Fluvial Processes and Landforms

Geomorphic significance of fluvial systems tied to the hydrological cycle

 - water evaporated from the ocean surface transported over continental regions where it condenses and precipitates

 - gravity returns some of this water back to the oceans as runoff

 - streams remaove excess water from the continents

 - streams erode, transport and eventually deposit weathered materials to form fluvial landforms

 - the geomorphic effect of flowing water displays significant spacial variations in its nature and intensity related to climatic and vegetative patterns

 - fluvial geomorphic features are also related to local soil and bedrock characteristics

 - flowing water is most effective as a landcape modifier in semiarid and subhumid regions

 - humid regions with abundant precipitation have only a moderate rate of development of fluvial features due to the protective influence of vegetation

 - the climatic zone of maximum fluvial effectiveness is one that receives enough rainfall to produce considerable storm runoff, but dry enough to support only a limited vegetation cover

Causes and Characteristics of Stream Flow

- land surfaces have infiltration capacities (maximum rate at which they can absorb water)

- water not absorded immediately collects at the surface (filling in depressions)

- if the land surface is sloped then water will begin to flow downhill under the influence of gravity

- flows increase in volume as the total upslope area being drained becomes progressively larger

- Channelization occurs where the water depth and speed enable flow to overcome the cohesion of soil particles

- on unprotected slopes (cleared of vegetation for agriculture) a parallel series of channels (rills) form first

- when rills coalesce they form steep sided gullies

- when gullies deepen enough to reach the water table of the area they begin tapping into groundwater supplies at which point they become a perennial stream channel

- rills and gullies normally do not form on undisturbed, well vegetated slopes

Drainage Systems and Patterns

- streams are organized into drainage systems (numerous interconnected stream segments)

- these stream segments act collectively to remove excess water and sediments from a drainage basin (total area drained by the stream system)

- drainage basin: main river (stream) fed by smaller tributaries (analogous to tree branches)

- drainage divides: separate the flow and direct it toward adjacent drainage systems (Continental Divide)

Stream Orders

- the size of a stream described as: length, width, volume of water discharged and drainage areas

- collectively these parameters can be conveyed by stream order

- streams are ranked from 1st to 12th order

- 1st order stream isa small stream with no tributaries

- 2nd order stream is formed when two 1st order streams form

- subsequent orders are formed when when two strams of the previous order join

- the physical dimensions of the streams increase with increasing stream order

- Mississippi River is a 10th order stream,

- Amazon River is a 12th order stream

Drainage Patterns

- the surface pattern collectively formed by the streams in a drainage system

- all drainage systems in a given area generally exhibit the same general pattern

- drainage patterns respond to bedrock types and preexisting surface features

- dendritic pattern: most common drainage pattern

- develops on gently sloping surfaces that are homogeneous in their resistance to erosion

- so structural control on stream location

- trellis pattern: typical of regions with lots of ridges and valleys where resistant and nonresistant sedimentary rock types alternate

- major streams in valleys flow parallel to ridges while the smaller tributaries flow down steep ridge flanks (perpendicular to the major streams) to join the major streams

(Appalachian Mountains, Valley and Ridge Regions)

- rectangular pattern: produced when the drainage pattern is controlled by intersecting fault and fracture systems (areas of crystalline rocks; granites)

- right angle bends in the streams

- radial pattern: outward flow of streams in all directions from a central pek or upland

- well developed near isolated volcanic peaks

- deranged pattern: large degree of spatial disorganization

- streams are highly irregular in their directions of flow

- they pass through depressions occupied by lakes and swamps

- areas of recent geologic orogin where drainage patterns have not had time to develop

(glacially scoured regions: Canadian Shield and Scandinavia)

 Erosion, Transportation and Deposition

- stream erosion results from the friction and shear forces generated between flowing water and nonmoving material with which it comes into contact

- if the shearing forces produced by the water exceed the cohesive strength of the nonmoving material, the material will be eroded and carried downstream

- 95 % of gravitational energy that produces stream flow is converted to heat through friction between water molecules

- the remaining 5 % is used for sediment erosion and transport

- erosive energy of a stream is related to its velocity, volume and the amount of friction between its banks and bed

- estimated that a stream's erosive power increases as the cube of its increase in velocity

(if the speed of the flow doubles, its erosive power increases by 8 times)

- critical erosion velocity: velocity of water needed to produce enough friction to erode material and produce eddies (turbulence) within the flow to lift the material and erode them

- sediment erosion by streams is accomplished in three different ways:

- hydraulic action: direct sweeping away of loose material by friction and turbulence by moving water

- abrasion: scraping of particles carried in the water against secondary materials

- corrosion: dissolving of rock materials so that they enter the flow in a molecular state

 Transportation

- turbulence lifts the sediments and keeps them from settling, allows the current to transport them downstream

- transport methods:

- dissolved load: transporting material in a dissolved state (particularly important in areas where the flow velocity is low, low turbulence)

- suspended load: consist of fine textured particles that are carried within the flow

- particles are kept from settling by the upward component of turbulent eddies generated by friction

- common where streams flow through unconsolidated areas

- bed load: the coarsest particles a stream is capable of moving

- particles are moved along the stream bed because they are too heavy for turbulence to transport them for great distances

- travel by saltation (roll slide or bounce along a stream bed)

- many bed loads are only transported during floods

 Deposition

- streams deposit sediments when their velocities slow sufficiently to reduce their turbulence

- this makes them incapable of transporting all of their sediment load

- coarsest materials are deposited first followed by finer sediments

- sediments are sorted or separated by size

- accumulation of thick deposits of sediments is common in the lower portions of stream systems (deltas)

- deposition also occurs locally and temporarily at numerous points along the courses of all streams

- accumulation of fine-textured stream-deposited sediments are called alluvium

- weathering, mass wasting and fluvial erosion act to erode the surface of the earth and produce most gradational landforms

- weathering acts first producing unconsolidated surface materials, mass movements transport these materials to stream valleys, and streams transport these materials to the sea

- during stream transport a variety of fluvial erosional and fluvial depositional features are formed

- the dominant fluvial feature is a valley (a linear lowland produced by erosion)

- the dimensions of a valley are determined by the volume of the river flow, the rate of stream erosion and the length of time over which the valley has developed

- Erosional Features:

- erosional valley refers to a valley still being actively deepened by a river downcutting toward base level (the elevation at any point along the stream's course where the gradient is sufficient to transport its sediment load past that point where there is no erosion or deposition)(flow equilibrium)

- a stream downcutting rapidly through rock material strong enough to maintain steep slopes produces a gorge

- a wider gorge is called a canyon

- the sides of gorges and canyons are determined by the material comprising the sides

- if the rock type is homogeneous a smooth slope will develop

- if the layers of rock alternate in erosive resistance an irregular slope will develop

- erosional valleys exist mostly in upland areas (plateaus, mountainous regions) in arid or semiarid regions

- erosional hills and mountains are the highland features that remain following the fluvial dissection of a plateau (carved out due to fluvial erosion)

- the pattern of hills or mountains within a region is determined by the fluvial drainage pattern that develops

- Appalachian and Ozark Plateaus form disorganized upland surfaces from dendritic drainage patterns

- in areas where folding and faulting have exposed rock sequences with different resistances to erosion, streams develop linear drainage patterns (trellis pattern)

- this will usually produce a parallel system of resistant linear ridgesand valleys (Appalachian Valley and Ridges)

- deposition occurs along the lower portions of drainage systems (gentle slopes)

- sediment supplies are in large volume

- stable long-term base level is more likely to be achieved than further upstream

- Floodplain is the dominant fluvial depositional feature

- floodplains contain secondary features such as meander deposits, natural levees, deltas and alluvial terraces

- floodplains are broad, flat-floored valleys covered by alluvium

- they are subject to flooding at all times of high water

- large quantities of sediment are transported by the stream during flooding and are deposited in a layer over the surface

- this adds valuable topsoil to the floodplain

Meanders

-the development of broad looping bends (meanders) in the courses of streams occupying flood planes is responsible for many floodplain features

-a meander begins as a slight bend in the channel

-the diversion in flow results in the erosion of unconsolidated material on the outside of the bend allowing it to become more pronounced

- as the bend increases in amplitude the river flow is directed toward the outer bank where erosion continues producing an undercut bank

- the meander slowly migrates down the valley as it grows

- the reduction of speed on the inner side of the developing meander causes the deposition of sediments as a point bar deposit

- continued erosion on the outer curve of the meander and deposition on the inner curve of the meander increases the amplitude of the meander

- the meander eventually forms a narrowing meander neck

- eventually meander sides intersect to form a meander cutoff

- the meander is eventually abandoned by the river (river takes the shorter route through the cutoff) and the meander is abandoned by the river to form an oxbow lake

oxbow lakes eventually fill with sediment becoming first a wetland (swamp) then dry land

Natural Levees

-water spilling over the banks of a river during a flood is subject to reduced current speeds and increased friction

- the coarsest sediments sometimes deposit atop the river banks where they accumulate to form a natural levee

- levees are broad ridges of fine sand and coarse silt that parallel each side of the stream channel

they increase the channel’s stability

Deltas

-deltas are deposits of alluvium formed when streams enter standing water bodies

- deposition takes place because the sudden reduction of flow that occurs as the stream enters the standing water body causes it to lose its sediment-carrying capacity

- all but dissolved sediments and clay sized particles settle out of the river flow

- the outward flow of sediments produces a fan shaped delta

- deltas are proportional in size to the sediment loads that a river carries

- river deltas are seaward extensions of floodplains

-a river that produces a flood plane has reached a stable base level

-if the base level is lowered again the river will continue down cutting its channel into the floodplain

- the remnant older floodplain forms a terrace with steep slopes well back from the current river channel

- a pair of terraces will be present on both sides of the river valley

- separation of the terraces on either side of the river channel depends on the width of the new river floodplain

Alluvial Fans

- these are fan-shaped deposits of alluvium that form in arid regions (deserts) when ephemeral streams (those that actually flow only rarely) flood and empty onto the desert plains out of the mountains

 Glacial Landforms

Glacier Formation and Distribution

- a glacier is a mass of freshwater ice formed on land that is in motion or has been in motion in the past

- glaciers move due to gravity

- source of glacial ice is compacted snow

- glaciers develop when the accumulation of snow during the year (winter) exceeds the loss of snow by melting and sublimation (summer)

- melting and sublimation processes are referred to as ablation

- need cold temperatures (high latitudes, alpine regions) and significant snowfalls

- glacier = snowfall accumulation > ablation

- snow gradually converted to glacial ice due to burial and compaction (change in density from fresh snow to a dense granular state called firn then to glacial ice)

- glaciers cover approximately 10% of the earth's land surface

continental glaciers - form in high latitudes and cover extensive land areas

alpine glaciers - form in high elevations in alpine areas

- two biggest continental glaciers: Antarctica (12, 588,000 km2) and Greenland (1,802,600 km2)

- these two continental glaciers contain ~ 75% of the world's freshwater supply

- if they melted sea levels would raise 45 meters

- alpine glaciers constitute a combined area of 508,000 km2

- most significant alpine glaciers: Alaska, Canadian Rockies, Canadian Arctic Islands, Andes, European Alps, Himalayas.......  

Glacier Processes and Features

- glacier motion is caused by gravity (compaction deforms ice which may melt a flow downhill)(river of ice)

- absolute motion is the actual movement of particles of ice in the glacier (very slow, usually imperceptible)

- surges are sudden increases in the rate of advance of a glacier

- surges are likely a result of abnormally high snowfall or are sometimes initiated by accumulations of subglacial meltwater

- relative motion of a glacier refers to advance or retreat of an ice front and is determined by the relationship between the absolute motion of a glacier and its rate of ablation

- a glacier that gains mass and advances usually has a positive mass balance (imputs exceed outputs)

- a glacier that loses mass and retreats usually has a negative mass balance (outputs exceed inputs)

- every glacier has a zone of accumulation and a zone of ablation (division between these zones is known as an equilibrium line)

- glacier movement has two main components: basal sliding (sliding of ice on bedrock) and internal flow (laminar flow of horizontally oriented ice crystals shearing over those below; plastic flow due to the weight of the overlying ice)

- the absolute motion of a glacier is the sum of basal sliding motion + internal flow motion

- internal flow rates are greatest near the center of the glacier than near the margins (next to bedrock)(similar to flow in a river)

- glaciers are very effective solid state eroders and transporters of regolith (weathered rock material) due to the tremendous pressure exerted by hundreds and even thousands of meters of ice

- Scraping is accomplished primarily by rocks frozen into the base of the ice and dragged over bedrock surfaces (large frozen in rocks sometimes produce striations or scratches in the bedrock which are valuable indicators of past ice flow direction; sand and silt sized particles smooth and polish some rock surfaces- like sandpaper)

- Plucking involves the excavation of angular, sometimes very large rock fragments from bedrock

- ice melting on the up-glacier side of an obstacle (hills, boulders) flows around or over the obstruction, freezes in rock fractures on the down-glacier side and plucks out rock fragments as the glacier moves on

- glaciers are most effective at eroding unconsolidated material (material from previous glaciations or soils) which may strip the unconsolidated cover down to bedrock

- glaciers pick up sediment, incorporate it into the ice flow and transport it away where it is eventually deposited as various features

- glaciers transport debris in a conveyor belt type fashion

- glacial drift is the general term for all deposits of glacial origin

- drift deposited directly by the glacier is known as till (unsorted, angular with particles ranging from clay to boulder sizes)

- glacial-fluvial sediments results from drift being reworked by glacial meltwaters after their initial deposition by glaciers (partly stratified and sorted)

- till is primarily deposited by a process of dumping at the glacier terminus (snout) or by leaving material on the surface beneath the glacier (produce deposits known as moraines)

- a terminal moraine is formed where the glacier terminates (conveying sediment to the end where it is dumped)(indicate the furthest extent of the glacier)

- when a glacier retreats, a series of moraines can remain marking the retreat of the glacier snout- these are called recessional moraines

- between terminal and recessional moraines a thin deposit of till is deposited by the retreating glacier - called a ground moraine

- drumlins are low elongated hills deposited by ice sheets (long axis of the drumlin parallel to the direction of ice movement)

- probably a result of reworking of previously deposited till by the readvancement of glacial ice

- composed of unsorted till

- usually occur in groups all parallel to one another (sometimes hundreds)

- as a glacial ice front recesses, sometimes large blocks of ice break off the main ice sheet and produce depressions called kettles in the ground moraine till

- if water from the isolated melting ice block is retained in the kettle depression, this is then known as a kettle lake

- glacio-fluvial deposits, generally called outwash deposits are usually deposited at or beyond the margins of the glacier or beneath the glacier (subglacial deposits)

- glacial meltwater is loaded with sediment that very often is deposited in braided stream deposits (multiple stream channels) across the area beyond the glacial front

- a lengthy deposit of glacio-fluvial alluvium confined to a valley is called a valley train

- sinuous ridges of glacio-fluvial deposited stratified drift are called eskers which are formed subglacially as meltwater flowing through tunnels in the interior of the ice sheet become choked off during a time when the ice sheet is stagnant (neither advancing or retreating)

- as water flow slows in these subglacial tunnels, sediment is deposited

- eskers are exposed when the glacier melts away

- kames are small, steep mounds or conical hills of stratified drift that form in pockets (rather than channels) in stagnant glacial ice through fluvial action as subglacial deltas or fans

Alpine Glaciers and Related Landforms

- most alpine glaciers originate in relatively small mountainside hollows called cirques

- if conditions are marginal for ice accumulation, the glacier may remain as a small, rounded cirque glacier

- if conditions are favourable for ice accumulation, the glacier will outgrow its cirque and flow down mountain side valleys as a long narrow river of ice called a valley glacier

- lateral moraines are stripes of rock debris that accumulate along the sides of alpine glaciers as a result of both glacial erosion and weathering along the sides of glacial valleys

- when two tongues of glacial ice meet their innermost lateral moraines coalesce to form a single medial moraine within the glacier

Alpine Glacier Erosional Landforms

- erosion by alpine glaciers tends to increase the relief and ruggedness of the topography and create a number of landform features

- cirques are bowl-shaped depressions eroded into the rock faces of mountainsides in glaciate areas

- consist of a steep headwall, a smooth central basin and a raised lip of threshold

- cirques that formed due to previous glaciations that are now devoid of ice and contain lakes are called tarns

- as cirques erode from opposite sides of a linear mountain they can eventually produce a narrow serrated ridge called an arête or erosion by three or more cirques can produce a sharp-crested pyramidal peak called a horn (Matterhorn, Switzerland)

- a glacial trough is a deep U-shaped valley, usually of fluvial origin, that has been subsequently occupied and modified by a valley glacier

- tributary glaciers entering a valley glacier (from the side) also carve out U-shaped valleys, but usually not as deep - the result (when the ice finally melts) is intersecting hanging valleys with steep walls

- at higher latitudes, glacial troughs may be eroded below sea level and occupied by ocean water to become fiords when the ice melts

Alpine Glacier Depositional Landforms

- same features moraine and glacio-fluvial features as ice sheet landforms but less extensive and not defined as well

The Pleistocene Epoch

The Pleistocene was a time when large ice sheets advanced and retreated over North America

Oxygen isotope records from marine core sediments suggest that global climate has gradually cooled from the beginning of the Eocene through the Pleistocene

Oxygen isotope data from deep sea cores reveal that during the past 2 million years there were at least 20 major glaciation periods

Stratigraphic evidence indicates that there were at least 4 major episodes of Pleistocene glaciation in North America as evidenced by glacial till deposits (separated by 4 interglacials as evidenced by soil horizons)

The most recent glacial event is called the Wisconsinan Glaciation which began about 70,000 years ago

Ice sheets that grew as a result of the cool temperatures imposed by the Wisconsinan Glaciation (Laurentide Ice Sheet in North America and the Fennoscandian Ice Sheet in Northern Europe) composed a volume of 43 million cubic kilometres during the Glacial Maximum

Ocean Sea level was lower by approximately 130 meters

The Laurentide Ice Sheet was composed of 4 main ice loges centred over Greenland, Baffin Island, the Keewatin region of the North West Territories and Labrador

There was also a separate Cordilleran Ice Sheet (ice free corridor between the Cordilleran and Keewatin Ice Sheets)

During the Wisconsinan Glaciation there were intervals of ice sheet advance and retreats

Tilt cycle: 41,000 years

Wobble cycle: 23,000 years

These effects occasionally reinforce one another to cause unusually great or small net solar radiation on the earth's surface (Milankovitch cycles)

Pluvial and Proglacial Lakes

The Laurentide Ice Sheet extended the Arctic front or jet stream (the boundary between cold air from the north and warm air from the south) south over the southwestern US during the Wisconsinan Glaciation

Weather systems track along the jet stream and therefore the result of the jet stream being further south during the Wisconsinan glaciation is more precipitation in the southwestern US

Water as a result of the increased precipitation collects in the basins defined by the Basin and Range geological province resulting in Pluvial Lakes (Lake Bonneville, Lake Lahontan, Lake Manly)

Once the Laurentide Ice sheet retreated north at the end of the Wisconsinan Glacial Period the jet stream assumed a more northerly route which reduced precipitation in southwestern US causing the Pluvial Lakes to evaporate leaving dry lake deposits in the basins (Playas)

Proglacial Lakes are formed by the meltwater accumulating along the margins of glaciers.

Numerous proglacial lakes existed during the Pleistocene (Lake Missoula, Lake Columbia, Lake Agassiz, Great Lakes)

The Scablands of Eastern Washington

Lake Missoula formed when an advancing glacier plugged the Clark Ford Valley at Ice Cork, Idaho causing glacial melt water to fill the western valleys of Montana.

When the ice dam impounding Lake Missoula failed (ice retreated back) the water drained south at a tremendous velocity draining south and southwest across Idaho and into Washington

The water carved out huge valleys in the bedrock (plucking off pieces of basalt 10 m across, channels contain giant ripple marks - 10 m high and 70 to 100 meters apart)

Lake Missoula formed, drained (producing scablands) and re-formed at least 4 times (possibly more)

Glaciers and Isostasy

When ice accumulates in great quantities in a particular area the earth's crust responds by gradually subsiding down into the mantle under the weight of the load (in some places 300 meters below previous elevations)

When the ice melts the weight of the ice is removed and the earth's crust slowly rebounds (isostatic rebounding)

The diagram below shows isostatic rebound in meters during the last 6000 years