Interpreting 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). • 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 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 Island, 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 years, when 30 million Confluence 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 Boundaries Research Unit training workshop No. 27, River Boundaries: Kosi fan - between 1736 and 1964, the Kosi River 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. • 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 Presentation by Keith Richards (University of Cambridge) at International Boundaries Research Unit training workshop No. 27, River Boundaries: Practicalities