Deltas: Deltas are depositional landforms found at the mouth of a river where the river meets a body of water with a lower velocity than the river (e.g. a lake or the sea). For a delta to develop, the body of water needs to be relatively quiet with a low tidal range so that deposited sediment isn’t washed away and has time to accumulate. When a river meets a stationary body of water, its velocity falls causing any material being transported by the river to be deposited. Deltas are made up of three sediment beds that have been sorted by the size of the sediment. The bottom most bed, the bottomset bed, is composed primarily of clay and some other fine grained sediments. Clay is the main constituent because when clay meets salt water a process called flocculation takes place where clay & salt particles clump together (flocculate) due to an electrostatic charge developing between the particles. This makes the clay particles sink due to their increased weight producing the bottomset bed. The bottomset bed stretches a fair distance from the mouth of the river as the fine sediments can be transported a reasonable distance from the river’s mouth. The foreset bed lies on top of the bottomset bed. The foreset bed is composed of coarser sediments that are deposited due to a fall in the river’s velocity and aren’t transported very far into the stationary body of water that the river flows into. The foreset bed makes up the majority of the delta and is dipped towards deep water in the direction that the river is flowing in. The topset bed is, as the name suggests, the topmost bed of the delta. It too is composed of coarse sediment but, unlike the foreset bed, the topset bed doesn’t dip, it’s horizontally bedded. Deltas can take on many different shapes. The three primary shapes of delta are cuspate, arcuate and bird’s foot. Arcuate deltas (e.g. The Nile Delta, Egypt) are shaped like a triangle (which is where the term delta comes from, the Greek letter delta Δ) and form when a river meets a sea with alternating current directions that shape the delta so that it looks like a triangle. Cuspate deltas (e.g. Ebro Delta, Spain) are vaguely shaped like a V with curved sides. Cuspate deltas form when a river flows into a sea with waves that hit it head on, spreading the deposited sediment out. Bird’s foot deltas (e.g. The Mississippi Delta) are shaped like (as the name suggests) a bird’s foot. They extend reasonably far into a body of water and form when the river’s current is stronger than the sea’s waves. Bird’s foot deltas are uncommon because there are very few areas where a sea’s waves are weaker than a river’s current.
Posted on: Sat, 02 Aug 2014 15:42:25 +0000
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