Braided riverbeds
Matukituki River, Otago (Susan Wiser)
A braided river flows in multiple, mobile channels across a gravel floodplain, with evidence of recent channel migration within the active riverbed and of historical movement of the active bed across the floodplain (Gray & Harding 2007). Braided rivers carry high sediment loads. The channels repeatedly branch and rejoin creating an intervening pattern of low islands and shallow bars. Globally, braided rivers occur only in mountainous, glaciated areas where natural gravel production, river flows and gentle gradients allow them to form (Peat & Patrick 2001). The riverbed’s substrate is hard and there is little cover in terms of vegetation or litter, because of the dynamic character of the habitat. A bank-to-bank flood can be expected about every decade. As such, opportunistic and scavenging invertebrates often colonise these sites after flooding.
Where do they occur?
Braided rivers occur throughout the South Island, particularly on the eastern side, and south of the Buller in the west. In the North Island they occur mainly on both sides of the axial ranges from East Cape southwards.
Notable flora and fauna
Rare and threatened plants include the nationally endangered climbing everlasting daisy (Helichrysum dimorphum), nationally vulnerable Crassula multicaulis, braided riverbed broom (Carmichaelia juncea) and Lady's tresses (Spiranthes novae-zelandiae), declining dwarf broom (Carmichaelia vexillata), Kirk’s broom (Carmichaelia kirkii), dwarf woodrush (Luzula celata), leafless pohuehue (Muehlenbeckia ephedroides), Coprosma pedicellata, Coprosma intertexta, and the naturally uncommon Myosotis uniflora and fierce lancewood (Pseudopanax ferox).
Rare and threatened birds include the nationally critical black-billed gull (Larus bulleri), black stilt (Himantopus novaezelandiae), nationally endangered black-fronted tern (Chlidonias albostriatus), nationally vulnerable banded dotterel (Charadrius bicinctus), crested grebe (Podiceps cristatus subsp. australis), wrybill (Anarhynchus frontalis), and declining white-fronted tern (Sterna striata).
Endangered invertebrates include the robust grasshopper (Brachaspis robustus), and the carabid beetle (Metaglymma aberrans (tersatum)).
Threat status
Endangered (Holdaway et al. 2012)
Threats
Human impacts (i.e. deforestation, dams, stopbanks, willow plantings) have changed the magnitude of flooding, sediment loads, and fire frequency. Braided riverbeds are very susceptible to a wide range of weeds, particularly hard-seeded legumes. Some of the more serious weeds are gorse (Ulex europaeus), Scotch broom (Cytisus scoparius), blackberry (Rubus fruticosus), butterfly bush (Buddleja davidii), pampas grass (Cortaderia selloana), crack willow (Salix xfragilis) and Russell lupin (Lupinus polyphyllus). Browsing impacts from both domestic stock and wild mammals have been reported (deer, goats, rabbits, hares), the latter very widespread and strongly impacting threatened species (Gruener & Norton 2006). Predatory mammals (stoats, cats, hedgehogs) have an impact on breeding of riverbed birds, such as wrybills, and by disturbing bird colonies. Gravel extraction and some recreational use (i.e. trail biking/quad driving) damage plant communities and disturb nesting birds. Abstraction of water for agriculture is a looming threat to the dynamics of braided rivers, for example by reducing the number of islands for breeding birds.
Further reading
Anon 2004. Forgotten Plants of the Shingle Riverbeds. Forest and Bird.
Caruso BS, Edmondson L, Pithie C 2013. Braided river flow and invasive vegetation dynamics in the Southern Alps, New Zealand. Environmental Management 52: 1–18.
Cockayne L 1912. On the peopling by plants of the subalpine riverbed of the Rakaia (Southern Alps of New Zealand. Transactions of the Botanical Society of Edinburgh 24: 104-125.
Cockayne L, Foweraker CE 1916. Notes from the Canterbury College Mountain Biological Station, Cass, No.4. Transactions and Proceedings of the New Zealand Institute 48: 166-187.
Cultural and Natural Heritage Advisory Groups 2006. Canterbury high country braided rivers and lakes. In: Our World Heritage: a tentative list of New Zealand cultural and natural heritage sites. Pp. 44-49. Wellington, Department of Conservation.
Daly I 2004. Inventory of instream values for rivers & lakes of Canterbury New Zealand: A desktop review. Report U04/13 Environment Canterbury.
Gray D, Harding JS 2007. Braided river ecology: a literature review of physical habitats and aquatic invertebrate communities. Wellington, Department of Conservation. 50 p.
Gruener IG, Norton DA 2006. Herbivory by hares as a threat to the native brooms Carmichaelia juncea and C. vexillata. New Zealand Journal of Ecology 30: 261-265.
O’Donnell CFJ, Hoare JM 2011. Meta-analysis of status and trends in breeding populations of black-fronted terns (Chlidonias albostriatus) 1962−2008. New Zealand Journal of Ecology 35: 30-43.
O'Donnell C, Sanders M, Woolmore C, Maloney R 2016. Management and research priorities for conserving biodiversity on New Zealand’s braided rivers. Wellington, Department of Conservation. 46 p.
Peat N, Patrick B 2001. Mackenzie’s braided rivers. Chapter 3 In: Wild Rivers: Discovering the natural history of the Central South Island. Dunedin, University of Otago Press.
Rebergen A, Keedwell R, Moller H, Maloney R 1998. Breeding success and predation at nests of banded dotterel (Charadrius bicinctus) on braided riverbeds in the central South Island, New Zealand. New Zealand Journal of Ecology 22: 33-41.
Steffens KE, Sanders MD, Gleeson DM, Pullen KM, Stowe CJ 2012. Identification of predators at black-fronted tern Chlidonias albostriatus nests, using mtDNA analysis and digital video recorders. New Zealand Journal of Ecology 36: 48-55.
Ullmann I, Bannister JM, Bannister P 2007. The Placopsis trachyderma – Raoulia – community, a special type of biological soil crusts in the braided rivers of southern New Zealand. Flora 202: 687–694.
Williams PA, Wiser SK 2004. Determinants of regional and local patterns in the floras of braided riverbeds in New Zealand. Journal of Biogeography 31: 1355-1372.
Links
Braided river (Wikipedia website)
Braided River Aid (BRaid Inc)
Braided Rivers - what's the story? (ECAN website)
Life on a braided river (Department on Conservation pamphlet)
Upper Waitaki braided rivers (Department of Conservation website)