Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial islands excised from existing floodplain or formed by within-channel or deltaic accretion. These rivers occupy a wide range of environments from low to high energy, however, their existence has never been adequately explained. They occur concurrently with other types of channel pattern, although specific requirements include a flood-dominated flow regime and banks that are resistant to erosion, with some systems characterized by mechanisms to block or constrict channels, thereby triggering avulsion. The fundamental advantage of an anabranching river is that, by constructing a semi-permanent system of multiple channels, it can concentrate stream flow and maximize bed-sediment transport (work per unit area of the bed) under conditions where there is little or no opportunity to increase gradient. On the basis of stream energy, sediment size and morphological characteristics, six types of anabranching river are recognized; types 1-3 are lower energy and types 4-6 are higher energy systems. Type 1 are cohesive sediment rivers (commonly termed anastomosing) with low Mildratio channels that exhibit little or no lateral migration. They are divisible into three subtypes based on vegetative and sedimentary environment. Type 2 are sand-dominated, island-forming rivers, and type 3 are mixed-load laterally active meandering rivers. Type 4 are sand-dominated, ridge-forming rivers characterized by long, parallel, channel-dividing ridges. Type 5 are gravel-dominated, laterally active systems that interface between meandering and braiding in mountainous regions. Type 6 are gravel-dominated, stable systems that occur as non-migrating channels in small, relatively steep basins. Anabranching rivers represent a relatively uncommon but widespread and distinctive group that, because of particular sedimentary, energy-gradient and other hydraulic conditions, operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges.
Anastomosing rivers are characterized by multiple channels separated by islands excised from the floodplain. Their status relative to the continuum concept of channel pattern is assessed with channel pattern defined in terms of three variables-flow strength, bank erodibility and relative sediment supply. Using an ordinal scaling (L(owkM(oderate)-H(igh)), the traditional forms of straight, meandering and braided have respective representations of (L,L,L), (M,L/M,L/M) and (H,H,M/H) in terms of those variables. The anastomosing pattern is on average represented by (L,L,M/H) but not so definitively as other forms. Specification of the third element (sediment supply) is particularly hampered by the paucity of data but aggradation, a characteristic of many anastomosing rivers, can be thought of as symptomatic of a moderately high rate of supply relative to the ability for onward transport. A sufficiently high rate of supply to a channel with low flow strength and resistant banks would induce shoaling and/or lateral constriction that locally forces flow out of the main channel and ultimately leads to the cutting of anabranches. A flow regime characterized by concentrated floods of relatively large magnitude is also regarded as highly conducive to the formation of new channels where low bank erodibility constrains channel capacity. Anastomosis may in certain cases represent a transitional form of channel pattern but there is no denying the longevity of some anastomosing systems. KEYWORDS
Cooper Creek is characterized by an exceptionally wide floodplain and a complex system of anastomosing channels which together can accommodate a large range of highly variable flows. In consequence flood frequency curves are unusually steep. With minimal tributary contribution over the two study lengths -Currareva-Nappa Merrie (> 400 km) and Nappa Merrie-Innamincka (32 km) -downstream output is largely controlled by upstream input, so that similar flow regimes and hydrograph forms can be maintained despite transmission losses and the variable activation of channels.Input-output relationships defined in terms of total flow volume and peak discharge suggest that, above a threshold flow of about 25% duration, transmission losses exceed 75% on average over the Currareva-Nappa Merrie length. The large difference in absorption between primary channels on the one hand and subsidiary channels and the floodplain surface on the other means that transmission losses vary non-linearly with stage. They are relatively low when flow is confined to the primary channels, but increase at higher stages as lesser channels and the floodplain are activated.Early levels do not recur until flows of < 0.5% duration are attained when output/input ratios increase rapidly with discharge. The influences of seasonality and background flow on output discharge are small relative to that of input discharge.Despite the long river distance, peak flows at Currareva and Nappa Merrie are well correlated. However, over the shorter Nappa Merrie-Innamincka length with its more confined system of channels, correlations are better and transmission losses less. They vary from 60% just above the threshold discharge to
The mining of alluvial tin in the Ringarooma basin began in 1875, reached a peak in 19W20, and had virtually ceased by 1982. During that time 40 million m3 of mining waste were supplied to the main river, quickly replacing the natural bed material and requiring major adjustments to the channel.Based on estimates of sediment supply from more than 50 widely scattered mines and the frequency of flows capable of transporting the introduced load, the river's transport history is reconstructed using a mass-conservation model. Because of the lengthy time period (1 10 years) and river distance (75 km) involved, the model cannot predict detailed change but it does reproduce the main pattern of sediment movement in which successive phases of aggradation and degradation progress downstream. Peak storage is predicted in that part of the river where braiding and anastomosis are best developed.Aggradation was most rapid in the upper reaches close to major supply points, becoming slower and later with distance downstream. Channel width increased by up to 300 per cent where the valley floor was broad and braiding became relatively common. Bridges had frequently to be replaced. While bed levels were still rising in lower reaches, degradation began in upper ones, notably after 1950, and by 1984 had progressed downriver over 30 km. Rates of incision reached 0 5 m yr-', especially in the early 1970s when record high flows occurred. As a result of degradation the bed material became gravelly through either reexposure of the original bed or lag concentration of coarser fractions. Also a narrower unbraided channel has developed. The river is beginning to heal itself and upper reaches now have reasonably stable beds but at least another 50 years will be required for the river to cleanse its channel of mining debris.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.