1. Channel geometry: controlled by discharge, caliber, and sediment load (independent factors); by slope (dependent factor); by channel width and depth, flow velocity, bed roughness (neither wholly dependent nor wholly independent)
2. Channel patterns
a. Braided: low sinuosity; marked by successive divisions and rejoining of flow around alluvial islands
b. Meandering: high sinuosity; more or less regular inflections in channel direction; scour pools at points of inflection.
c. Straight: as aggrading streams best developed on delta plains as distributary channels; thalweg shifts but channel does not meander
3. Flow of water in channels
a. Reaches: maximum velocity in center, decreases to sides; maximum turbulence toward sides of channel and channel floor
b. Bends: channel asymmetrical, gentle on convex side, steep on concave side; in low stage, maximum velocity close to concave bank, maximum turbulence close to foot of concave bank; convex side is shoal and irregular
c. Flow at surface and depth: excepting high stage, maximum velocity swings from concave bank across reach to opposite concave bank; during flood and increased velocity, flow less sinuous, maximum velocity swings out from concave bank to bar, may scour bar to give chute channels
4. Load types: Bed load and suspended load
Lateral erosion rates: up to 2,500 ft per year reported; 500ft per year in lower Mississippi River
b. Levee formation: in flood stage, channel overflows banks; resulting decrease in velocity causes deposition; coarsest sediment adjacent to channel, fines carried to floodbasin; levee and floodbasin built through vertical accretion in contrast to lateral accretion of point bars
c. Straight streams: where aggrading associated with extensive levees that lead to channel stability and confinement: channel cross section symmetrical; shift of thalweg occurs with minor point bars developed but principal deposition along channel floor
d. Braided streams: broad, shallow streams characterized by incompetence in transporting coarsest fraction; high bed load, low suspended load; during flood, load moved as a series of transverse bars; longitudinal bars built from nucleus of coarse material not moved by strearn
e. Crevassing: during flood, excess water from channel leaves through isolated low section in natural levee; once initiated, breach is scoured and deepened; sediment load deposited on lower slope of levee; result is a crevasse splay.
f. Cutoffs: chute cut off, channel shortened by cutting new channel along a swale in meanderbelt neck cutoff, abandonment of meander, either by gouging new channel across narrow neck or through capture of one loop by the next upstream, resulting in oxbow lake
g. Avulsion: sudden abandonment of part or whole of meanderbelt by strearn from a new course at lower level on floodplain; local crevassing may lead to permanent change of stream course and major shift or avulsion
5. Processes
Meandering streams: shift of thalweg results in erosion on concave bank and deposition on next downstream convex bank to form point bar; with complementary cut-and-fill process stream meanders and shifts laterally, result is accretion topography formed by a series of point bars.
Point bars: aggradation of bed load against shoal convex bank; build up to about 80 ft depending on bank stability and, in turn, on bed-load/suspended-load ratio
Formation: shift in maximum velocity to concave bank results in super-elevation of water surface toward concave bank, cross-channel current is set up with velocity about 15 percent of downstream velocity, flows toward convex bank; bed load carried up the convex bank; as bar is ascended during flood, progressively coarser materials are dropped out, giving upwardfining sequence in texture and scale.
6. Flow regime and bed forms
a. Lower flow regime: resistance to flow large, sediment transport small; lower part gives small scale ripples with small-scale cross stratification; upper part of lower flow gives large-scale ripples or dunes with large-scale cross stratification; transport in discrete steps; bed form roughness predominates; water surface and stream bed out of phase
b. Transitional flow regime: characterized by washout dunes
c. Upper flow regime: resistance to flow small, sediment transport large; resulting bed forms are plane bed and antidune; internal structures marked by flat bedding, either horizontal, plane, or gently undulating, surfaces of laminae show primary current lineation; transport is continuous; grain roughness predominates; water surface and stream bed in phase.
7. Fluvial environments and facies
a. Channel floor: deeper part of strearn; coarsest sediment in system left as residual acumulation or lag; moved only during exceptional floods; fllls scours in channel; resulting deposit is commonly conglomeratic, shows large- to moderate-scale trough cross beds, and forms basal unit in channel sequence
b. Point bar: extends up from lag deposite; as thick as channel in which it was formed; in fine grained systems up to 80 ft, in coarse-grained systems about 20 ft; made tip predominantly of sand; has shape of individual meanderbelt, asymmetrical in cross section; in fine-grained point bars, sequences fine upward in texture and scale of stratification, lower part with large-scale troughs, tabular crossbeds common on downstrearn end, upper part cornmonly with horizontal bedding; in coarse-grained point bars, no vertical trend in texture or structures, main structures large- to moderate-scale troughs, few tabulars, and comnnonly large-scale foreset cross stratification developed as chute-front deposits; lateral shift of meanderbelt gives sand unit of varying width, up to 30 mi; fine-grained rneanderbelts commonly multistoried and with complete point bar sequences; coarse-grained meanderbelts commonly multilateral with incomplete point-bar sequences
c. Channel bar: bars formed within channel, common to braided streams; common types are transverse bars and longitudinal bars; marked by few stratiflcation types, generally plane bedded flanked by small tabulars or foresets and minor small-scale troughs; lateral discontinuity of sedimentation units; resulting deposit a highly irregular multilateral sand.
d. Swale: chiefly fine-grained sediments fllling arcuate depressions between point-bar ridges in meanderbelt; overbank deposition; generally 10 to 15 ft thick, 30 to 100 ft wide, depending on relief of point-bar topography; fill mostly mud, laminated, suspension deposition.
e. Chute cut off: formed by scouring swale during flood, swale tends to straighten; bed load may be transported through cutoff and be deposited as large-scale foresets at downstream end; abandoned chute off similar to swale off.
f. Levee: sinuous, ribbon-like, prismatic body of sediment, triangular in cross section; size depends on size of river and amount of suspended load; best developed on concave bank and slightly upstream of point bars in fine-grained meanderbelts; flanks both sides of aggrading straight stream; coarsest sediments adjacent to channel, fine away from channel; rapid interbedding of coarse and fine sediments; primary structures small-scale troughs, wavy laminations; primary structures generally destroyed; root mottling common; commonly oxidized with concretions abundant
g. Crevasse (crevasse splay): narrow to broad localized tongues of sediments, sinuous to lobate in plane, deposits over lower slopes of levees and outer margins of flood basins; range in thickness from few inches to several feet; as coarses or commonly coarser than associated Ievee deposits
h. Flood basin and floodplain: accumulation of fines suspended in floodwaters in low areas marginal to channels and levees; finest sediment in fluvial system; primary structures mostly uniform lamination; shrinkage cracks common; root mottling; homogenized soil zones; swamps may be common, giving local organic deposits
i. Abandoned channel (channel off): channels that have been abandoned or reduced in slope and depth due to reduced sediment supply; common occurrence is in fine-grained meanderbelts where cutoff takes place; shape is that of channel at time of abandonment, thicker sequence of fines adjacent to concave bank; marked by sand plug adjacent to active channel, remainder of channel filled with fines; laminated muds, may be burrowed; organics common
8. Fluvial models: The four basic fluvial models outlined below represent end points in a spectrum of fluvial systems; they have been synthesized from salient features of both modern and ancient case studies.
a. Braided stream model
1. Setting: best developed as upstream very high bed-load fluvial facies where slope is relatively high (though here not well preserved except as parts of alluvial fans); and deposited under low flash-discharge conditions; local downstream facies where stream traverses non-cohesive fil or along zone of tectonic or physiographic-induced slope increase (these local facies more likely to be preserved); prominent constituent facies in alluvial-fan systems
2. Geometry of channel units: essentially flat bedded, discontinuous, lenticular, tabular; longitudinal bars commonly about 40 ft wide, 400 ft long, tapered; units normally less than 2 ft thick; axis of longitudinal bar parallel to flow, transverse bar normal to fiow
3. Structures: predominantly parallel bedding and tabular cross beds; minor trough crossbeds and ripple cross-laminations; structures of moderate scale, normally less than 2 ft; troughs commonly associated with debris piles and longitudinal bars; parallel beds flanked downstream and laterally by tabular crossbeds, ripple cross-lamination below tabulars (if sediment no larger than medium sand); small troughs in upper part of bar. Transverse bars:
chiefly tabular or planar crossbeds, commonly of flner texture than longitudinal bars
4. Composition and textures: commonly but not invariably coarse-grained, very few fines or topstratum deposits accumulated or preserved; poorly to moderately sorted; longitudinal bar deposits fine and become better sorted distally
5. Degree of channel scour: slight, individual channels delineated only by textural differences or discontinuities in structural trend; scour chiefiy marginal to longitudinal bar when it becomes emergent
6. Distribution of sand facies: form multilateral sand facies; width/thickness of facies and individual sand units high
7. Current pattern and directional features: low-sinuosity streams, anastomosing; entire facies elongate in direction of flow; variance in specific directional features will depend on relative abundance of structural types (may be very high, up to 900)
8. Depositional units: longitudinal and transverse bars
9. Associated fluvial facies: poorly developed topstratum muds except as local channel fill; -no levee deposits; no in situ organics preserved though longitudinal bars commonly vegetated; sand-to-mud ratio of system high
10. Main distinguishing features: few stratification types, lateral discontinuity of individual sedimentation units; may be dominated by either parallel laminae or beds or tabular crossbeds (as number of tabular crossbeds increases, trough crossbeds increase); few ripples; high sand facies, multilateral
1. Setting: delta plain of high-constructive delta systems; built by high, continuous discharge streams with balance of mud and sand for levee construction and facility for differential compaction; very low gradient streams, aggrading
2. Geometry of channel units: elongate symmetrical channel sands, convex downward base, fiat top; up to 300 ft thick and about 4 to 5 mi wide; thickness commonly due to superposition of multiple channel sands
3. Structures: chiefly troughs with climbing ripples toward channel margins, few tabulars; original structures commonly obliterated by compactional distortion
4. Sequence of structures (base to top): (1) massive to highly distorted with moderate to small scale injections and convolutions; (2) troughs of moderate to large scale; (3) troughs with a few tabular crossbeds preserved; (4) climbing ripple cosets, migration away from thalweg, transitional with levees; (5) laminated channel-fill muds abruptly overlying entire sand body (fill muds commonly burrowed). No measurable trend in scale of structures; abandoned channel flll muds commonly scoured by reactivated distributary channel
5. Composition and textures: fine- to medium-grained sand, poorly to moderately well sorted, mud clasts and wood fragments common; coarsest grained sediment conflned to channel. No definable upward fining in texture, characterized by vertical uniformity
6. Degree of channel scour: generally slight; principal accumulation by downward accretion through subsidence and differential compaction in high mud framework; basal contact marked by distortion rather than by extensive scour
7. Distribution of sand facies: width/thickness ratio of sand body low; multistorying common due to differential compaction and subsidence and tendency for distributaries to reoccupy abandoned distributaries
8. Levees: very well developed, but rarely overlying channel sand body or channel fill; commonly root mottled and concretionary
9. Current pattern and directional fea tures: very low sinuosity streams; unidirectional with relatively low variance
10. Depositional units: bed-load deposition, chiefly on drop of flood stage; channel confined and ernbanked without significant lateral rnigration; insignificant development of point bars
11. Associated facies: well-developed levee deposits, crevasse deposits, and interdistributary muds with abundant in situ organics. Sand/mud ratio of delta-plain facies low
12. Isolith pattern and regional trend: parallel or angular to depositional slope; distributive
13. Main distinguishingfeatures: syrnmetrical channel sands enclosed by muds with abundant bedded organics and minor sands; distributive patterns; uniformity in vertical sequence; flanking position of levee deposits; multistorying comrnon; individual channel units thick; basal contortion; unidirectional trends
1. Setting: lower parts of rnoderate to high bed-load fluvial systems under conditions of locally higher gradient (tectonic or physiographic control) or over high sand terraces under lower to rnoderate discharge; would probably braid if not stabilized by vegetation
2. Geometry of channel units: multilateral sand body buiit of a compiex of partly preserved channel and meanderbelt units; individual rneanderbelt units 10 to 25 ft thick and up to 5 mi wide; erosively bounded; mappabie sand units built up of superposed and adjacent meanderbelt sand bodies up to 200 ft thick and 30 mi wide
3. Structures: sands — predominantly moderate- to large-scale troughs, few tabular crossbeds with srnail-scale troughs, sorne horizontal beds, abundance and variety of ripple forms; ievee
deposits (sands and silts) extensively root rnottled and concretionary; overbank muds laminated to mottled; delicate ripple cross-laminae, shallow scour and fu, sorne crayfish burrows, root mottles
4. Sequence of structures (base to top): (1) large troughs (channel lag); (2) rnoderate troughs and tabulars with minor small troughs; (3) wedge sets or avaianche beds; (4) parallel-inclined laminae with minor scour and fil and thin tabulars. No upward fining in scale of structures; in fact, avalanche structures comrnonly largest scale
5. Composition and textures: fine to chiefly coarse-grained sand, granule to fine pebble gravel common; very little mud or top stratum. No upward flning in textures; coarsest grain associated with wedge sets and lower part of tabulars
6. Degree of channel scour: rnoderate
7. Distribution of sand facies: width/thickness ratio of sand body very high; multilateral
8. Current pattern and directional features: rnoderate-sinuosity streams; relatively low directional variance, lower than fine-grained meanderbelt and braided facies
9. Depositional units: troughs fil in channel and adjacent convex bank by advancing sand dunes; tabulars formed by sand waves advancing on relatively smooth surface; wedge sets formed as chute-front deposite (foreset beds) of numerous chute cutoffs; bankfull conditions involve entire valley width with upper flow regime near tree une developing parallel-Iarninated, coarse-grained units with minor trough fill and tabulars; complex accretes laterally
10. Associated fluvial facies: poorly developed or preserved topstratum deposite with most fines carried through the facies; principal topstratum deposite represented by mud filled with sand or not preserved; very poorly developed levees, stabilized by vegetation
11. Isolith pattern and regional trend: individual channel units parallel to depositional slope; resultant multilateral sand body normal to slope
12. Main distinguishing features: moderately coarse-grained sand facies with little mud; incomplete point-bar sequence (lower part only); abundant (up to 1/3) wedge-set crossbeds; multilateral sand body; may include some braided-stream structures
9. Mineral reservoirs: Fluvial sand facies provide potentially good reservoirs for oil and gas from the standpoint of primary permeability and porosity; structural traps commonly owing to differential compaction, especially in high-mud systems, and stratigraphic traps arising from irregular depositional topography are, in some cases, significant. Commonly, a principal limitation is the distance of fluvial channel sands from potential marine source beds. Many so-called channel-sand reservoirs are, in reality, elongate delta sand bodies. Other important mineral accumulations are a variety of placer deposits, especially base metals. Special ceramic clays such as kaolinites are common adjuncts. Fluvial coals commonly flank high-mud fluvial channel deposits. Uranium roll-front deposits are commonly formed in channel-sand facies.
1. Setting: low gradient, moderately high discharge, mixed-load streams on aggrading alluvial plains; (under relatively uniform discharge sand) deltas
2. Geometry of channel units: commonly multistoried; asymmetric channel belts, convex downward (base and cutbank side of channel erosively bounded), upper part transitional; average thickness of individual meanderbelt sand unit 40 to 80 ft, if thicker involves channel superposition; width 5 to 10 mi, commonly less
3. Structures: sands — predominantly moderate- to large-scale troughs, few tabular crossbeds with small-scale troughs, some horizontal beds, abundance and variety of ripple forms; levee deposits (sands and silts) extensively root mottled and concretionary; overbank muds laminated to rnottled; delicate ripple cross-laminae, shallow scour and fill, some crayfish burrows, root rnottles
4. Sequence of structures (base to top): (1) moderate- to large-scale troughs with basal channel lag; (2) tabular crossbeds with minor small-scale troughs; (3) horizontal or parallel beds with parting lineation; (4) current ripples. Overlain by laminated muds of swaley ful and/or mottled and concretionary levee deposits. Overbank rnuds larninated to rnottled. Complete point-bar sequence cornmonly developed; upward fining in scaie of sedirnentary structures
5. -Composition and textures: chiefly fine-grained sand, ranging from gravel at base to muds at top. High clay-pellet sand and plant debris in upper point bar. Upward fining of textures
6. Degree of channel -scour: prominent along cutbank or steep side of channel unit or meander belt unit due to lateral cutting; well-developed basal scour
7. Distribution of channel and meanderbelt units: width/thickness ratio of sand body moder
ate; multistorying common »
8. Levees: moderately well developed, especially aboye and lateral to steep lateral margin of meanderbelt sand facies; veneers and overlies channel sands
9. Current pattern and directional features: moderate to very high sinuosity streams; average vector parallel to sand-body trend but with a high variance
10. Depositional units: lateral erosion and accretion in a meandering stream with point bars accreted on sedimentation side during flood stage
11. Associated fluvialfacies: well-developed topstratum or overbank muds endose meanderbelt sand body, levee deposits and channel-fill muds; levee deposits overlie channel sand; channel fill best developed on cutbank side of channel unit and commonly with abundant woody organics; coals parallel trend of sand body. Sand-to-mud ratio mod-erate to iow
12. Isolith pattern and regional trend: sand bodies roughiy paraiiel to depositionai siope, cornmonly tributary, may be distributary
13. Main distinguishing fea tures: fine-grained sand associated with abundant muds and sorne organics; complete point-bar sequence with troughs dorninant; levees overlie channel sand; multistorying of rneanderbeit sand comrnon; individual channel units rnoderateiy thick
