Positive estuaries have a river at the back and get less saline going away from the sea. The true halophytes are found at the front, less salt tolerant wetland plants are found at the back. In a positive estuary the back zone is colonized by Juncus, Scirpus , and Phragmites . The middle zone is dominated by Spartina or mangroves, with patches of halophytes like Batis and Salicornia . The front is colonized by annual Salicornia before Spartina can get going, this is because Salicornia can easily colonize mud flats.
Negative estuaries are like the Colorado River delta with no current river flow.
These areas are saltiest near their backs. In a negative estuary, the back zone is covered in Allenrolfia, Atriplex , and Distichlis . The front zone has algae and seaweed as well as Salicornia . A salt marsh will typically only have about a dozen species living in it. The middle zone is occupied by the bushes Salicornia subterminalis a nd Salicornia virginica .