Eutrophication occurs in water bodies when too many nutrients are introduced. Over-fertilization of water causes algae to grow blocking sunlight for plants.
Streams carrying too much sediment clogs the entrance into the basin. As sediments dump in, water flow decelerates and widens the entrance forming a delta.
Ocean currents are like giant conveyor belts moving huge amounts of water all the time. Wind, temperature and salt gradients all influences ocean currents.