Changes

==Abandoned distributary deposits==

==Abandoned distributary deposits==

The distributary channel is the natural flume which accommodates and directs a part of the water and sediment discharged from the parent river system to the receiving basin. In most deltas, the distributary channels are rather stable and do not display a tendency toward lateral migration, thereby preventing the formation of [[point-bar deposit|point-bar]] or [[meander-belt deposit]]s. In some deltas, for example, with high bedload streams or in those environmental settings where tidal range is high, migration of the distributary channel can take place, resulting in formation of deposits similar to channel deposits described in the [[delta plain, upper|upper delta plain]].

The distributary channel is the natural flume which accommodates and directs a part of the water and sediment discharged from the parent river system to the receiving basin. In most deltas, the distributary channels are rather stable and do not display a tendency toward lateral migration, thereby preventing the formation of [[point-bar deposit|point-bar]] or [[meander-belt deposit]]s. In some deltas, for example, with high bedload streams or in those environmental settings where tidal range is high, migration of the distributary channel can take place, resulting in formation of deposits similar to channel deposits described in the [[delta plain, upper|upper delta plain]].

Although little research has been conducted, the lack of channel migration in the lower delta plain is undoubtedly due to the fact that most river channels scour down through their distributary-mouth-bar deposits into underlying [[marine clay]]s. This scouring provides an entrenchment of the distributary channel with minimal tendencies for lateral migration. Active distributary channels vary considerably in size, some only a few meters wide and 1 to 2 m deep, and others of a large major river delta system with channels reaching 1 km in width and 30 m in depth. Depth within the channel decreases rapidly as the river-mouth bar is approached, and water depths over most distributary-mouth bars rarely exceed 3 m.

Although little research has been conducted, the lack of channel migration in the lower delta plain is undoubtedly due to the fact that most river channels scour down through their distributary-mouth-bar deposits into underlying [[marine clay]]s. This scouring provides an entrenchment of the distributary channel with minimal tendencies for lateral migration. Active distributary channels vary considerably in size, some only a few meters wide and 1 to 2 m deep, and others of a large major river delta system with channels reaching 1 km in width and 30 m in depth. Depth within the channel decreases rapidly as the river-mouth bar is approached, and water depths over most distributary-mouth bars rarely exceed 3 m.

Abandonment of a distributary channel is an extremely complex process, and in many areas is simply an accident. Log jams, loss of gradient advantage, infilling during a catastrophic period (such as a hurricane), or changes in the upstream part of the river will cause the channel to deteriorate and infill. Deprived of an active influx of sediment and water, the channel will then undergo an infilling process in which only local sediments derived from both upstream and downstream will infill the abandoned hole in the ground. The lower parts of the channel are commonly filled with poorly sorted sands and silts containing an abundance of transported organic debris. As a channel shoals, the water becomes more stagnant, and lower current velocities are maintained; soon, fine-grained material begin to infill the channel proper. With time and continued subsidence the channel often fills entirely with fine-grained, poorly sorted sediments. Organic debris, logs, and clays with extremely high water content often form the upper part of the channel fill. Thus in many deltaic regions of low tides and high suspended sediment load, no process is available to infill the channels with sand or other coarse debris.

Abandonment of a distributary channel is an extremely complex process, and in many areas is simply an accident. Log jams, loss of gradient advantage, infilling during a catastrophic period (such as a hurricane), or changes in the upstream part of the river will cause the channel to deteriorate and infill. Deprived of an active influx of sediment and water, the channel will then undergo an infilling process in which only local sediments derived from both upstream and downstream will infill the abandoned hole in the ground. The lower parts of the channel are commonly filled with poorly sorted sands and silts containing an abundance of transported organic debris. As a channel shoals, the water becomes more stagnant, and lower current velocities are maintained; soon, fine-grained material begin to infill the channel proper. With time and continued subsidence the channel often fills entirely with fine-grained, poorly sorted sediments. Organic debris, logs, and clays with extremely high water content often form the upper part of the channel fill. Thus in many deltaic regions of low tides and high suspended sediment load, no process is available to infill the channels with sand or other coarse debris.