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 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). • 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
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
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… 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. Xora, South Africa Presentation by Keith Richards (University of Cambridge) at International Drysdale River, Australia 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 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 Rio Parana-Rio Paraguay Waimakariri, South I sland, New Zealand
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 Kumbh Melas, every 3 Confluence years, when 30 million pilgrims bathe on Jan 24 at the confluence • The network node where two tributaries join (Sangam) of three Different flows mix along a shear layer • sacred rivers (Ganges, • Within a channel, there are confluences at the Yamuna, Saraswati) downstream ends of mid- channel bars where flows re-combine
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) Cirque du Fer a Cheval, French Alps Presentation by Keith Richards (University of Cambridge) at International Kosi fan - between 1736 and 1964, the Kosi River Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities and Solutions , Durham University, 19-21 September 2005 shifted 110 km from east to west
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) Breton Sound delta Bayou Lafourche 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)
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 Accretion The deposition of sediment – • may be lateral accretion (point bar, scroll bar - sand) or vertical accretion (overbank sediment – silt, clay) Lateral accretion may result in • a gradual addition of (initially unproductive) land 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? 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
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- occupy an old channel • One form of avulsion occurs when a meander bend is cut off. River Rapti, Gangetic Plain
Recommend
More recommend
