Sed Strat Ch. 9 Rivers & Alluvial Fans

What are the three geomorphic zones that are recognized in fluvial and alluvial systems?

1. erosional

2. transfer

3. depositional

Define channels

depressions or scours in the land surface that contain a flow

Define overbank area/floodplain

area of land between or beyond the channels that receives water only when the river is in flood

What is in the fluvial environment

channel and overbank/floodplain settings

Define alluvial

a general term for land surface processes that involve the flow of water

Define alluvial plain

a general term for low-relief continental area where sediment is accumulating

Define drainage basin/cachment area

area of ground that supplies water to a river system

Define discharge

the volume of water flowing in a river in a time period

Define perennial fluvial system

a network of rivers or streams that maintains continuous water flow within its defined channels throughout the entire year under normal climatic condition

Define ephemeral river

if they are dry for a long period of time and only experience flow after sufficient rain in the drainage basin they are called this

Define terminal fan

a fan-shaped deposit that forms in an arid region after it loses water and sediment due to evaporation and absorption before reaching a larger body of water

Define distributive fluvial systems

a fan-shaped geological deposit formed by a river network that radiates and splits outward from a single apex across a basin floor

Define low flow stage

when water level is well below the level of the channel banks

Define high flow stage/bank-full flow

water flowing close to or at the level of the bank

Define river flood and overbank flow

when the volume of water exceeds the volume which can be contained in the channel, this occurs

What are the three principle variables in rivers

1. how straight or sinuous

2. presence or absence of sand/gravel bars

3. number of separate channels

Define braided river

contains mid-channel bars that are covered at a bank-full flow

What is anabranching/anastomosing river

consists multiple interconnected channels separated by areas of floodplain

Define meandering river

channel that is sinuous and have depositional bars only on the insides of bends

Define longitudinal bars

elongate along the axis of the channel

Define transverse bars

wider than they are long spreading across the channel

Define linguoid bars

crescentic bars with their apex pointing downstream

Define coarse lag

accumulation of larger clasts since flow is strongest between bars

Define compound bars

mixture of clast sizes

Define braidplains

regions where braided rivers repeatedly migrate and change position on the alluvial plain

Basal lag

A concentration of coarse clasts deposited on the channel floor at the base of a channel, above an erosional surface.

Linguoid dune

An arcuate or tongue-shaped subaqueous dune that migrates with the current; these dunes commonly create trough cross-bedding.

Thalweg

The line along the deepest part of a river channel, where water velocity is generally highest.

Bedload

Sediment transported along the channel bed; when flow slows, the river may no longer be able to carry it.

Mixed load

The mixture of suspended sediment and bedload transported and deposited by a river.

Chute channel

A short-cut channel that forms when high-stage flow crosses a point bar and cuts across the inner bank of a meander.

Sinuosity

The ratio of channel distance along a river reach to the straight-line distance between the reach's endpoints; the chapter calls a river sinuous when this ratio is greater than 1.5.

Meandering

A channel pattern in which sediment accumulates on the inside of bends while erosion affects the outer banks.

End-members

The contrasting extremes of a range of channel forms; in this chapter, meandering and braided rivers are the end-members.

Lateral migration

Sideways movement of a river channel through erosion of one bank and deposition on the opposite bank.

Point bar

A sediment deposit on the inner side of a meander bend, commonly showing a fining-upward sequence from coarser sediment at the base to finer sediment above.

Lateral accretion surface

An inclined surface within a channel-fill deposit that records the successive growth and lateral movement of a point bar.

Epsilon cross-stratification

The inclined stratification associated with lateral accretion on a migrating point bar; the surfaces slope from the river bank toward the deeper channel.

Oxbow lake

A lake formed when a river cuts off a meander bend, leaving the former channel loop isolated from the active river.

Wadi

A river or stream in a desert that may flow temporarily after rainfall and remain dry for long periods.

Ephemeral flow

Temporary or intermittent river flow; streams may dry up between rainstorms or during a dry season.

Wadi gravels

Poorly sorted gravel deposits from ephemeral flows, commonly containing angular or subangular clasts in a sand-and-mud matrix.

Suspended load

Sediment carried within the water rather than along the channel bed; it commonly includes clay and silt and may include fine sand.

Levée (levee)

A raised sediment bank beside a river channel, built by repeated flood deposition and standing higher than the surrounding floodplain.

Crevasse splay

A low, fan-shaped deposit of sediment spread across the floodplain when floodwater breaks through a levee.

Alluvial fan

A roughly cone-shaped sediment body formed where water and sediment leave a steeper source area, spread out at a break in slope, and deposit sediment at a basin margin.

Unchannelised flow

Flow that is not confined to a river channel; some definitions of alluvial fans emphasize deposits formed by this type of flow on relatively steep slopes.

Humid fan

An alluvial-fan-like deposit described in the chapter for glacial outwash deposited by braided rivers in wetter settings.

Megafan

A very large, fan-shaped sediment deposit that can form through the lateral migration of a river.

Distributive fluvial system

A river-and-sediment system in which channels and sediment spread outward across a basin rather than being dominated by one trunk channel.

Colluvial fan

A sediment body made from a mixture of talus deposits and debris-flow deposits, commonly found in sub-polar regions.

Feeder canyon

Funnels drainage toward a basin margin and opens onto an alluvial fan, where the flow loses energy and deposits sediment.

Fan apex

The highest, most proximal part of an alluvial fan, next to the feeder canyon, from which the fan spreads outward.

Fan-head canyon

A canyon that may be cut into the surface of an alluvial fan near its apex.

Fan toe

The outer edge of an alluvial fan, marking the limit of deposition of coarse detritus.

Debris flow

A dense slurry of water and sediment that moves downslope and deposits sediment, often in lobes, with poor sorting and little separation of clast sizes.

Sheetflood

A rapid, shallow, sheet-like flow across a fan surface; the chapter associates sheetflood processes with fast, turbulent flow on relatively steep slopes.

In situ

In place; fossil remains preserved where the organism lived or grew rather than being transported elsewhere before burial.

Palynomorph

Highly resistant plant fossil material such as pollen and seeds, which can survive transport and help with dating or correlating terrestrial deposits.

Trace fossil

Evidence of an organism's activity, such as tracks, burrows, or nests, preserved in sediment; trace fossils can provide information about past environments.

Palaeoenvironment

An ancient environment reconstructed from evidence such as sediments, fossils, and traces of animal activity.

Pedogenesis

The physical, chemical, and biological processes that form and modify soil.

Redox conditions

Oxidising and reducing conditions in soil, particularly those affecting iron minerals.

Horizonation

The development of distinct layers, or horizons, within a soil profile.

Illuviation

The redistribution of clay, iron, and organic material into different soil layers.

Entisol

A very immature soil with little soil-profile development; the chapter associates it with a relatively short period of surface exposure before more sediment is deposited.

Inceptisol

A soil showing some early development but remaining less mature than more developed soil orders.

Andisol

A soil order whose formation is restricted to volcanic substrates.

Histosol

A peaty soil associated with waterlogged conditions.

Spodosol

One of the forest-associated soil orders discussed in the chapter; together with Alfisols and Ultisols, it indicates a longer period of surface exposure than very immature soils.

Alfisol

One of the forest-associated soil orders discussed in the chapter; the chapter groups it with Spodosols and Ultisols as indicating a longer period of soil development.

Ultisol

One of the forest-associated soil orders discussed in the chapter; the chapter groups it with Spodosols and Alfisols as indicating a longer period of soil development.

Oxisol

A soil order most commonly associated with humid tropical conditions; the chapter links oxisols with extensive weathering and iron- and aluminium-rich material.

Vertisol

A soil order associated in the chapter with sub-humid to semiarid climates that have pronounced seasonality.

Mollisol

A soil type associated with grassland settings in the chapter.

Aridisol

A soil order associated with arid to semiarid conditions; calcium carbonate precipitation and calcrete formation may occur in these soils.

Gelisol

A soil order that the chapter associates with cold climatic conditions.

Laterite

Highly weathered soil material derived from bedrock and consisting mainly of iron and aluminium oxides; the chapter describes laterites as oxisols.

Ferricrete

A cemented, iron-rich oxisol; ferricretes are a type of hardened soil profile.

Duricrust

A hardened, highly resistant soil surface or profile layer that develops over a long period.

Silcrete

A silica-rich hardened soil crust, described alongside iron-rich duricrust forms.

Calcrete

A soil horizon or crust formed by the precipitation and accumulation of calcium carbonate, especially in arid or semiarid environments.

Rhizocretions

Carbonate encrustations associated with roots within a soil profile.

Glaebules

Small soil nodules; these may be carbonate nodules formed as calcium carbonate precipitates.

Tepee structures

Dome-like forms in a calcrete layer produced as the layer expands during calcium carbonate precipitation.

Palaeosol

A fossil soil formed in the geological past; these can reveal information about ancient landscapes, vegetation, climate, and surface exposure.

Avulsion

A change in the position of a river channel in which part of the old course is abandoned and a new course is cut across the floodplain.

Architecture of fluvial successions

The three-dimensional arrangement of channel and overbank deposits within a fluvial sedimentary succession.

Palaeoclimate

Climate conditions in the geological past, inferred in part from evidence such as ephemeral river flow, flood deposits, and palaeosols.

Aeolian dune

A dune formed by wind; the chapter notes that alluvial-fan deposits may be associated with aeolian dunes.

Lithologies

The rock and sediment types present in a deposit; the chapter lists conglomerate, sandstone, and mudstone as examples.

Mineralogy

The mineral composition of a sediment or deposit; the chapter notes that it can vary and may be compositionally immature.

Texture

The grain-size and sorting characteristics of sediment, ranging in the chapter from very poorly sorted debris-flow deposits to moderately well-sorted river sands.

Bed geometry

The shapes and arrangement of sediment beds, such as sheet-like deposits on fans or overbank areas and lens-shaped river-channel units.

Sedimentary structures

Features such as cross-bedding and lamination that form during sediment deposition and can help identify channel deposits.

Palaeocurrents

Indicators of past flow direction in a channel and of depositional slope.

Fossils

Preserved remains or traces of organisms; animal fossils are uncommon in fluvial settings while plant fossils may be common in floodplain deposits.

Colour

yellow, red, and brown colours with oxidising conditions.

Facies associations

Groups of sedimentary facies that occur together and help indicate connected depositional environments, such as fans with ephemeral lakes or aeolian dunes, or rivers with lake, delta, or estuarine facies.

Bajada

merging alluvial fans