I. Fluvial Processes and Associated Landforms

A. Hydrologic Cycle

Hydrologic Cycle

• the atmosphere links the oceans and the continents

• water evaporates into the atmosphere from the oceans (to a lesser extent from the continents), and is transported by the wind

• some precipitates back into the ocean -- some is carried over the continents, where it falls-out, & has to make it back to the ocean

• of that which falls on the land, some soaks into the ground - "infiltration"

• infiltrating water flows laterally and downward, eventually seeping into lakes and streams, or directly into the ocean

• if the rate at which water falls is greater than the earth's ability to absorb it, the water flows across the earth's surface - "runoff"

• 96,000 cubic kilometers of precipitation falls on the continents, only 60,000 cubic kilometers of this evaporates, leaving 36,000 cubic kilometers that runs-off from the land to the ocean causing tremendous erosion:

• this moving water is the single most important agent that sculpts Earth's land surface

B. Introductory Ideas

• the flow of water through the Earth's waterways is the chief agent of landmass denudation

• world's rivers with greates discharge (flow volume/time): Amazon & Parana (S.A.), Zaire (Africa), Ganges (India), Chang Chiang (Asia), Misssouri-Ohio-Mississippi, St. Lawrence, & Mackenzie (N.A.)

• "fluvial processes" - stream (river) related processes

• geographers seek to describe the patterns associated with fluvial systems, & the processes that created the patterns

• fluvial sysytems behave in characteristic ways (processes) & produce characteristic landforms

• fluvial systems operate within the hydrologic cycle, & under the influence of gravity

• factors which influence individual streams: climate, surface composition, topography, vegetation, & length of time in operation

• streams are mixtures of water & solids - carried in solution, suspension, & by mechanical transport

• streams modify the landscape

• landforms are created by the erosive action of flowing water & the deposition of stream-transported materials

Streams Modify Landscape

(1) Terms:

• "erosion" - denudation by wind, water, & ice, which dislodges, dissolves, or removes surface material

o ("fluvial erosion" - denudation by streams)

• "transport" - the actual movement of denuded material by air, water & ice

• "deposition" - the process whereby weathered (eroded), and transported sediments are laid down. Specific depositional landforms are created form the actions of air, water, and ice

• "alluvium" - the general descriptive term for clay, silt, & sand transported by running water & deposited in sorted or semisorted sediment on a floodplain, delta, or in a stream bed

C. Streamflow Characteristics

• streams are supplied by surface runoff and underground water - which come from rain & snow

• runoff initially flows in broad, thin sheets across the ground - "sheet flow"

• the amount of sheet flow is a function of the soil's "infiltration capacity" =

(1) intensity and duration of precip.

(2) soil wetness

(3) soil texture

(4) slope of the land

(5) vegetative cover

• after flowing as a thin, unconfined sheet, the water begins to form small channels called "rills", which carry the water to a stream

Streamflow:

• "laminar flow" - a streamlined flow of water in which individual sediment particles (and water particles) move along evenly in generally parallel flows - this type of flow is uncommon - only occurs when water is moving very slowly through a smooth channel

• "turbulent flow" - small eddies are produced by friction between streamflows, & channel sides & bed - more common - is a multidirectional movement

• turbulent flows increase the action of suspension, traction & saltation, thus increasing channel abrasion

• stream velocities are greatest in the center near the surface (corresponding w/the deepest part of the stream)

• velocities decrease closer to the bottom & sides of the stream because of frictional drag on the flowing water

• in a curving stream the maximum velocity line tends to migrate from side to side along the channel, deflected by the sides

• the ability of a stream to erode and transport material is directly related to its velocity

Factors which determine a stream's velocity (and thus its erosional capability):

(1) gradient (slope); (2) shape, size, and roughness of the channel; (3) discharge

(1) gradient (vertical drop over a distance) - all else being equal, the steeper the gradient the greater the velocity

(2a) cross-sectional shape - determines the amount of water in contact with the channel, and thus the frictional drag

• the most efficient channel is the one with the least perimeter for its cross-sectional area
• all else being equal, water will flow with a greater velocity through a semicircular channel

(2b) size - an increase in channel size reduces the ratio of perimeter to cross-sectional area and increases the efficiency - when the size of the channel increases, proportionally less of the water is in contact with the bed and banks of the channel

(2c) roughness - the smoother the channel, the greater the velocity

(3) discharge - the amount of water flowing past a point in unit time

• units are cubic meters per second
• discharge = cross-sectional area x velocity
• discharge (m3/sec) =

channel width(m) x channel depth(m)

x velocity (m/s)

• as discharge increases, width, depth, and velocity all incease in an orderly fashion

recall: as the velocity increases, the carrying capacity of the river will increase!!

Downstream Changes:

• longitudinal profile - from a rivers source area (headwaters), to where it enters into another body of water (mouth)

• a stream's gradient constantly decreases from its head to its mouth - yields a concave-upward curve

• discharge increases toward the mouth, thus width, depth, and velocity all increase systematically toward the mouth of the river

• there is thus an inverse relationship between discharge and gradient: when gradient is high, discharge is small; and when gradient is low, discharge is high

• the average flow velocity in a wide, placid river is often greater than that of a turbulent mountain stream - the narrow channel, rough bed and consequent great drag imposed on the mountain stream, send the water in all directions, lowering average velocities

D. Stream Erosion

• Stream Erosion - streams carve & shape the landscape through which they flow

Erosional Processes:

• hydraulic action - the erosive work accomplished by the turbulence (eddies) of water; causes a squeezing & releasing action in the joints of bedrock; capable of prying & lifting rocks

• abrasion - the mechanical wearing and erosion of bedrock accomplished by the rolling and grinding of particles & rocks carried in a stream

• both volume & velocity are important determinants of the erosive & transport capability of streams

E. Stream Transport

• competence - the streams ability to move particles of a certain size - is a function of velocity

• capacity - the total possible load that a stream can transport - is a function of discharge

Transportation Processes:

• solution - refers to the dissolved load of a stream (the chemical soln. derived from mineral (e.g., limestone); from chem. weathering)

• suspended load - fine-grained particles physically held aloft in the stream, with the finest particles not deposited until the stream's velocity approaches zero

• bed load - coarser materials that are dragged along the bottom by "traction" - or rolled/bounced along by saltation

Sediment Transport

F. Channel Patterns

• stream velocities are greatest in the center near the surface (corresponding w/the deepest part of the stream)

• velocities decrease closer to the bottom & sides of the stream because of frictional drag on the flowing water

• in a curving stream the maximum velocity line tends to migrate from side to side along the channel, deflected by the sides

• stream channels assume a snakelike form weaving across the landscape - producing a "meandering stream" - found in "old stage" valleys

• the outside portion of each curve receives the most erosion (greatest H2O velocity), & a steep bank is formed - a "cut bank"

• the inner portion of the curve receives sediment - a "point bar"

• if the sediment load of a stream exceeds the streams capacity, the stream channel is filled - "aggradation"

• when aggradation occurs "braidded streams" can form - a stream comprised of a maze of interconnected channels laced with excess sediments

G. Stream Deposition

• (weathering, mass movement, erosion, transportation……. now deposition)

• the stream deposits alluvium (unconsolidated sediments), creating depositional landforms

• stream meanders migrate downstream through the landscape

• the landscape near a meandering river contains residual deposits from the previous river channels - "former point bar deposits"

• "cutoffs" & "oxbow lakes"

• "floodplains" - a low lying area near a stream channel , subject to recurrent flooding, alluvial deposits generally mask the underlying bedrock

• floodplains are good for agriculture - fresh supply of nutrients with flooding; risky to live on

Meandering Stream Processes and Oxbow Lake Formation

• natural “levees” are formed as a river overflows its channel and alluvium is deposited on either side of the stream channel. As the water overflowing the channel moves laterally onto the flood plain, its velocity decreases with distance from the channel, and since a streams ability to move particles of a certain size (competence) is a function of velocity, the larger pieces of unconsolidated alluvium are deposited first, directly adjacent to the channel, thus building banks (i.e., levees) on either side of the stream channel. The finer sediment (alluvium) is carried farther away from the stream channel and is deposited onto the floodplain.

Flooding and Levee Development

• "alluvial terraces" - level areas that appear as topographic steps above the river, created by a stream as it scours with renewed downcutting into its flood plain - comprised of unconsolidated alluvium

Alluvial Terraces

• "delta" - a depositional plain formed where a river enters a lake or an ocean; (named after the triangular shape of the greek letter delta )

H. The Drainage Basin System

Terms & Concepts:

• "drainage basins" - the basic spatial geomorphic unit of a river system

• "drainage divides" - the ridges & highlands that separate one drainage basin from another - they delimit the "catchment area" of a drainage basin

• "continental divides" - a large-scale ridge or elevated area that determines the drainage patterns of drainage basins; e.g., the ridge in N.A. that separates drainage to the Pacific in the west from drainage to the Atlantic & Gulf in the east and to Hudson Bay & the Arctic Ocean in the north

• inputs to drainage basins: precipitation, minerals & rocks of the regional geology, & changes in energy as a result of uplift/subsidence due to tectonic activity

• changes in any portion of the basin can affect the entire system as it adjusts to carry the appropriate load relative to discharge & velocity

• a stream drainage system attempts to reach an equilibrium between "discharge", "transported load", "channel characteristics", & "channel slope"

(1) Drainage Patterns - the geometric arrangement of streams in an area

• determined by: slope, differing rock resistance to weathering & erosion, climatic & hydrologic variability, & structural controls of the landscape

Most Common Drainage Patterns

• "dendritic" - a tree-like pattern; energy expended is efficient because the overall length of the branches is minimized - the underlying (level) siltstone, sandstone, & shale is easily eroded

• "trellis" - characteristic of dipping & folded topography - the streams are directed by parallel folded structures - the smaller streams join the main streams at right angles

• "parallel" - is associated with steep slopes & some relief

• "rectangular" - formed by a faulted & jointed landscape which directs streams in patterns of right angle turns

• "radial" - results form streams flowing off a central peak or dome, e.g., a volcanic mountain

• "annular" - produced by structural domes; concentric patterns of rock strata guide the streams

• "deranged" - occur where the surface patterns have been disrupted (e.g., glaciated shield regions) - no clear geometry in the drainage pattern

• these drainage patterns are a function of the structure & relief of the land