Cinematic 4K. A river delta is a landform that forms from deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or standing water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or another river that cannot transport away the supplied sediment. Aerial air sky best beautiful amazing geography geographic formation mainland continent alluvium alluvial plain river mouth stream creek brook valley rivulet gully mountain hill hills mountains straight forward motion from 1000 meter high altitude swamp quicksand marsh bog quagmire morass forest trees hurst creek material bed 3840×2160 UHD 4K.