I nterpreting terms used in river boundary definition Keith - - PowerPoint PPT Presentation

I nterpreting terms used in river boundary definition

Keith Richards Department of Geography University of Cambridge Rivers as boundaries are doubly problematic – they follow a shifty linear feature, and split an areal one in two!

River

• A river is a large natural waterway.
• A watercourse that flows at all times, receiving

ground or surface water, for example from other streams or rivers. The terms “river” and “stream” are

• ften interchangeable, but are indicative of size.
• A river is a channelised flow of water, draining part of

the rain (or snowmelt) that falls on a sloping area of land down that slope towards a low point (such as a lake or sea).

• I s a RI VER by definition PERENNI AL? Can an

EPHEMERAL flow be a river?

• I NTERMI TTENT rivers (time and space)? What if

groundwater pumping lowers watertable – natural?

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

River basin

• A river basin, watershed* or

catchment is the area of land whose excess water drains through a river network into a body of water such as a lake or ocean.

* US = basin; UK = divide

• A river basin has topographic unity; hillslopes, river channels

and drainage networks that transfer water and sediment through the landscape.

• A river basin is a fundamental accounting unit of the water

balance and water resource of an area.

• Overspill (the Yazoo Backwater project)
• Underground rivers in limestone regions

Chagres river basin, Panama http://skagit.meas.ncsu.edu/~helena/measwork/panama/panama.html

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

River network

• Rivers form a structured,
• rganised system of components –

nodes (sources, junctions) connected by segments; this is the river network.

• But headwaters are ephemeral
• Map scale/ DEM resolution affects

network properties

• Underground streams again…

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Source

• The source is the start, or beginning, of a river.

The source of a river is usually found in the hills or

• mountains. A river can have more than one source

(so what criteria determine “THE source”?)

When does a slope turn into a stream (the stream head) When does a rill become a gully? When does a gully become a stream/river?

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

River Mouth

• Can be morphologically simple, but complex processes –

tide and river flood interactions; salt marsh accretion; tidal channel migration. Dynamic and unpredictable.

Drysdale River, Australia Xora, South Africa

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Tributary

• Tributary river may change behaviour of main river if

water quality and sediment load differ; and control or disturbance of tributary may impact on main river.

• Tributary catchment may be in the neighbouring domain

if boundary follows main river centre-line or thalweg.

Wahoe tributary to Waimakariri, South I sland, New Zealand Rio Parana-Rio Paraguay

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Confluence

• The network node where two tributaries join
• Different flows mix along a shear layer
• Within a channel, there are confluences at the

downstream ends of mid- channel bars where flows re-combine

Kumbh Melas, every 3 years, when 30 million pilgrims bathe on Jan 24 at the confluence (Sangam) of three sacred rivers (Ganges, Yamuna, Saraswati)

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Alluvial fan

• A half-cone-shaped sedimentary deposit where a

river emerges from a mountain catchment. Forms because channels shift the locus of deposition (as a result of avulsion)

Kosi fan - between 1736 and 1964, the Kosi River shifted 110 km from east to west Cirque du Fer a Cheval, French Alps

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Distributary

• A channel which divides from the main channel of a river
• n a fan or delta
• A diffluence – a channel in which the flow divides

around a mid-channel bar (it may re-combine downstream at a confluence)

Bayou Lafourche Breton Sound delta

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Meanders

Sinuosity – self formed v inherited (bends created by valley axis) Wavelength, radius of curvature Meander bend migration – cut-bank, point bar, scroll-bar, bar-and- swale

1841 Texas – Arkansas border(Andrew Alden, geology.about.com)

• The deposition of sediment –

may be lateral accretion (point bar, scroll bar - sand)

• r vertical accretion (overbank

sediment – silt, clay)

• Lateral accretion may result in

a gradual addition of (initially unproductive) land

Accretion

http:/ / pubs.water.usgs.gov/ fs-004-03

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Multi-thread river (i)

• A braided river:
• The flow passes through a number of

interlaced branches that divide and rejoin, around bars created by bedload transport in the channel itself

• Looks very different at high flow, but where

is the thalweg then?

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Multi-thread river (ii)

• An anastomising river:
• The flow is in channels

that divide and rejoin around a number of floodplain islands

• Channels may change

– by avulsion – because of obstruction (eg tree throw)

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Avulsion

• The diversion of a river

channel into a new course.

• This may be because

floodplain aggradation causes a river to divert to an area of low elevation.

• I t may be because a channel

bar encourages overspill to re-

• ccupy an old channel
• One form of avulsion occurs

when a meander bend is cut

• ff.

River Rapti, Gangetic Plain

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Avulsion (continued)

• A notorious dispute involving avulsion concerned McKissick’s I sland two miles south of

the I owa line in the northwest corner of Atchison County. McKissick’s is not really an island at all but more than 5,000 acres of fertile farmland originally on the Nebraska side of the river but now on the Missouri side. I t started out as a tract of land within a sharp bend on the west bank of the Missouri River, but a flood in 1867 changed its

• location. The river cut across the neck of the narrow bend and dug itself a new channel,

shortening its course and isolating the acreage on the Missouri side. Gradually, the former riverbed dried up and became a part of the east bank. "Today, it is nearly impossible to determine where the old riverbed used to be. I t’s just a bean field," said Norman Brown, a surveyor for the Department of Natural Resources’ Land Survey Program.

• Problems with the change became acute by the year 1900 because people on both

sides of the river claimed ownership of the McKissick’s I sland acreage. The problem was worsened by the actions of taxing authorities in the counties on both sides of the river who also claimed it. The Missouri people using the land refused to pay taxes to Nemaha County, Neb., and the land was eventually sold on the Nemaha County courthouse steps to a Nebraska farmer for delinquent taxes, hence initiating the battle between farmers. I n 1905, the two states sued in the U.S. Supreme Court to determine which state owned McKissick’s I sland. Nebraska won.

• http:/ / www.dnr.state.mo.us/ magazine/ 1999_summer/ mo-historic-border-battles.htm

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Main channel

• The channel in which the

greatest proportion of flow is transported

• Deepest? Most navigable?
• At what flow stage? What if

the main channel varies with stage?

• How stable over time is the

“main” channel?

• Can there be a “main channel”

(ie, the most navigable, dredged route) within a “main channel” (ie, the distributary with most flow)

The Brahmaputra – where is the main channel?

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Bankfull discharge

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

• Alluvial channels are adjusted to the bankfull discharge, which is

the flow that just fills the channel before flooding the floodplain – drowns the bedforms, simplifies the channel appearance

• Channel-forming discharge
• Approximately equal to the mean annual flood (average annual

maximum discharge)

• Varies in frequency from once every

0.5 to once every 20 years

• Aggradation and incision of channel

changes inundation frequency

• f floodplain (as does dredging)

River Bank

Bank lines migrate because of both erosion and deposition Depositional banks are gently sloping so defining the bank line is arbitrary and difficult Channels often have complicated cross- sections, with two-stage channels, terraces etc.

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Thalweg

• The line of maximum depth along a river channel, valley or lake (from

German: tal, valley, and weg, way). I n practice, the minimum depth along the thalweg may be a critical property (or the local bed elevation maximum).

• Also the line of maximum current velocity; “the thalweg is a path marking the

greatest surface velocity and the deepest flow in a meandering stream” (Rivers Council of Minnesota). But these do not always coincide!

• “The middle of the chief navigable channel of a waterway that forms the

boundary line between states.” (!)

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005

Median/ Centre line

• The centre-line along a river channel (mid-point between the banks)
• Easily extracted automatically? (Much bank line data; not easy if

point bars!)

• What if medial bars? (Can’t automate the decision about which is

the main channel if only have bank line data?)

• Stage dependent (and medial bars are drowned!)

http:/ / gis.esri.com/ library/ userconf/ proc99/ proceed/ papers/ pap972/ p972001.htm

Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions, Durham University, 19-21 September 2005