It is not in the national park
This needs saying first, because the delta appears on Nyerere itineraries and a traveller can reasonably assume it is somewhere a boat from a camp might reach. It is not. The Rufiji delta is on the Indian Ocean coast, in Rufiji District in Pwani Region, and it is a long way downstream of the park. Boat safaris from Nyerere work the channels and oxbow lakes of the upper Rufiji inside the park; they do not go to the delta.
The delta is protected as part of the Rufiji-Mafia-Kilwa Marine Ramsar Site, designated on 29 October 2004, covering 596,908 hectares across Rufiji and Mafia districts in Pwani Region and Kilwa District in Lindi Region. The Ramsar documentation records that the northern part of the delta and its mangroves can be reached by road from Dar es Salaam in about four hours, roughly a hundred kilometres to the south — which is the practical route, and it has nothing to do with a safari.
What is actually there
The Ramsar record puts the delta at approximately 1,400 square kilometres, with a north-south extent of 65 kilometres, protruding 15 kilometres into the Mafia Channel. Most of it is mangrove: the site’s mangrove management plan gives 57,893 hectares of mangrove across the whole Ramsar site, of which 53,255 hectares are in the Rufiji delta itself, with Mafia holding 3,473 and northern Kilwa 1,165. Nine mangrove species grow here, and the documentation describes the stands as the most extensive along the eastern African coast and the largest continuous mangrove forests in eastern Africa.
Beyond the mangroves are intertidal flats, sandbanks, marshes of varying salinity and, seaward, seagrass beds. Rice is farmed in the delta, and fishing, mangrove cutting and seaweed farming are the working activities of the people who live in it. This is not a wilderness; it is an inhabited, worked landscape that happens to be internationally important.
Birds and dugong
A first full waterbird count in the delta, made in December 2001, produced 40,160 waterbirds of 62 species — a figure the documentation itself treats as a minimum, since birds hide in the mangroves and upstream. Numbers during the October-to-March migration are higher.
The Ramsar record also carries the most striking single fact about this coast: of 32 dugong sightings reported along the Tanzanian coast from January 2000, 22 — sixty-nine per cent — came from the Rufiji-Kilwa area, indicating the largest remaining concentration of dugong in Tanzanian waters. Most of those were incidental captures in gillnets rather than sightings of live animals, which is the whole conservation problem in one sentence.
Visiting, realistically
There is no visitor infrastructure in the delta and no published excursion. Access is by boat from the coastal villages and by road from the Dar es Salaam side, and everything is arranged locally. Mangrove trips on this coast are far more developed around Mafia and Kilwa Masoko, both of which are inside the same Ramsar site and both of which have places to stay.
The honest recommendation: a traveller who wants the Rufiji as a river should do the boat safari inside the national park, where the channels, sandbanks, hippo and crocodile are and where the logistics exist. A traveller who wants the delta should treat it as a separate coastal trip from Dar es Salaam or Mafia, not as an add-on to a safari, and should not expect a lodge to be able to arrange it.