Geosystems Ch. 15

alluvial terrace

level areas that appear as topographic steps above a stream, created by the stream as it scours with renewed downcutting into its flooplain; composed of unconsolidated alluvium.

Specifically, what is a flood? How are such flows measured and tracked, and how are they used in floodplain management?

"A flood is defined as a highwater flow that passes over the natural bank along any portion of a stream." - Pg. 478. "FLOOD PROBABILITY - Maintaining extensive historical records of discharge during precipitation events is critical for predicting the behaviour of present streams under similar conditions. The Water Survey of Canada and U.S. Geological Survey have detailed records of stream discharge at stream-gauging stations for only about 100 years - in particular, since the 1940s. Flood predictions are based on these relatively short-term data. Scientists hope that adequate funding to support hydrologic monitoring by stream-gauging networks will continue, for the data are essential to flood hazard assessment. On the basis of these historical data, flood discharges are rated statistically according to the time intervals expected between discharges of similar size. Thus, a "10-year flood" has a recurrence interval of 10 years, a "50-year flood" has a recurrence interval of 50 years, and so on. ... a 10-year flood has a discharge that is statistically likely to occur once every 10 years on average, based on discharge data for that particular stream. This also means that flood of this size has only a 10% likelihood of occurring in any one year and is likely to occur about 10 times each century. The use of historical data works well where available; however, complications are introduced by urbanization and dam construction, which can change the magnitude and frequency of flood events on a stream or in a watershed. These statistical estimates are probabilities that events will occur randomly during a specified period. For example, 2 decades might pass without a 50-year flood; or a 50-year level of flooding could occur 3 years in a row. Recently, scientists are describing floods and precipitation events using the annual exceedance probability (AEP), a term that better represents the statistical likelihood of occurrence. By this measure, a 100-year flood has a 1% annual exceedance probability." - Pg. 483.

What is the basic organisational unit of a river system? How is it identified on the landscape? Define the several relevant key terms used.

'... Every stream has its own DRAINAGE BASIN, or WATERSHED, ranging in size from tiny to vast. A major drainage basin system is made up of many smaller drainage basins, each of which gathers and delivers its runoff and sediment to a larger basin, eventually concentrating the volume into the main stream.' - Pg. 454. In any drainage basin, water initially moves downslope as OVERLAND FLOW, which takes 2 forms: 1) Sheetflow: a thin film spread over the ground surface. 2) Rills: small-scale grooves in the landscape made by the downslope movement of water. Rills can develop into deeper GULLIES and then into stream channels leading to the valley floor. Regarding the terrain that surrounds drainage basin. Interfluve: the high ground that separates one valley from another and directs sheetflow. Catchment: Water-receiving area of every drainage basin. Defined by drainage divides. Drainage divides: ridges that are the dividing lines that control into which basin the surface runoff drains.

In Figure 15.3, follow the Allegheny-Ohio-Mississippi River system to the Gulf of Mexico. Analyze the pattern of tributaries and describe the channel. What role do continental divides play in this drainage?

'A special class of drainage divides, CONTINENTAL DIVIDES, separates drainage basins that empty into (455/456) different bodies of water surrounding a continent. ... . The principal drainage divides and drainage basins in the United States and Canada are mapped in Figure 15.3.' 'A major drainage basin system is made up of many smaller drainage basins. Each drainage basin gathers its precipitation and sediment and delivers it to a larger river, concentrating the flow into the main stream. Every smaller drainage basing gathers gathers its precipitation and sediment and delivers it all to a larger river, concentrating the flow into the main stream. ... from Figure 15.3 is the Peace-Athabasca-Mackenzie river system. The total area drained by this system is 1.8 million km^2, 18% of Canada's total land mass. Precipitation failing in an area collects in streams and may travel vast distances. Consider the precipitation that falls in central Alberta as far south as Jasper National Park. This water flows in hundreds of small streams that join the Athabasca River. At the same time, precipitation in northern Alberta feeds hundreds of streams that flow into the Peace and Athabasca rivers flow into Lake Athabasca, and then into the Slave River, which, in turns, empties into Great Slave Lake. Water from Great Slave Lake flows into the mackenzie River. The Liard River collects the outflow of streams that gathered precipitation in northeastern British Columbia and then joins the Mackenzie at Fort Simpson, and continues northward past Inuvik into the Beaufort Sea and the [456/457] Arctic Oean. Each tributary, large or small, adds its flow and sediment load to the larger river it joins. When water enters a body of standing water such as a lake or the sea, sediment accumulates. Sediment weathered and eroded in central Alberta and northeastern British Columbia is carried by rivers and is eventually deposited in the Peace-Athabasca, Slave, and mackenzie river deltas.

What role is played by rivers in the hydrologic cycle?

'Earth's rivers and waterways form vast arterial networks that drain the continents, and at any moment, approximately 1250 km^3 of water is flowing through them. Even though this volume is only 0.003% of all freshwater, the work performed by this energetic flow makes it a dominant natural agent of landmass denudiation. Rivers shape the landscape by removing the products of weathering, mass movement, and erosion and transporting them downstream. Rivers also serve society in many ways. They not only provide us with essential water supplies, but also receive, dilute, and transport wastes, provide critical cooling water for industry, and form critical transportation networks.' - Pg. 454.

suspended load

Fine particles held in suspension in a stream. The finest particles are not deposited until the stream velocity nears zero.

sediment transport

Fine-grained mineral matter that is transported and deposited by air, water, or ice.

Explain these statements: (a) All streams have a gradient, but not all streams are graded. (b) Graded streams may have ungraded segments.

'GRADED STREAMS - The changes in a river's gradient from its headwater to its mouth are usually represented in a side view called a LONGITUDINAL PROFILE. The curve of a river's overall gradient is generally concave (Figure 15.19). As mentioned earlier, a river characteristically has a steeper slope nearer the headwaters and a more gradual slope downstream. The causes of this shape are related to the energy available to the stream for transporting the load it receives. [469/470/471/472]The tendency of natural systems, including streams, to move toward a state of equilibrium causes stream channels, over a period of years, to adjust their channel characteristics so that the flow is able to move the sediment supplied form the drainage basin. A GRADED STREAM is one in which the channel slope has adjusted, given the discharge and channel conditions, so that stream velocity is just enough to transport the sediment load. A graded stream has the characteristic longitudinal profile illustrated in Figure 15.19. Any variaton, or bump, in the profile, such as the steep drop of a waterfall, will be smoothed out over time as the stream adjusts toward a graded condition. Attainment of a graded conditions does not mean that the stream is at its lowest gradient, but rather that it has achieved a state of DYNAMIC EQUILIBRIUM between its gradient and its sediment load. This balance depends on many factors that work together on the landscape and within the river system. An individual stream can have both graded and ungraded portions, and it can have graded sections without having an overall graded slope. In fact, variations and interruptions are the rule rather than the exception. Disturbances in a drainage basin, such as mass wasting on hillslopes that carries material into stream channels, or overgrazing of riparian vegetation and associated streambank instability, can cause disruptions to this equilibrium condition. The concept of stream gradation is intimately tied to stream gradient; any change int he characteristic longitudinal profile of a river causes the system to respond, seeking a graded condition. The following discussions of tectonic uplift and of nickpoints delve further into this concept.' Pg. 469-472. So basically, streams tend toward DYNAMIC EQUILIBRIUM, wherein its gradient and its sediment load are equal: over time, steep gradients have been smoothed over, and the velocity is JUST enough to transport its sediment. A graded slope has GRADIENT, in that it has rise over run, but it's gradient and sediment load are roughly equal. Not all slopes are graded, in that they may have uneven gradients. Also, different sections of an individual stream may be graded.

What are the five largest rivers on Earth in terms of discharge?

'Of the world's rivers, those with the greatest DISCHARGE (the streamflow volume past a point in a given unit of time, discussed in this chapter) are the Amazon of South America, the Congo of Africa, the Yangtze (Chang Jiang) of Asia, and the Orinoco of South America.' - Pg. 454.

Define the term FLUVIAL. What is a fluvial process?

'Processes that are related expressly to streams and rivers are termed FLUVIAL'

What is a river delta? What are the various deltaic forms? Give some examples.

'RIVER DELTAS - The mouth of a river is where the river reaches a base level. There the river's velocity rapidly decelerates as it enters a larger, standing body of water. The reduced stream energy causes deposition of the sediment load. Coarse sand is deposited closest to the river's mouth. Finer materials, such as silty mud and clays, are carried farther and form the extreme end of the deposit, which may be SUBAQUEOUS, or underwater, even at low tide. The level or nearly level depositional plain that forms at the mouth of a river is a DELTA. ARCUATE (Arc-shaped) DELTA: Extensive lower plain covered by an intricate maze of distributaries formed in an ARCUATE (arc-shaped) pattern. Ex: Ganges and Brahmaputra Rivers in South Asia ESTUARINE DELTA: A delta that is in the process in filling an ESTUARY, the body of water at a river's mouth where freshwater flow encounters seawater.

How does stream discharge do its erosive work? What are the processes at work in the channel?

'STREAM CHANNEL PROCESSES - the geomorphic work performed by a stream includes EROSION and DEPOSITION, which depends on the volume of water and the total

Describe the Ganges River delta. What factors upstream explain its form and pattern? Assess the consequences of settlement on this delta.

'The combined delta complex of the Ganges and Brahmaputra Rivers in South Asia is the largest in the world at some 60,000 km^2. This delta features an extensive lower plain covered by an intricate maze of distributaries formed in an ARCUATE (arc-shaped) pattern. ... Owing to the high sediment load of these rivers, deltaic islands are numerous.' - Pg. 475.

What is the sequence of events that takes place as a stream dislodges material?

1. EROSION: water dislodges, dissolves, or removes weathered surface material. 2. Material transported to new locations. 3. DEPOSITION: Materials are deposited into a new location.

bajada

A continuous apron of coalesced alluvial fans, formed along the base of mountains in arid climates; presents a gently rolling surface from fan to fan.

delta

A depositional plain formed where a river enters a lake or an ocean; named after the triangular shape of the Greek letter delta.

drainage pattern

A distinctive geometric arrangement of streams in a region, determined by slope, differing rock resistance to weathering and erosion, climatic and hyrologic variability, and structural controls of the landscape.

hydrograph

A graph of stream discharge (in m^3 x s^-1( over a period of time (minutes, hours, days, years) at a specific place on a stream. The relationship between stream discharge and precipitation input is illustrated on the graph.

flood

A high water level that overflows the natural riverbank along any portion of a stream.

base level

A hypothetical level below which a stream cannot erode its valley-and thus the lowest operative level for denudation processes; in an absolute sense, it is represented by sea level, extending under the landscape.

oxbow lake

A lake that was formerly part of the channel of a meandering stream; isolated when a stream eroded its outer bank, forming a cutoff through the neck of the looping meander.

natural levee

A long, low ridge that forms on both sides of a stream in a developed floodplain; a depositional product (coarse gravels and sand) of river flooding.

drainage density

A measure of the overall operational efficiency of a drainage basin, determined by the ratio of combined channel lengths to the unit area.

continental divide

A ridge or elevated area that separates drainage on a continental scale; specifically, that ridge in North America that separates drainage to the pacific on the west side from drainage to the Atlantic and Gulf on the east side and to Hudson bay and the Arctic Ocean in the north.

yazoo stream

A small tributary stream draining alongside a floodplain; blocked from joining the main river by its natural levees and elevated stream channel.

braided stream

A stream that becomes a maze of interconnected channels laced with excess sediment. braiding often occurs with a reduction of discharge that reduces a stream's transporting ability or with an increase in sediment load.

How does a stream transport its sediment load? What processes are at work?

A stream transports its sediment load when the force of flowing water is GREATER than resistant forces. Via hydraulic action, the stream erodes, loosens and lifts rocks. Hydraulic Action: a type of erosive work performed by flowing water alone, a squeeze-and-release action that loosens and lifts rock. •Transport: if force of flowing water > resistant forces 'SEDIMENT LOAD - When stream energy is high and a supply of sediment is present, streamflow propels sand, pebbles, gravel, and boulders downstream in the process known as SEDIMENT TRANSPORT. The material carried by a stream is its SEDIMENT LOAD, and the sediment supply relates to topographic relief, the nature of rock and soil through which the stream flows, climate, vegetation, and human activity in a drainage basin. Discharge is also closely linked to sediment transport-increased discharge moves a greater amount of sediment, often causing streams to change from clear to murky brown after a heavy or prolonged rainfall. Sediment is moved as DISSOLVED LOAD, SUSPENDED LOAD, or BEDLOAD.' - Pg. 463. Bedload •Large grain sizes (gravel, boulders, cobble) •Travel along the bed by TRACTION and SALTATION Can form complex structures such as dunes, cross-bedding, and chains Dissolved Load •Very small grain sizes •Carried within solution •Often enters stream through weathering Suspended Load •Small grain sizes (clay, silt, sand) •Rarely touch the bed •Travel in suspension

flash flood

A sudden and short-lived torrent of water that exceeds the capacity of a stream channel; associated with desert and semiarid washes.

traction

A type of sediment transport that drags coarser materials along the bed of a stream.

What processes are involved in the formation of an alluvial fan? What is the arrangement, or sorting, of alluvial material on the fan?

ALLUVIAL FAN: Fan-shaped fluvial landform at the mouth of a canyon; generally occurs in arid landscapes where streams are intermittent. (SEE bajada). BAJADA: A continuous apron of coalesced alluvial fans, formed along the base of mountains in arid climates; presents a gently rolling surface from fan to fan. (SEE Alluvial fan.) PLAYA: An area of salt crust left behind by evaporation on a desert floor, usually in the middle of a desert or semiarid bolson or valley; intermittently wet and dry. Alluvial fans occur in arid or semi-arid climates, commonly at the mouth of a canyon where an ephemeral stream channel exits the mountains into a flatter Valley (See Figure 15.26). They can also occur in humid climates along mountain fronts and at the margins of steep-sided glaciated valleys. Alluvial fans are produced when flowing water (such as a flash flood) abruptly loses velocity as it leaves the constricted channel of a canyon and therefore drops layer upon layer of sediment along the base of the mountain block. Water then flows over the surface of the fan and produces a braided drainage pattern, sometimes shifting from channel to channel. A BAJADA may form if individual fans conjoin into one sloping surface. The Sediment composing alluvial fans is sorted by size. 1. The coarsest materials (gravels) @ the mouth of the canyon, the apex of the fan. 2. Pebbles and finer gravels further out. 3. Sands and silts. 4. Finest clays and dissolved salt carried in suspension and solution to the valley floor. As water evaporates, salt crusts may be left behind on the desert floor in a PLAYA (see Figure 13.14c).

playa

An area of salt crust left behind by evaporation on a desert floor, usually in the middle of a desert or semiarid bolson or valley; intermittently wet and dry.

graded stream

An idealized condition in which a stream's load and the landscape mutually adjust. This forms a dynamic equilibrium among erosion, transported load, deposition, and the stream's capacity.

Explain the base level concept. What happens to a stream's base level when a reservoir is constructed?

BASE LEVEL: The level below which a stream cannot erode its valley. ULTIMATE BASE LEVEL: Sea level. LOCAL BASE LEVEL: When a reservoir is constructed, the stream's base level goes up.

bed load

Coarse materials that are dragged along the bed of a stream by traction or by the rolling and bouncing motion of saltation; involves particles too large to remain in suspension.

erosion

Denudiation by wind, water, or ice, which dislodges, dissolves, or removes surface material.

Describe drainage patterns. Define the various patterns that commonly appear in nature.

Drainage patterns are open systems. Inputs: Precipitation and the minerals and rocks of the regional geology. Energy & materials are redistributed as the stream constantly adjusts to its landscapes Outputs: water and sediment thorugh the mouth of the stream or river, a lake, ocean, DRAINAGE PATTERN: the arrangement of channels in an area. There are 7 common types of drainage patterns: -DENDRITIC (Greek word DENDRON, meaning "tree"): a treelike pattern similar to natural systems like capillaries in the human circulatory system, or the veins in leaves or the roots of trees. Energy expenditure in the moving of water and sediment through the drainage system is efficient because the total length of the branches is minimized. -TRELLIS: characteristic of dipping or folded topography. -RADIAL: streams flow off a central peak or dome, such as occurs on a volcanic mountain. -RECTANGULAR: formed by a faulted and jointed landscape, which directs stream courses in patterns of right-angle turns. -ANNULAR: occur on structural domes, with concentric patterns of rock strata guiding stream courses. DERANGED: with no clear geometry and no true stream valley occurs in areas such as the glaciated shield regions of Canada and northern Europe.

Describe the formation of a floodplain. How are natural levees, oxbow lakes, riparian marshes (backswamps), and yazoo tributaries produced?

FLOODPLAIN: A flat, low-lying area long a stream channel, created by and subject to recurrent flooding: alluvial deposits generally mask underlying rock. NATURAL LEVEES: A long, low ridge that forms on both sides of a stream in a developed floodplain: a depositional product (coarse gravels and sand) of river flooding. OXBOW LAKES: A lake that was formerly part of the channel of a meandering stream; isolated when a stream eroded its outer bank, forming a cutoff through the neck of the looping meander (SEE Meandering stream). RIPARIAN MARSHES: YAZOO STREAM: A small tributary stream draining alongside a floodplain: blocked from joining the main river by its natural levees and elevated stream channel. "...YAZOO STREAM, also known as YAZOO TRIBUTARIES, which fllow parallel to the main river but are blocked from joining it by the presence of natural

alluvial fan

Fan-shaped fluvial landform at the mouth of a canyon; generally occurs in arid landscapes where streams are intermittent.

Describe the flow characteristics of a meandering stream. What is the pattern of flow in the channel? What are the erosional and depositional features and the typical landforms created?

Flow characteristics of a meandering stream include: 1) A gradual channel slope 2) Streams with a more sinuous (snakelike) form. 3) Remarkable looping pattern. 4) The outer portion of each meandering curve is subject to the fastest water velocity, it undergoes the greatest erosive, or SCOURING. SCOURING can create a steep UNDERCUT BANK, or CUTBANK. 5) The inner portion of each meander experiences the slowest water velocity and thus is a zone of fill (or aggradation) that results in a POINT BAR, an accumulation of sediment on the inside of a meander bend. 6) Laterial erosion creating MEANDER SCARS. 7) Actively meandering streams erode their outside banks as they migrate, often forming a narrow neck of land that eventually erodes through and forms a CUTOFF. 8) Oxbow Lakes, creating when meanders become isolated from the rest of the river. 9) Thalweg = zone of maximum depth.

Differentiate between a natural stream hydrograph and one from an urbanized area.

HYDROGRAPH: A graph of stream discharge. 'Following urbanization, stream discharge dramatically increases.' - Figure 15.9 •Urbanization activities: -Can decrease lag time between storm event and peak discharge -Can increase magnitude of peak discharge

artificial levee

Human-built earthen embankments along river channels, often constructed on top of natural levees.

point bar

In a stream, the inner portion of a meander, where sediment fill is redeposited.

internal drainage

In regions where rivers do not flow into the ocean, the outflow is through evaporation or subsurface gravitational flow. Portions of Africa, Asia, Australia, and the western united States have such drainage.

undercut bank

In streams, a steep bank formed along the outer portion of a meandering stream; produced by lateral erosive action of a stream; sometimes called a CUTBANK.

dissolved load

Materials carried in chemical solution in a stream, derived from minerals such as limestone and dolomite or from soluble salts.

fluvial

Stream-related processes.

sheetflow

Surface water that moves downslope in a thin film as overland flow; not concentrated in channels larger than rills.

Why is Niagara Falls an example of a nickpoint?

NICKPOINTS (KNICKPOINTS): The point at which the longitudinal profile of a stream is abruptly broken by a change in gradient; for example, a waterfall, rapids, or cascade. "NICKPOINTS - when the longitudinal profile of a stream contains an abrupt change in gradient, such as at a waterfall or an area of rapids, the point of interruption is a NICKPOINT (also spelled KNICKPOINT). Nickpoints can result when a stream flows across a resistant rock layer or a recent fault line or area of surface deformation. Temporary blockage in a channel, caused by a landslide or a logjam, also could be considered a nickpoint; when the logjam breaks, the stream quickly readjusts its channel to its former grade. A nickpoint is a relatively temporary and mobile feature on a landscape. .... In the region of Niagara Falls on the Ontario-New York border, glaciers advanced and then receded some 13,000 years ago. In doing so, they exposed resistant rock strata that are underlain by less-resistant shales. The resulting tilted formation is a CUESA, which is a ridge with a steep slope on one side (called an escarpment) and beds gently sloping away on the other side (Figure 15.22a). The Niagara escarpment actually stretches across more than 700km from east of the falls, it extends northward through Ontario, Canada, and the Upper Peninsula of Michigan and then curves south through Wisconsin along the western shore of Lake Michigan and the Door Peninsula. As the less-resistant: material continues to weather, the overlying rock strata collapse and Niagara Falls erodes upstream toward Lake Erie (this is the process of headward erosion, described earlier in the chapter). Engineers occasionally use control facilities upstream to reduce flows over the... [472/473] ... American Falls at Niagara in order to inspect cliff erosion, which has moved the location of the falls more than 11 km upstream from the steep face of the Niagara escarpment (Figures 15.22b and c).

meandering stream

The sinuous, curving pattern common to graded streams, with the energetic outer portion of each curve subjected to the greatest erosive action and the lower-energy inner portion receiving sediment deposits.

gradient

The drop in elevation from a stream's headwaters to its mouth, ideally forming a concave slope.

hydraulic action

The erosive work accomplished by the turbulence of water; causes a squeezing and releasing action in joints in bedrock; capable of prying and lifting rocks.

aggradation

The general building of land surface because of deposition of material; opposite of degradation. When the sediment load of a stream exceeds the stream's capacity to carry it, the stream channel becomes filled through this process.

discharge

The measured volume of flow in a river that passes by a given cross section of a stream in a given unit of time; expressed in cubic metre per second or cubic feet per second.

estuary

The point at which the mouth of a river enters the sea, where freshwater and seawater are mixed; a place where tides ebb and flow.

degradation

The process occurring when sediment is eroded along a stream, causing channel incision.

deposition

The process whereby weathered, wasted, and transported sediments are laid down by air, water, and ice.

hydrology

The science of water, including its global circulation, distribution, and properties-specifically water at and below Earth's surface.

saltation

The transport of sand grains (usually larger than 0.2 micrometres) by stream or wind, bouncing the grains along the ground in asymmetrical paths.

nickpoint

a.k.a. (knickpoint) The point at which the longitudinal profile of a stream is abruptly broken by a change in gradient; for example, a waterfall, rapids, or cascade.

drainage basin

the basic spatial geomorphic unit of a river system; distinguished from a neighbouring basin by ridges and highlands that form divides, marking the limits of the catchment area of the drainage basin

Differentiate between stream competence and stream capacity.

•Competence: maximum particle size that a river can move (based on average discharge) •Capacity: maximum amount of solid sediment that a river can move at a given discharge