Deltaic lobe

Delta lobes in the Mississippi River Delta. Each lobe was the primary outlet for a while, but was abandoned when the flow shifted.
 4600 yrs BP,
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 current

A deltaic lobe is a wetland formation that forms as a river empties water and sediment into other bodies of water. As the sediment builds up from this delta, the river will break away from its single channel and the mouth will be pushed outwards, forming a deltaic lobe.[1]

When the rate of water discharge and lobe progradation are sufficiently high, a river can form a deltaic lobe. A single deltaic lobe includes a network of shallow channels called distributaries that make up a distributary network that branches off from the mainstream of the river. These networks can be the blueprint for a future progradational deltaic lobe when the initial deltaic lobe is abandoned.[2] As the deltaic lobe progresses, heavier and coarser sediments settle first. As heavier sediments are deposited at the top of the deltaic lobe, smaller and finer sediments get deposited out, creating the beginning of a deltaic fan. When the alluvium, the smallest sediment carried by the deltaic lobe, is deposited and new land is formed, the resulting formation is considered a delta.[1]

Lobes are important in forming river deltas over time by amalgamation of channel avulsions.[3] When a lobe is prograded the frequency of avulsion decreases, and the avulsion length increases relative to a non-progradational deltaic lobe. As the deltaic lobe progrades, the channel bed gradient is lowered, resulting in a sedimentary push upstream. This shifts the location of the avulsion forwards creating a completed deltaic lobe on which overlies a delta.[1] Lobe formation is determined by a relationship between water discharge and lobe progradation. A model must take into account both factors in order to accurately predict avulsion timing and location.[3]

  1. ^ a b c David E. Frazier. "Recent Deltaic Deposits of Mississippi River: Their Development and Chronology: ABSTRACT." AAPG Bulletin, vol. 51, 1967, doi:10.1306/5d25c219-16c1-11d7-8645000102c1865d.
  2. ^ Delta. (October 10, 2016). In National Geographic Society. Delta
  3. ^ a b Moodie, Andrew J., et al. "Modeling Deltaic Lobe‐Building Cycles and Channel Avulsions for the Yellow River Delta, China." Journal of Geophysical Research: Earth Surface, vol. 124, no. 11, 2019, pp. 2438–2462., doi:10.1029/2019jf005220.