Chapter 14:River Systems and Landforms

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Hydrology

science of water (circulation, distribution, properties)

Fluvius

'river' (processes and landforms)

Driving forces

gravity (insolation)

• 3 processes

: erosion, transportation and deposition

Alluvium

clay, silt and sand deposited by stream

Streams

mixture of solids and water

o Fluvial

Stream related process; from the Latin fluvius for "river" or running water

o Hydrology:

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

o Erosion

Denudation by wind, water, or ice, which dislodges, dissolves, or removes surface material.
o Water dislodges, dissolves, or removes surface material in the erosion process.
o Thus a stream is a mixture of water and solids; the solids are carried in suspension, by mechanical transport, and in dissolved solution.

o Transport

The actual movement of weathered and eroded materials by air, water and ice.

o Deposition

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

o Alluvium

The general term for clay, slit, sand, gravel, and mineral fragments transported by running water as sorted or semi sorted sediment on a floodplain, delta, or streambed.

o Base level:

is a level below which a stream cannot erode its valley. In general, the ultimate base level is sea level, the average level between high and low tides.
o 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.
o • base level (Powell; USGS): the level below which a stream cannot erode its valley
o • imaginary surface extending inland from sea-level (lowest level of denudation processes)
o • local base-level: river, lake, resistant strata (lower limit of local streams)

o Drainage Basins

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.

o Sheetflow

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

o Continental divides:

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 Artic Ocean in the north.

o 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.

o 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. Determined by dividing the total length of all stream channels in the basin by the area of the basin.

o 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 structure controls of the landscape.

o Discharge

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

o Exotic stream

A river that rises in a humid region and flows through an arid region, with discharge decreasing toward the mouth; for example, the Nile River and the Colorado River.
o • rivers have played important role in agriculture, transportation, industrial uses, removal of excess waste, etc.  human pop'n grew alongside
o • e.g. North Carolina (1999): Hurricanes Denis, Floyd and Irene (Sept-Oct) deluged NC with several feet of pptsaturated ground; flushed out waste lagoons (each hog 2 tons/year of waste) set on floodplains sent into the ocean  dead zone
o • e.g. Bangladesh: 3⁄4 of area on floodplain, densely populatedupstream forest harvesting of Ganges-Brahmaputra watershed leads to increased run-off  higher sediment loadsdeposited in delta regions forming island used as nascent farming communities (150,000 dead during flooding 1988, 91)
o • rating floodplain risk: flood - high water flow, over-topping levees; statistically rated for time interval between floods of a certain magnitude - e.g. 10 yr flood (the greatest level of flooding likely to occur every 10 years; 10% chance every year)

exotic stream: Q decreases with distance (high POTET rates arid environments) - e.g. Nile River, Colorado

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.
o Erosion: turbulence and abrasion= modify landscape
o • Hydraulic action = work done by flowing water alone (Running water causes squeeze-release action that loosens and lifts rocks) = further erosion of streambed: (mechanically) abrasion
o • Upstream: hydraulic influence vs. downstream: sediment load

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.

o Four processes Transport Eroded materials: solution, suspension, saltation, and traction.

solution, suspension, saltation, and traction.

Dissolved load

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

suspended load

fine particle held in suspension in a stream. The finest particles are not deposited until the stream velocity nears zero. Consists of fine-grained, clastic particles (bits and rock).
o Turbulence in the water, with random upward motion, is an important mechanical factor in holding a load of sediment in suspension.

bed load

refers to coarse materials that are dragged, rolled, or pushed 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.

traction

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

saltation

The transport of grains (usually larger than0.22mm or 0.008in) by stream or wind, bouncing the grains along the ground in asymmetrical paths.

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.

braided stream

A stream that becomes a maze of interconnected channels because it is 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.
o • laminar - streamlined, smooth flow with suspended particles move in parallel (deep and smooth channels)
o • turbulent - mixing flow, with increased transport (shallow streams, rough channel). Flow becomes turbulent is shallow stream or where channel is rough

Fluvial Transport

o • sediment load: f(x) of topography, geology, climate, vegetation and human activity
o • competence - ability to move particles of specific size; f(x) of velocity • capacity - total possible load stream can transport
o • solution- dissolved load (chemical weathering - limestone, soluble salts)
o • suspended - fine-grained clasts; flowing water
o • bed load - coarse material dragged along streambed: traction; rolled or bounced: saltation
o • stream load > capacity = aggradation
o • braided stream - reduced Q (reduces transport ability); landslides, glacial meltwater

Meandering stream

The sinuous, curving pattern common to graded streams, with the energetic outer portion of each curved subjected to the greatest erosive action and the lower-energy inner portion receiving sediment deposits.
o • Highest v = deepest part of channel
o • Three types channels: braided, straight and meandering
o • Meandering - sinuous pattern develops when slope is gradual
o • Undercut bank - outer portion of each meander (high v, scouring action)
o • Point bar - low v = sediment deposited; migrate downstream (landscape)
• The outer portion of each meandering curve is subject to the fastest water velocity

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 cut bank.

Point bar

In a stream the inner portion of a meander, where sediment fill is redeposited because the inner portion of the a meander experiences the slowest velocity and this receives sediment fill, forming a point bar.

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. In Australia known as the billabong. (Easier to understand: When the former meander becomes isolated from the rest of the river; the resulting oxbow lake may gradually fill with organic debris and silt or may again become part of the river when it floods.)
o Mississippi River is many miles shorter today than it was in 1830s because of artificial cutoffs that were dredged across meander necks to improve navigation and safety.
o Oxbow lake: erodes outer bank (neck)=neck narrows=erodes, forms cutoff = isolated from channel =stream becomes straighter

o Stream Gradient

A graded stream is one in which over a period of years, slope is delicately adjusted to provide, with available discharge and with prevailing characteristics, just the velocity required for transportation of the load supplied from the drainage basin. By J.H Mackin
o • stream gradient: elevation decline over distance from headwaters to mouth; concave shape
o • graded stream - balance (dynamic equilibrium) btw erosion, transportation, and deposition
o • stream gradients may be affected by tectonic uplift of the landscape which changes the base level balance (gradient) =further down-cutting=rejuvenation

Gradient

The drop in elevation from a stream's headwaters to its mouth, ideally forming a concave. Every stream runs downhill under the pull of gravity. In the course of this process, each stream develops its own gradient: which is the rate of elevation decline from its headwaters to its mouth.

Graded stream

An idealized condition in which a stream's load and landscape mutually adjust load, deposition, and the streams capacity.

o 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.
o Conversion of potential energy in the water at the lip of the falls to concentrated kinetic energy at the base works to eliminate the nickpoint interruption and smooth out the gradient.
o • nickpoints - abrupt change in gradient (e.g. waterfall) • potential energy  kinetic energy: eliminate nickpoint (geology)
o • Waterfall - water in freefall =erosion at base =undercuts, collapse and shift upstream
o • Waterfall = series of rapids
o • Niagara Falls - 11km retreat from the Niagara Escarpment in 12,000

Floodplain

The flat low-lying area flanking many stream channels that is subjected to current flooding. Alluvial deposits generally mask underlying flood.

o Natural levees

A long low ridge that forms on both sides of a stream in a developed floodplain; they are depositional products of river flooding.
o When floodwater rises the river over flows its banks, loses stream competence and capacity as it spreads out, and drops a portion of its sediment load to form the levees. Larger sand sizes particles drop out first, forming the principal component of the levees, with finer silts and clays deposited farther from the river.

Yazoo tributary

A small tributary stream draining alongside a floodplain; blocked from joining the main river by its natural levees and elevated stream channel so it flows parallel to the river not with it.

Backswam

A low -lying, swampy area of a floodplain; adjacent to a river, with the river's natural levee on one side and higher topography on the other.

Alluvial terraces

: Level areas that appear as topographic steps above a stream, created by the stream as it scours with renewed down cutting into its floodplain; composed of unconsolidated alluvium.
o Represents what originally was a depositional feature (a floodplain) that subsequently has been eroded by the rejuvenated (tectonic uplift) stream.

Delta

A depositional plain formed where a river enters a lake or an ocean; named after the triangular shape of the Greek letter delta.
o Delta: mouth of river reaches base-level (large body of water)= decrease in v (transported load > capacity) =sediments deposited (coarse=fine)
o distributaries - reverse dendritic pattern through delta
o • Types of deltas: braided - Ganges River =excess sediment (deforestation)
o arcuate - arc-like (Nile River)
o estuarine - Tiber river (mix of seawater with freshwater)

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.
o

o Mississippi Delta River

o Mississippi Delta River
• • 120 million years collecting sediment from drainage basin = Gulf of Mexico
• • 5000 years = 7 distinct deltaic complexes
• • most recent deltaic complex last 500 years (bird-foot delta)
• • very complex, dynamic system = main channel exists due to heavily engineered flood and sediment control structures (Army Corp of Engineers)
• • isostatic changes (3.25 million sq km drainage basin)  extend the delta 90 m/year
• • New Orleans below river level (some below sea-level)
• Figure 14.26
• • Atchafalaya River may capture Mississippi=shorter route to GM with steeper gradient (30% of Q): save New Orleans from flood; damage financially

o Floods and River Management

o Rivers have played important role =agriculture, transportation, industrial uses, removal of excess waste, etc. = human pop'n grew alongside \
o • e.g. North Carolina (1999): Hurricanes Denis, Floyd and Irene (Sept-Oct) deluged NC with several feet of ppt >saturated ground; flushed out waste lagoons (each hog 2 tons/year of waste) set on floodplains sent into the ocean  dead zone
o • e.g. Bangladesh: 3⁄4 of area on floodplain, densely populated > upstream forest harvesting of Ganges-Brahmaputra watershed leads to increased run-off > higher sediment loads>deposited in delta regions forming island used as nascent farming communities (150,000 dead during flooding 1988, 91)
o • rating floodplain risk: flood - high water flow, over-topping levees; statistically rated for time interval between floods of a certain magnitude - e.g. 10 yr flood (the greatest level of flooding likely to occur every 10 years; 10% chance every year)

Engineering Failures in New Orleans, 2005 Rating Floodplain Risk
o Hydrograph:

o Hydrograph: A graph of stream discharge (in cms or cfs) 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.
• Hydrographs: stream Q over time > relates ppt input and stream Q
• • base-flow: dry season, low water flow (groundwater input)
• • peak-flow: ppt event in watershed; f(x) amount, location and duration of event and surface characteristics in watershed
• • urbanization: increases/hastens peak flow (~ runoff pattern to deserts)

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