interpreting terms used in river boundary definition -...

Interpreting terms used in river boundary definition

Keith RichardsDepartment of GeographyUniversity 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 often 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).

• Is a RIVER by definition PERENNIAL? Can an EPHEMERAL flow be a river?

• INTERMITTENT 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 landwhose excess water drainsthrough a river networkinto a body of water suchas 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


River network• Rivers form a structured,

organised 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…


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?


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


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 Island, New Zealand

Rio Parana-Rio Paraguay


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 flowsre-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)


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


Distributary• A channel which divides from the main channel of a river

on 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



MeandersSinuosity – 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 belateral accretion (point bar, scroll bar - sand)or vertical accretion (overbanksediment – silt, clay)

• Lateral accretion may result in a gradual addition of (initially unproductive) land

Accretion

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


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?


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)


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.

• It may be because a channel bar encourages overspill to re-occupy an old channel

• One form of avulsion occurs when a meander bend is cut off.

River Rapti, Gangetic Plain


Avulsion (continued)• A notorious dispute involving avulsion concerned McKissick’s Island two miles south of

the Iowa 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. It 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. It’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 Island 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. In 1905, the two states sued in the U.S. Supreme Court to determine which state owned McKissick’s Island. Nebraska won.

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


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 distributarywith most flow)

The Brahmaputra – where is the main channel?


Bankfull dischargePresentation 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 frequencyof floodplain (as does dredging)

River BankBank 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.


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

German: tal, valley, and weg, way). In 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.” (!)


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.htmPresentation 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

interpreting terms used in river boundary definition -...

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