Chapter 12 Physical Geography: River Systems

What is a correct match?

Gullies-->developed rills

Artificial Levee

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

The term for processes that are stream-related is

Fluvial

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.

Yazoo stream

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

Flash flood

A sudden and short-lived torrent of water that exceeds the capacity of a stream channel; associated with desert and semiarid washes. --These channels may fill in a few minutes and surge briefly during and after a storm.

Traction

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

What is a true match?

Capacity--total load that a stream can support

As a stream's competence decreases, which size of sediment would be deposited last?

Clay

Internal drainage

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

Alluvial terraces

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

Storm Hydrographs

May cover only a period of days, reflecting changes in discharge caused by specific precipitation events that lead to local flooding.

Watersheds are defined by

Multiple drainage divides

What is a correct pairing?

Radial drainage--central peak

The movement of particles along the streambed in short hops and jumps is referred to as

Saltation

Fluvial

Stream-related processes; from the Latin fluvius for "river" or "running water".

Sheetflow

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

Sediment Transport

The movement of rocks and sediment downstream when energy is high in a river or stream.

Nickpoint

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.

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.

The folded topography of the Ridge and Valley Province is characterized by which drainage pattern?

Trellis

Gradient facts

Within its drainage basin, every stream has a degree of inclination or gradient. • This is also known as the channel slope. • The gradient of a stream is defined as the drop in elevation per unit of distance. • Usually this is measured in meters per kilometer or feet per mile.

An abrupt change that interrupts the gradient of a stream is

a Nickpoint

When a river reaches a base level, its velocity decreases and its competence and capacity also decrease, which leads to the formation of

a delta

Flood

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

The ultimate base level is

sea level

Annual Hydrographs

show discharge over the course of an entire year, usually with the highest discharge occurring during the spring snowmelt season.

Relative to a meandering stream,

the inner portion of a meander features a point bar.

Saltation

the transport of salt grains (usually larger than 0.2mm, or 0.008 in.) by stream or wind, bouncing the grains along the ground in asymmetrical paths.

What is the correct sequence from smallest to largest?

--Rills, gullies, streams, river

The unconsolidated material deposited by streams is called

Alluvium

Rivers that have their headwaters in humid regions and subsequently flow through arid regions, like the Nile River, are termed

Exotic Streams

Point Bar

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

Given that Q=wdv, what will happen to Q (discharge) if w and d (width and depth) increase but v (velocity) remains constant?

Q will increase.

Drainage basin

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

Peak Flow

The highest discharge that occurs during a precipitation event; determined by the amount, location, and duration of the rainfall episode.

Flood Protection

• Usually, the term levee connotes an element of human construction, and these engineered features are common across the United States and throughout the world. • Artificial levees are earthen embankments, often built on top of natural levees. • They run parallel to the channel (rather than across it, like a dam) and increase the capacity in the channel by adding to the height of the banks. • For efficient use of time and materials, channels are often straightened during levee construction. • Levees are meant to hold floods within a channel, not prevent them completely. • Eventually, given severe enough conditions, an artificial levee will be overtopped or damaged in a flood.

In a meandering stream, the

Outside bank is the cut bank

Dissolved Load

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

Abrasion

Mechanical wearing and erosion of bedrock accomplished by the rolling and grinding of particles and rocks carried in a stream, removed by wind in a "sandblasting" action, or imbedded in glacial ice.

Mississippi River Deltas

Over the past 120 million years, the Mississippi River has transported alluvium throughout its vast basin into the Gulf of Mexico. • During the past 5000 years, the river has formed a succession of seven distinct deltaic complexes along the Louisiana coast. • Each new complex formed after the river changed course, probably during an episode of catastrophic flooding. • The history of the Mississippi River delta shows a dynamic system with inputs and outputs of sediment and shifting distributaries. • Mississippi drainage basin produces 550 million metric tons of sediment annually—enough to extend the Louisiana coast 90 m (295 ft) a year.

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. • A graded stream is one in which the channel slope has adjusted so that stream velocity is just enough to transport the sediment load. • Attainment of a graded condition 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.

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. ---at its maximum in upstream tributaries of a drainage basin, where sediment load is small and flow is turbulent.

Floodplains

The flat, low-lying area adjacent to a channel and subjected to recurrent flooding is a floodplain. • It is the area that is inundated when the river overflows its channel during times of high flow. • When the water recedes, it leaves behind alluvial deposits that generally mask the underlying rock with their accumulating thickness. • Stream meanders tend to migrate laterally across a valley; over time, they produce characteristic depositional landforms in the floodplain. • On either bank of some rivers, low ridges of coarse sediment known as natural levees are formed as by-products of flooding. • Coarser, sand-sized particles (or larger) are deposited first, forming the principal component of the natural level

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.

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 hydrologic variability, and structural controls of the landscape.

Floodplain

A flat, low-lying area along a stream channel, created by and subject to recurrent flooding; alluvial deposits generally mask underlying rock.

Floods and River Management

A flood is defined as a high water flow that passes over the natural bank along any portion of a stream. • Floods in a drainage basin are strongly connected to precipitation and snowmelt, which are, in turn, connected to weather patterns. • Floods can result from: − periods of prolonged rainfall over a broad region, − intense rainfall associated with short-lived thunderstorms, − rapid melting of the snowpack, − and rain-on-snow events that accelerate snowpack melting. • Floods vary in magnitude and frequency, and their effects depend on many factors.

Tectonic Uplift

A graded stream can be affected by tectonic uplift that changes the elevation of the stream relative to its base level. • A previously low-energy river flowing through the newly uplifted landscape becomes rejuvenated. • River gains energy and actively returns to downcutting • The associated degradation of the channel can eventually form entrenched meanders that are deeply incised in the landscape. • Such a stream is called an antecedent stream because it downcuts at the same rate at which the uplift occurs, thus maintaining its course.

Hydrograph

A graph of stream discharge (in m3/s or cfs, ft3/s) 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. --A graph for a specific portion of a stream changes after a disturbance such as a forest fire or urbanization of the watershed, with peak flows occurring sooner during the precipitation event.

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 logging meander. In Australia, known as a billabong (meaning "dead river" as an Aboriginal word).

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 ration 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.

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.

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.

processes of the stream channel

As this debris moves along, it mechanically erodes the streambed further through the process of abrasion, with rock and sediment grinding and carving the streambed like liquid sandpaper. • Fluvial erosion by hydraulic action and abrasion causes streams to erode downward (deepen), erode laterally (widen), and erode in an upstream direction (lengthen). • The process whereby streams deepen their channel is known as channel incision. • The process whereby streams lengthen their channels upstream is called headward erosion. • This type of erosion occurs when the flow entering a main channel has enough power to downcut, such as what occurs at the break in slope where a gully enters a deep valley

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.

Mississippi River Deltas

Compaction and the tremendous weight of the sediment in the Mississippi River create isostatic adjustments in Earth's crust. • These adjustments are causing the entire delta region to subside, a natural process that has occurred throughout the evolution of the river basin. • In the past, subsidence was balanced by additions of sediment that caused areal growth of the delta. • With the onset of human activities such as upstream dam construction and the excavation of canals and waterways through the delta for shipping and oil and gas exploration, the supply of alluvial sediment has decreased. • The delta is now subsiding without sediment replenishment. • The present main channel of the Mississippi River persists in large part from the effort and expense directed at maintaining the extensive system of artificial levees

Erosion

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

Changes in Discharge over time

Discharge varies throughout the year for most streams, depending on precipitation and temperature. • Rivers and streams in arid and semiarid regions may have perennial, ephemeral, or intermittent discharge. --Discharge changes over time at any given channel cross section ---when rainfall occurs in some portion of the watershed, the runoff is concentrated in streams and tributaries in that area.

Alluvial Fan

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

Suspended Load

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

Floodplains

Finer silts and clays are deposited farther from the river. • On meandering river floodplains, wetlands known as riparian marshes (sometimes called backswamps) often form in the poorly drained fine sediments deposited by overbank flows. • Another floodplain feature is yazoo streams, also known as yazoo tributaries, which flow parallel to the main river, but are blocked from joining it by the presence of natural levees. • Low-lying ridges of alluvium that accumulate on the inside of meander bends as they migrate across a floodplain often form a landscape referred to as bar and swale topography.

Single Thread Channels

For example, the Ohio, Missouri, and Mississippi Rivers, which form boundaries of several states, can shift their positions quite rapidly during times of flood. • Consider the Nebraska-Iowa border, which was originally placed mid-channel in the Missouri River. • In 1877, the river cut off the meander loop around a town, leaving the town "captured" by Nebraska. • The new oxbow lake was called Carter Lake and remains the state boundary. • Today, the city of Carter Lake is the only part of Iowa that lies west of the Missouri River.

Alluvium

General descriptive term for clay, silt, sand, gravel, or other unconsolidated rock and mineral fragments transported by running water and deposited as sorted or semisorted sediment on a floodplain, delta, or streambed.

The downstream portion of a river in humid areas

Generally is of higher velocity, which is masked by reduced turbulence.

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.

Flood Protection

In the United States, floods cause an average of about $6 billion in annual losses. • The catastrophic floods along the Mississippi River and its tributaries in 1993 and 2011 produced damage that exceeded $30 billion in each occurrence. • In 2010, floods occurred in 30 U.S. states and in at least as many countries in the world, as record land and ocean temperatures energized air masses, producing excessive rainfall totals. • Flood protection, when in place, generally takes the form of dams, spillways, and artificial levee construction along river channels.

Stream Channel Processes

The geomorphic work performed by a stream includes erosion and deposition, and depends on the volume of water and the total amount of sediment in the flow. --Hydraulic action is a type of erosive work performed by flowing water alone, a squeeze-and-release action that loosens and lifts rocks --The downstream portions of a river move much larger volumes of water past a given point and carry larger amounts of sediment.

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 meters per second or cubic feet per second.

River Deltas

The mouth of a river is where the river reaches a base level. • The combined delta complex of the Ganges and Brahmaputra Rivers in South Asia is the largest in the world.

Fluvial Processes and Landforms

The ongoing interaction between erosion, transportation, and deposition in a river system produces fluvial landscapes. -Erosion, transport, and deposition are affected by discharge and channel gradient. --Running water is an important erosional force. --Erosion in fluvial systems is the process by which water dislodges, dissolves, or removes weathered surface material. --This material is then transported to new locations, where it is laid down by the process of deposition.

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.

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.

Humans and Floodplains

Throughout history, civilizations have settled on floodplains and deltas, especially since the agricultural revolution of 10,000 years ago, when the fertility of floodplain soils was discovered. • Villages generally were built away from the area of flooding, or on stream terraces, because the floodplain was dedicated exclusively to farming. • Over time, as commerce grew and river transportation became more important, development near rivers increased. • Water is a basic industrial raw material used for cooling and for diluting and removing wastes; thus, industrial sites along rivers became desirable—and remain so. • Despite our historical knowledge of flood events and their effects, floodplains continue to be important sites of human activity and settlement.

What is an effect of urbanization on stream flow?

Urbanization decreases the lag time between a storm event and the peak flow.

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 is determined by: − topographic relief, − the nature of rock and soil through, which the stream flows, − climate, − vegetation, − and human activity in a drainage basin. • The dissolved load of a stream is the material that travels in solution, especially the dissolved chemical compounds derived from minerals, such as limestone or dolomite or from soluble salts. • The suspended load consists of fine-grained clastic particles (bits and pieces of rock).

Single-Thread Channels

Where channel slope is gradual, streams develop a sinuous (snakelike) form, weaving back and forth across the landscape in a meandering stream pattern and acquiring distinctive flow and channel characteristics. • The tendency to meander is evidence of a river system's propensity (like any natural system) to find the path of least effort toward a balance between self-equilibrating order and chaotic disorder. • In a straight channel or section of channel, the greatest flow velocities are near the surface at the center, corresponding to the deepest part of the stream. • Velocities decrease closer to the sides and bottom of the channel because of the frictional drag on the water flow. • Because the outer portion of each meandering curve is subject to the fastest water velocity, it undergoes the greatest scouring.

The suspended load of a stream consists of particles that

are held aloft in the stream flow.

Perennial Streams

flow all year, fed by snowmelt, rainfall, groundwater, or some combination of those sources

Intermittent Streams

flow for several weeks or months each year and may have some groundwater inputs

Ephemeral Streams

flow only after precipitation events and are not connected to groundwater systems

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.

Basic Fluvial Concepts

• In general, streams, a mixture of water and solids, provide resources and shape landforms. • They create fluvial landscapes through the ongoing erosion, transport, and deposition of materials in a downstream direction. • The energy of a stream to accomplish this geomorphic work depends on a number of factors, including: − gradient − base level − volume of flow (discharge)

Drainage Basins and Drainage Patterns

• Streams, which come together to form river systems, lie within drainage basins, the portions of landscape from which they receive their water. • 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.