the fan the flow overtops the channels or avulses to leave a deposit in the form of a tongue or sheet that has a small areal extent & is poorly-bedded sieve deposits also occur coarse sediments without matrix deposited in lobate masses while the water that transported the sediment infiltrates Fans are of interest as a record of changes in sediment yield through time a fan is more likely to have a complete depositional record than a channel, where any change is likely to destroy a part of the record preserved in sediments

44 Bull (1960s) thick alluvial fans are generally tectonically-controlled, climatic fans tend to be thinner fans in Fresno County, CA (USGS Professional Paper, 1964) fans derived from mudstone or shale-rich basins are generally 35-75% steeper than fans of similar area derived from sandstone-rich basins, & roughly twice as large as fans from sandstone basins of comparable size Bull attributed this to the greater erodibility of shale

45 Schumm, 1977, The Fluvial System

46 Death Valley, CA

47 Coastal Peru Annapurna region, Nepal Sinai, Egypt

48 Annapurna region, Nepal Pleasant Valley, NV

49 Bajada (alluvial apron), w US Bright Angel Creek, AZ

50 Oi River debris cone, Japan

51 Red Sea coast, Sinai, Egypt coastal Peru Tributary fan, Grand Canyon, AZ playa, Death Valley, CA

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54 Kluane Provincial Park, Canada

55 Quito, Ecuador

56 Federal Emergency Management Agency image

57 Pediment, Mohave Desert, CA

58 Deltas delta: a depositional plain or cone formed by a river at a local or ultimate base level sediment accumulation results in irregular progradation of the shoreline, & the river divides into distributaries at the delta apex delta deposition differs from that of fans 1) delta deposition occurs as a result of reduced flow velocity when river enters body of standing water 2) delta expansion is vertically limited by water level 3) delta surface gradient much flatter than that of fan

59 sediment-lade flow entering standing water mixed in 3d (axial jet) or in 1d (plane jet), depending on the density contrast between the two water masses (dense flows = plane jet = turbidity current) deltas are divided into two types based on the balance among the fluvial system, climate, tectonic stability, & the shoreline dynamics 1) high-constructive deltas develop when fluvial action is the prevalent influence, and can be elongate high mud content & rapid subsidence when inactive lobate sink slowly on abandonment, so that upper sand is reworked by marine processes

60 2) high-destructive deltas occur where ocean or lake energy is high, so that fluvial deposits are reworked wave-dominated sediment accumulates as arcuate sand barriers near river mouth tide-dominated tidal currents arrange sand into units radiating linearly from river mouth deltas shift with time as levees are breached at crevasses, or base level changes

61 High-constructive deltas High-destructive deltas Types of deltas (Ritter et al., 1988)

62 topset bottomset foreset bottomset younger delta older delta

63 Mississippi River deltas (Schumm, 1977)

64 Estuary, northern California

65 Colorado River delta

66 Cook Inlet, coastal Alaska