Under what conditions do braided rivers form?
Braided rivers are a type of river that form a network of many branches within a channel. They often form when the bedload sediment is high compared to the suspended load, which then helps the development of bars, creating the braided character. Fast flow and steep gradients characterize braided rivers.
Where do braided rivers form?
Braided river formations are common where water flow is slow and there is a build up of sediment in the river. This can cause changes in the direction of the river and create new channels. This can occur to form a river delta, which is a form of braided river.
Where would you expect to find braided streams and why?
Braided streams can develop anywhere there is more sediment than a stream is able to transport. One such environment is in volcanic regions, where explosive eruptions produce large amounts of unconsolidated material that gets washed into streams.
Where are braided streams found most commonly?
Most braided streams occur where there are almost no lateral confining banks, as on large alluvial piedmont ALLUVIUM; ROCK FANfans or OUTWASH PLAIN, FAN, TERRACE, SANDURsandurs, but in certain regions they occur in confined valleys (often “underfit”; see Streams—Underfit).
Why are braided rivers formed?
In big floods the rocks and sediments are carried out across the plains toward the coastline. Braided rivers form when these rocks and sediment build up on the riverbed. In time the build-up becomes so high that the water, seeking the lowest path, begins to flow down a new channel.
What are the characteristics of braided river?
Braided streams and rivers have multi-threaded channels that branch and merge to create the characteristic braided pattern. Braided channels are highly dynamic with mid-channel bars which are formed, consumed, and re-formed continuously.
Under what conditions does a braided stream form quizlet?
Braided streams form where channels are choked by sediment and the flow is forced around sediment obstructions, flow occupies multiple channels across a valley.
Where do braided channels occur?
Braided channels form in non-cohesive sand and gravel and are often associated with glaciated mountain regions but also occur on alluvial fans, interior and coastal plains, and in a variety of climatic regions. A reach of the braided gravel-bed Sunwapta River, Canada.
Why are braided rivers rare?
Globally, braided rivers are rare. They occur only where a very specific combination of climate and geology allows rivers to form ever-changing and highly dynamic 'braided' channels across a wide gravelly riverbed (see 'braidplains').
What is a common characteristic of all braided streams?
Deposits of Braided Rivers tend to be coarse-grained and contain abundant amalgamated channels. Braided river systems are characterized by braiding parameters greater than 1, low sinuosity, steep slopes, abundant sediment supply, high and variable discharge and high width/depth ratios (Miall,1982).
What is river braiding geography?
A braided channel is one that is divided into smaller channels by temporary islands called eyots. Braided channels tend to form in rivers that have a significant amount of sedimentary load, a steep profile and where discharge regularly fluctuates.
Where are streams most commonly found?
Larger seasonal streams are more common in dry areas. Rain-dependent streams (ephemeral) flow only after precipitation. Runoff from rainfall is the primary source of water for these streams. Like seasonal streams, they can be found anywhere but are most prevalent in arid areas.
Where do braided channels occur?
Braided channels form in non-cohesive sand and gravel and are often associated with glaciated mountain regions but also occur on alluvial fans, interior and coastal plains, and in a variety of climatic regions. A reach of the braided gravel-bed Sunwapta River, Canada.
Why does NZ have braided rivers?
The entire Canterbury Plain was formed by gravel carried from the Southern Alps by braided rivers. The movement of gravels, and working and reworking of sediments to form islands, plains, river mouths and coastal beaches, all contribute to sustaining the mauri (life force) of that braided river system.
Which river has a braided?
The Brahmaputra is a river of many names. In its upper courses, where it winds through a maze of narrow gorges in Tibet, it is the Yarlung Tsangpo.
Introduction
Braided rivers are a type of river that form a network of many branches within a channel. They often form when the bedload sediment is high compared to the suspended load, which then helps the development of bars, creating the braided character.
Sediment Transport Processes
Both the flow speeds and transport capacity vary dramatically within a braided river, which means the Reynolds number (Re) varies widely throughout the channel.
Vertical Sequence of Facies
As channels migrate back and forth, they leave a layer/sheet of sand containing abundant cross stratification. These sheets tend to fine upward because the channels migrating due to bar migration. Bar migration occurs due to more erosion upstream and more deposition on the opposite side, downstream. Sediment is finer grained higher up on the bars.
Summary
Braided rivers are characterized by their fast flow and steep gradients, forming when the bedload sediment is high compared to the suspended load. They form a network of many branches within a channel. These branches are separated by the formation of bars. These bars will migrate through time, causing the migration of the river channel.
What are braided rivers?
Braided rivers have a multithreaded planform comprising many confluences and diffluences (Miall, 1977 ). Braid bars and channels can be classified as a hierarchy depending upon their relative size, and three orders of braiding can be seen in proglacial rivers with constant discharge ( Williams and Rust, 1969; Figure 6 ). First-order braid bars are those largest bars that are separated by major channels. First-order bars are made up of a series of smaller, second-order braid bars, which are separated by smaller channels. Third-order braid bars are contained within second-order bars. The complexity of braid pattern is however, strongly stage-dependent, with rivers losing their braided character during peak flood conditions. Mid-channel or central bars, which include crescentic, longitudinal, and transverse bars, bank-attached, point, or unit bars, and larger composite forms known as compound bars or braid bar complexes, have been described in braided river systems on a range of scales ranging from meters to kilometers. All bars are subject to constant dissection via splays and chutes ( Figures 6 and 7 ), remobilization and migration ( Miall, 1977, 1983 ). Only the largest bar forms produced by the most powerful flow events such as jökulhlaups may become stable over periods of years.
Why are braided rivers important?
It is important for species to increase their reproductive potential by timing periods of recruitment and maturation to the periodic floods. Because of the periodic creation of recruitment habitats, seedling regeneration with a short life span should be favored in braided rivers.
What are the characteristics of a proglacial river?
Proglacial rivers exhibit patterns of flow and discharge behavior that are either seasonally-coupled with melt cycles (“normal” rivers) and/or inundated by jökulhlaups (glacial outburst floods). These differing hydrological conditions give rise to distinctive morphological and sedimentological signatures: essentially, low-magnitude, high-frequency signals are imparted on the proglacial outwash sediments by normal melt cycles, whereas the sedimentary records of jökulhlaup-fed rivers are dominated by high-magnitude, low-frequency events ( Marren, 2005 ). Not only do jökulhlaups contribute substantial quantities of sediment and meltwater to proglacial river systems, but they can also contribute to the formation of pitted sandar (or kettled outwash plains). Hollows within sandar are created by the melt-out of isolated buried ice blocks, which may originate as icebergs transported by the jökulhlaups.
How do glaciers control erosion?
Glaciers exert a strong control on depositional and erosional processes operating in river systems beyond glacier margins (i.e. proglacial rivers) through the delivery of sediment and meltwater. Most proglacial rivers transport large quantities of both suspended sediment and bedload, which is typically deposited in extensive, gently sloping outwash plains (referred to by the Icelandic term sandar; singular: sandur ), or as narrow, linear tracts of outwash in valley settings (called valley trains or valley sandar ). The proglacial river systems responsible for depositing sandar and valley sandar are usually characterized by migrating braided channels owing to a combination of their steep gradients, abundant bedload, cohesionless bank and bed material, and fluctuating discharges ( Miall, 1992 ).
What is the complexity of a braided river?
Complexity of braid pattern is, however, strongly stage dependent, with rivers losing their braided character during peak flood conditions. Braid bars, mid-channel bars, unit bars, and islands have been described in braided river systems on a range of scales ranging from meters to kilometers.
What are the challenges of mapping braided rivers?
A particular challenge associated with mapping braided rivers is surveying the inundated channel beds. Inundated areas may account for less than 10% of a braidplain at low flow, as has been observed in a range of braided river contexts (Brasington et al., 2000, 2003; Ashmore and Sauks, 2006 ). These inundated areas are, however, highly susceptible to topographic evolution during flood events, so it is critical to map these areas with techniques associated with minimal vertical error. Laser ranging technologies hold promise to bridge this missing link through the application of high-power, high-frequency water penetrating lasers. However, the power limitations on such systems restrict the collection of data to large footprint scans acquired from distance on airborne platforms and at significant cost. Although such ‘bathymetric LiDAR’ systems have been widely applied to monitor coastal topography at coarse resolution ( Wedding et al., 2008; Chust et al., 2010 ), to date only one prototype system, the experimental advanced airborne research LiDAR ( Brock et al., 2004) has been developed to map topography at the spatial resolution (<2 m spacing) required to reliably represent fluvial topography ( McKean et al., 2008 ). As such, current strategies to survey river systems require sub-aerial data to be supplemented with externally sourced bathymetric data so that channel change in both exposed and inundated areas can be adequately quantified.
What is the main sand body of the braided river delta front?
The main sand body of the braided river delta front is from a distributary channel, which is a natural extension of the braided channel to the lake. As underwater distributary channel is not fixed and often erodes underlying sediments, however, the mouth bar of the braided river delta front is not well developed.
What makes a braided river unique?
Read on to learn why all the factors that make braided rivers unique – their multiple, shifting channels and banks, varying flows, variety of habitats, and ability to move over the landscape – also make it difficult to determine where a braided riverbed begins and ends. What are braided rivers?
Which rivers are braided?
The seven alpine rivers that contribute 88 per cent of the flow within the region are all braided – namely, the Waiau Toa/Clarence, Waiau Uwha, Hurunui, Waimakariri, Rakaia, Rangitata and Waitaki Rivers.
Why is the Environment Canterbury decision on braided river beds important?
Environment Canterbury is engaged in a process of clarifying the rules on braided river beds in order to protect their natural character, ecosystem health and biodiversity values. This process is aimed at providing clarity for landowners, planners and communities about what is and is not appropriate in braided river environments. Part of this is ensuring a clear understanding for all parties of how to apply the legal definition of riverbed (under the Resource Management Act) to a braided river. This is why Environment Canterbury has appealed a recent High Court decision on this matter to the Court of Appeal.
What is the purpose of the Waimakariri and Ashley Rakahuri River Regional Parks?
Through the provision and management of the Waimakariri and Ashley Rakahuri River Regional Parks, Environment Canterbury is able to provide a range of recreation services while also providing for protection and enhancement of biodiversity on plains sections of these two rivers.
Why are braided river mouths important?
Braided river-mouths in particular are culturally significant for Ngāi Tahu, provide an important habitat for many species, and are important for coastal gravel nourishment. The Mahaananui Iwi Management Plan states: “Tangata whenua have a particular interest in the beds of lakes and rivers and their margins.
What does the decision mean in braided rivers?
As things stand, the court's decision means that there is too much legal uncertainty to continue with the plan changes intended by BRIDGE.
What are the braided rivers in Canterbury?
Canterbury’s braided rivers are unique river ecosystems. They provide an outstanding habitat for many rare birds, fish, plants and other species. The entire Canterbury Plain was formed by gravel carried from the Southern Alps by braided rivers. The movement of gravels, and working and reworking of sediments to form islands, plains, ...
What are the physical drivers of rivers?
The main physical drivers of rivers are the discharge of water and the transport of sediment. The flow of water is driven by gravity and counteracted by resistance due to grain texture, bedforms, obstacles and vegetation. Sediment is transported by tractive forces exerted on the alluvial river bed or by forces on grains kept in suspension by upward motions in turbulent flow. The transport of coarser sediment depends on flow strength, leading to erosion of the bed in accelerating flows and sediment deposition in decelerating flows. Variations in flow velocity thus drive morphological changes that, on their turn, affect the variations in flow velocity. These close interactions regard the coarser bed-material load that can roll, slide and jump over the bed (bedload) or move in suspension (suspended load). Finer washload is transported in suspension too, but this transport does not depend on flow strength but on the supply of material upstream. Washload travels unaffected by variations in flow strength through the river and settles only in stagnant water bodies of wetlands or floodplains after floods. Fine cohesive sediments can also settle when they flocculate in the saline environment of estuaries. Mixtures of sediments with different grain sizes or different mass densities can segregate and give rise to spatial sorting patterns.
Why are rivers important?
Rivers have been providing a wide variety of benefits to mankind for millennia, making rivers the cradle of civilization. Rivers have always been an important source of water as well as an essential transport route for humans as well as aquatic life. For mankind, rivers also have great recreational and cultural values. These benefits, and others, are known as ecosystem services, commonly divided into:
How do rivers help the oceans?
Rivers are the conveyor belts that collect and transport excess precipitation and denudation products from the continents to the oceans. By transporting water and sediment, they create their own shapes through processes of erosion and sedimentation. Rivers transport also nutrients and seeds. Moreover, they sustain the lifecycles of fish by providing the routes for migration.
How does building with nature help the ecosystem?
Building with Nature involves measures or interventions that optimize the use of ecosystem services to achieve new functionality, sustainability and new opportunities for nature. All over the world, rivers face challenges for which Building with Nature could offer solutions. Many rivers have lost space in the past by interventions to reclaim agricultural land, to avoid ice jams, to eradicate malaria, to improve navigation and log transport, and to expand urban areas. The resulting confinement increased flood levels and the resulting concentration of flows in the main channel eroded the river bed, often several metres in just one or two centuries. Sediment mining and interruption of natural sediment transport by constructing dams had similar effects (Kondolf, 1997), adding up to the effects of narrowing by river training. The lower water levels resulting from river bed erosion draw down surrounding groundwater levels, drain connected wetlands, and reduce inundation frequencies of floodplains, obliterating valuable riverine habitats and exacerbating the effects of droughts.
