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Distributary stream patterns occur when a river flows into a large body of water (ocean or lake) and a delta forms. The distributaries maybe either braided (interconnected channels) or sinuous, but usually several large independent branches channel the water across the delta. Lobes or larger splays of prograded sediment form at the mouth of each distributary, and as these lobes prograde out and increase the length of the channel, the gradient may drop enough so that flow will begin to wane in that distributary. If the gradient becomes too low, the stream flow may seek an alternate channel with a higher gradient which results in a shorter stream-flow path to the open water. These independent channels often form as crevasse splays through a natural levee.
Although many different facies occur in the delta plain, the depositional sequence is essentially progradational with well formed bottomset, foreset, and topset layers. The relative density difference between the river and ocean or lake will also result in a different depositional sequence. With fresh-water lakes, the denser stream water (sediment laden) will flow turbidly along the bottom so that the bed load settles more basinward. In salt water, the less dense discharge will float on top resulting in bed-load deposition nearer to the top of the delta slope. A salt water wedge often develops beneath the emerging river which helps separate the flow and causes rapid deposition of the bed load. This wedge may extend a considerable distance upstream beneath the main flow of the channel.
In an alluvial system, a distributary drainage pattern may develop where the stream enters a body of water. The alluvial fan, in this case, might be considered a small delta; however, the alluvial fan consists of very distinctive debris flow deposition.
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