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27-474: The Manuherikia Group is a fluvial - lacustrine sedimentary fill in the Central Otago area of New Zealand, at the site of the prehistoric Lake Manuherikia . The area consists of a valley and ridge topography , with a series of schist - greywacke mountains at roughly ninety degrees to each other. The Manuherika Group occurs in the current basins, and occasionally on the mountains themselves. One of

54-481: A coherent genetic context. He drew attention to Manuherikia Group sediments on the top of mountain ranges, and to the observation that sequences in distinct basins were similar. His conclusion was that the Manuherikia Group is the result of sedimentation in a single (except for the earliest stages) very large basin that was later intruded by the growth of the mountain ranges. This basin ultimately grew into

81-538: A fossil fauna (the Saint Bathans Fauna ) of birds, fish, and a crocodilian, New Zealand's first known terrestrial mammal, as well as bats. Stromatolites are also present. At two locations around the depocenter , river deltas developed and large thicknesses of coal built up on upper delta plains. These are termed the Blackstone Delta and Teviot Delta. Fully lacustrine conditions ensued with

108-707: A large river is enormous. It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea, the Yellow River 796 million tons, and the Po River in Italy 67 million tons. The names of many rivers derive from the color that the transported matter gives the water. For example, the Yellow River (Huang He) in China is named after the hue of the sediment it carries, and

135-619: A new one ( avulsion (river) ). A braided river may form as small threads come and go within a main channel. The intensity and frequency of both drought and rain events are expected to increase with climate change. Floods , or flood stage , occur when a stream overflows its banks. In undisturbed natural areas, flood water would be able to spread out within a floodplain and vegetation of either grassland or forest , would slow and absorb peak flows. In such areas, streambeds should remain more stable and exhibit minimal scour. They should retain rich organic matter and, therefore continue to support

162-597: A rich biota ( river ecosystem ). The majority of sediment washed out in higher flows is "near-threshold" sediment that has been deposited during normal flow and only needs a slightly higher flow to become mobile again. This shows that the streambed is left mostly unchanged in size and shape over time. In urban and suburban areas with little natural vegetation, high levels of impervious surface , and no floodplain, unnaturally high levels of surface runoff can occur. This causes an increase in flooding and watershed erosion which can lead to thinner soils upslope. Streambeds can exhibit

189-454: A short distance then settling again). If the upwards velocity is higher than the settling velocity, the sediment will be transported high in the flow as wash load . As there are generally a range of different particle sizes in the flow, it is common for material of different sizes to move through all areas of the flow for given stream conditions. Sediment motion can create self-organized structures such as ripples , dunes , or antidunes on

216-410: A single huge lake – Lake Manuherikia – that extended over some 5,600 km (2,200 sq mi). Palynological work by Couper, Mildenhall, Mildenhall and Pocknall has indicated that the Manuherikia Group is basically Miocene in age. The plant macrofossils , common in the lower Manuherikia Group, are mostly Early Miocene , perhaps with some in the earliest Middle Miocene . Following Douglas,

243-509: Is the best exposed. The St Bathans paleovalleys eventually filled, sedimentation spread out over a broader area, and the fluvial character became dominantly meandering . This material is grouped as the Fiddlers Member. Extensive coal swamps developed between the rivers and their flood basins. Individual flood basin lakes began to coalesce into a true lacustrine system – Lake Manuherikia . Its muddy and sandy sediments are grouped as

270-473: Is the bottom of a stream or river ( bathymetry ) and is confined within a channel , or the banks of the waterway. Usually, the bed does not contain terrestrial (land) vegetation and instead supports different types of aquatic vegetation ( aquatic plant ), depending on the type of streambed material and water velocity. Streambeds are what would be left once a stream is no longer in existence. The beds are usually well preserved even if they get buried because

297-469: The White Nile is named for the clay it carries. The main kinds of fluvial processes are: The major fluvial (river and stream) depositional environments include: Rivers and streams carry sediment in their flows. This sediment can be in a variety of locations within the flow, depending on the balance between the upwards velocity on the particle (drag and lift forces), and the settling velocity of

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324-400: The stream bed exerts a shear stress directly onto the bed. If the cohesive strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow. In addition, if the river carries significant quantities of sediment , this material can act as tools to enhance wear of

351-745: The Bannockburn Formation. Around the lake margins, distinctive shoreline facies developed. The most widespread of these is the Kawarau Member which contains diverse plant fossils. Distinctive facies within the Kawarau Member have been termed the Cromwell Submember and the Ewing Submember. The Ewing is typically intercalated with the Lauder Member, an informal unit of the Bannockburn Formation. This contains

378-412: The banks and canyons made by the stream are typically hard, although soft sand and debris often fill the bed. Dry, buried streambeds can actually be underground water pockets. During times of rain, sandy streambeds can soak up and retain water, even during dry seasons, keeping the water table close enough to the surface to be obtainable by local people. The nature of any streambed is always a function of

405-421: The bed ( abrasion ). At the same time the fragments themselves are ground down, becoming smaller and more rounded ( attrition ). Sediment in rivers is transported as either bedload (the coarser fragments which move close to the bed) or suspended load (finer fragments carried in the water). There is also a component carried as dissolved material. For each grain size there is a specific flow velocity at which

432-429: The earliest geologists to work in the area, McKay , understood that the Manuherikia Group was probably originally continuous. Although some workers came to believe the sediments were deposited in a series of small, interconnected basins between the mountain ranges, e.g. Park, later workers, like Cotton argued that the sediments had been isolated by later mountain growth. Douglas placed the Manuherikia Group sediments into

459-666: The flow dynamics and the local geologic materials. The climate of an area will determine the amount of precipitation a stream receives and therefore the amount of water flowing over the streambed. A streambed is usually a mix of particle sizes which depends on the water velocity and the materials introduced from upstream and from the watershed. Particle sizes can range from very fine silts and clays to large cobbles and boulders ( grain size ). In general, sands move most easily, and particles become more difficult to move as they increase in size. Silts and clays, although smaller than sands, can sometimes stick together, making them harder to move along

486-543: The grains start to move, called entrainment velocity . However the grains will continue to be transported even if the velocity falls below the entrainment velocity due to the reduced (or removed) friction between the grains and the river bed. Eventually the velocity will fall low enough for the grains to be deposited. This is shown by the Hjulström curve . A river is continually picking up and dropping solid particles of rock and soil from its bed throughout its length. Where

513-572: The particle. These relationships are shown in the following table for the Rouse number , which is a ratio of sediment settling velocity (fall velocity) to upwards velocity. Rouse = Settling velocity Upwards velocity from lift and drag = w s κ u ∗ {\displaystyle {\textbf {Rouse}}={\frac {\text{Settling velocity}}{\text{Upwards velocity from lift and drag}}}={\frac {w_{s}}{\kappa u_{*}}}} where If

540-578: The primary subdivision of the Manuherikia Group is into a lower, fluvial (commonly with coal) Dunstan Formation and an upper, lacustrine Bannockburn Formation . The oldest unit of the Manuherikia Group is the Saint Bathans Member of the Dunstan Formation, consisting of the braided-river fill of valleys incised into the basement rocks. Traces of three St Bathans paleovalleys are known – one at Blue Lake , adjacent to St Bathans ,

567-467: The river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium . Even small streams make alluvial deposits, but it is in floodplains and deltas of large rivers that large, geologically-significant alluvial deposits are found. The amount of matter carried by

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594-429: The river or stream bed . These bedforms are often preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the flow that deposited the sediment. Overland flow can erode soil particles and transport them downslope. The erosion associated with overland flow may occur through different methods depending on meteorological and flow conditions. Stream bed A streambed or stream bed

621-422: The stream meanders downhill. Pools can also form as water rushes over or around obstructions in the waterway. Under certain conditions a river can branch from one streambed to multiple streambeds. For example, an anabranch may form when a section of stream or river goes around a small island and then rejoins the main channel. The buildup of sediment on a streambed may cause a channel to be abandoned in favor of

648-399: The streambed. Deposition usually occurs on the inside of curves, where water velocity slows, and erosion occurs on the outside of stream curves, where velocity is higher. This continued erosion and deposition of sediment tends to create meanders of the stream. In streams with a low to moderate grade, deeper, slower water pools ( stream pools ) and faster shallow water riffles often form as

675-424: The streambed. In streams with a gravel bed, the larger grain sizes are usually on the bed surface with finer grain sizes below. This is called armoring of the streambed. The streambed is very complex in terms of erosion and deposition. As the water flows downstream, different sized particles get sorted to different parts of a streambed as water velocity changes and sediment is transported, eroded and deposited on

702-654: The submergence of these deltas and for much of the mid Miocene, Lake Manuherikia was without known bounds. Rising mountains in the Late Miocene - Pliocene eventually deluged Lake Manuherikia with gravel – the Maori Bottom, or Maniototo Conglomerate . An extremely diverse macroflora flora, based on well-preserved cuticle. There are two cycad-like taxa: The conifers total 16 species in 12 genera, including: Other gymnosperms: Monocots include: The dicots include: Fluvial The movement of water across

729-404: The upwards velocity is approximately equal to the settling velocity, sediment will be transported downstream entirely as suspended load . If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move (see Initiation of motion ), it will move along the bed as bed load by rolling, sliding, and saltating (jumping up into the flow, being transported

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