Evaluating the Impact of the Stiegler’s Gorge Dam on Ecosystem Services and Fisheries of the lower Rufiji floodplain and Delta, Tanzania

After the Zambezi (4500 m³s^-1), the Rufiji (800 m³s^-1) is the second most important river flowing into the Western Indian Ocean. Its delta’s 500 km² mangrove system, is key to Tanzania’s shrimp fisheries and coastal livelihoods in the Rufiji-Mafia-Kilwa Marine Ramsar Site. Between 2019 and 2024, a 2115 MW hydropower dam was constructed at Stiegler’s Gorge, within the 30,000 km² Nyerere National Park. Since the early 2000s, participatory research and monitoring have been conducted in the Rufiji floodplains and delta, to assess potential dam impacts, in particular on ecosystem services related to flood-dependent production systems.

The data collected, in collaboration with a network of local observers, both hydrometric and socio-economic, have been used in hydrodynamic modelling, land use mapping and fisheries assessments. The findings were discussed during stakeholder workshops, including government institutions. Based on previous research, the perception of post-dam ecosystem services delivery was evaluated and the fisheries assessment were redone in two floodplain lakes, well-connected Lake Zumbi and non-connected Lake Mtanza within the DIDEM project. Tools were focus group discussions and household questionnaire surveys. Fish data were collected at lake-adjacent landing sites.

Results revealed a relationship between ecosystem service demand satisfaction and river connectivity. The Zumbi connection offered more reliable water supply and supported greater fish diversity, though catch levels were lower than in Mtanza due to unsustainable fishing practices. Fish diversity in Lake Mtanza has declined (down to 3 species) whose size distribution suggests overfishing. Users there have mostly shifted to farming. Lake Zumbi maintained a higher diversity (15 species) and fisheries remain important for livelihoods. With lower water levels, crocodiles are an issue. Catch per Unit Effort in both lakes increased in comparison to the predam period, a sign of the use of more intensive (but also more destructive) techniques. The perception is that the upstream hydraulic infrastructure has interfered with fish migration and 80% of the interviewees at Lake Mtanza find the lake’s supply is insufficient while in Lake Zumbi this is 40%. There was no difference in salinity between connected and unconnected lakes; however, a comparison with previous data before dam construction revealed an increasing trend.

Our findings seem to confirm the findings from pre-dam research that managed flood releases from the dam, amounting to a minimum of 4500 m³s^-1 i.e. substantially higher than the design flood of 2500 m³s^-1, could be highly beneficial to biodiversity and ecosystem service delivery to the vulnerable downstream communities strongly dependent on the floodplain lakes, especially their fisheries. The flood release hydrograph would preferably show substantial inter-annual variability to simulate the pre-dam floods but guarantee connecting the river to the main floodplain lakes annually. Such releases, especially if they are planned to contain a maximum of fine sediment, i.e. at the start of the wet season - when the river has a high sediment charge – would also help to reduce saltwater intrusion and the expected loss of mangrove systems in the delta.