I infer that a valley is formed by physical weathering. The river flow gradually slices through the land (physical weathering), creating a valley. The weathered material gets carried by the river (erosion) and into a river delta.
River Delta Formation
I infer that a river delta is formed by water erosion. The river carries sediment away (water erosion), and gets deposited near the ocean or a lake. The sediment is from valleys the river goes through and carries away. It takes a long time for sediment to erode and build up into a river delta.