Where are deltas formed and why are they formed there?

Where a river which transmits a great deal of material arrives at the sea and there is little tidal range a delta will form. Example the Nile into the Mediterranean or the Mississippi into the Gulf of Mexico. The Amazon & the Congo, which flow into an Ocean, the Atlantic in these examples, do not form deltas because the ocean currents distribute the outflow of material into & along the ocean currents.