An analysis of remotely sensed imagery reveals that fluvial planform geometries within aggrading continental areas are dominated by distributive fluvial systems (DFSs). We documented the gradient, length, apex location, planform, termination type, and tectonic and climatic setting of 415 examples of fluvial systems which in planform display a radial, distributive channel pattern and have an apex-toe distance . 30 km (large DFSs). The longest of these DFSs is 704 km in length, with the majority (72%) ranging between 30 and 100 km in length. Gradients on individual systems range from 0.00003 (0.0018u) to 0.02656 (1.5u). Six planform types are recognized, those with: (1) a single braided channel that bifurcates downstream into braided and/or straight channels, (2) a single dominant braided channel, (3) a single dominant braided channel which becomes sinuous downstream often bifurcating, (4) a single dominant sinuous channel, (5) a single sinuous channel that bifurcates downstream into smaller sinuous channels, and (6) multiple sinuous channels. Of the studied examples 58% occur within exorheic basins and 42% in endorheic basins, with seven different termination types recognized. In many examples, channel planform changes downstream from a distributive pattern to a contributory pattern. In others, channels terminate at an axial fluvial system, at the coast, in eolian dune fields, playa lakes, permanent lakes, or wetlands. Large DFSs and their catchments are developed in all climatic regimes, including drylands, tropical, subtropical, continental, and polar climates. Large DFSs occur in all tectonic settings, including extensional, compressional, strike-slip, and cratonic tectonic regimes.General trends and relationships between the different studied parameters can be observed, leading to a broad understanding of the main controls on large DFS development. All of the planform types occur in all tectonic settings and all climate zones. Braided planforms dominate all tectonic settings, but particularly compressional regimes. High-gradient braided systems tend to be associated with areas of high relief and are well developed in dryland climates where discharge is inferred to be intermittent in comparison to tropical climates. Large DFSs with sinuous planforms do occur in dryland climates but tend to predominate in wetter, more tropical climates where discharge is more constant and the fluvial systems can distribute bedload more efficiently. Sinuous systems also tend to have significantly lower gradients than braided systems. Although these general observations can be made, there are significant variations from these trends, which are inferred to be controlled by variations in (1) discharge related to climate and (2) sediment supply, which is a function of climate, catchment size, catchment lithology, and catchment relief.Large DFS length is controlled by the available horizontal accommodation space, which in turn is strongly related to tectonic setting. The longest DFSs occur in peripheral foreland basins and cratonic se...
We propose that the so-called ''terminal fan'' facies model should be abandoned since it is flawed on several counts and it is leading to misunderstanding and poor communication. Rivers in drylands may experience excessive downstream discharge reduction such that they terminate subaerially rather than reach the sea or a lake. The facies model predicts that the distal reaches of such rivers form a network of bifurcating distributary channels producing a fan-shaped sediment body, with downstream thinning and fining of sedimentary units, ending in sand-filled ribbons encased in mud.Extensive review of modern rivers has failed to turn up convincing examples that fit the model. Rivers in drylands do not ubiquitously end in fans. Fan-shaped fluvial bodies are common wherever rivers are released from confinement and the discharge conditions promote frequent avulsion. Channels on such fans generally do not repeatedly bifurcate downstream. Where they are seen to do so, it can usually be shown they are lacustrine deltas inherited from wetter times. The term ''distributary'' is being used carelessly and is conveying incorrect understanding of sediment geometry and architecture. The proposed synonym of ''fluvial distributary systems'' is unsatisfactory as it perpetuates the same misunderstandings. Reliance on planform alone in analogue selection is highly risky.The fluvial fan is a composite sediment body resulting from frequent nodal avulsions in a setting without horizontal constraints. Channels on fans range in planform as much as any other river. The resultant sedimentary record differs little from that expected from non-fan fluvial systems except having a regionally radiating orientation when viewed over geological time scales. Contrary to the implications of the facies model, there is no distinctive ''terminal fan'' sedimentary succession.
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