Rivers deposit sediments at their mouths to form deltas.
Deltas form where a river enters a larger body of water or a broad basin. Losing some of its capacity to carry sediments as its gradient decreases and its channel widens, the river deposits its load, building deltas of various shape from these materials. The existence of a delta attests to a substantial enough rate of river deposition to counteract erosion by ocean or lake waves.
Delta sediments are diverse, dependent on the loads that their constituent rivers transport. The largest are rock fragments derived from weathering and erosion in the drainage system. A stream in full flood easily has the capacity to drag and tumble large rocks along its channel; even big boulders too massive to transport can be worn down by the abrasion of smaller rocks and sediments in the flow, thus rendering them into mobile fragments. Granular sand might be composed of any kind of material but most often stems from the weathering of granite into quartz crystals. Smaller yet are particles of silt (between 1/16 and 1/256 millimeters) and clay (smaller than 1/256 millimeters). The origin of these river sediments varies from the simple abrasion of channel-edge rocks to volcanic deposits drained from mudflows called lahars that are associated with volcanic eruptions.
All manner of nongeological debris can accumulate in a delta, washed out from points upstream. Large rivers draining forested areas tend to carry a substantial burden of woody materials, from twigs and branches to huge logs. The delta of the Mackenzie River, Canada’s lengthiest, contributes tree trunks and branches to the Beaufort Sea, whose currents wash such driftwood along the Arctic coast.
Often a basic gradient of deposition is distinct in a delta, with large, coarse gravels deposited first, then progressively finer-grained particles throughout the delta and beyond. Coastal deltas are often divided into three sections: an upper delta plain at their head, dominated mainly by river influence; a lower delta plain occasionally submerged in brackish tidal waters and thus impacted both by freshwater and marine currents; and a subaqueous zone beneath sea level. Materials deposited in the lower delta plain may not all be riverine in origin: For example, a mangrove swamp occupying such a delta zone might be awash in nutrients and flotsam from the ocean that cycle in during high tide.
Deltas are classified by their shape and formation, which depend greatly on coastal topography, river characteristics and coastal wave energy. The Mississippi River each year deposits more than 200 million tons of sediment at its mouth in the Gulf of Mexico, creating a distinctive, ever-shifting “bird’s-foot” delta. The delta of the Nile is arcuate, or curved, as it’s sculpted mainly by wave action. A few large inland deltas exist, forming where a river flows into a broad topographic depression; Mali’s Inner Niger Delta and Botswana’s Okavango Delta are examples.