Scientific and local knowledge combined for the design of socio-environmental flows for restoring the productivity of the Incomati Delta (Mozambique)

Deltas play an important role at the interface between river basins and the ocean and provide numerous economically valuable ecosystem services

The Delta of the Incomati River a transboundary catchment with 47,000 km², shared between the Republic of South Africa, Eswatini and Mozambique, experienced profound changes over the past 70 years, resulting in reduction of its average discharge from 200 m³ per second (m³s^-1) in 1950 to currently 3.2 m³s^-1 per second. Dam construction, expansion of irrigated sugarcane and plantations of exotic trees in the uplands are the main drivers of change, while climate change, will likely induce further changes in the hydrodynamic equilibrium of the Delta.

As a result, saltwater incursion into the Delta has extended and can reached 60 km inland during survey conditions (2021-2023). This has negatively affected key productive habitats such as its mangroves and floodplains, its biodiversity and ecosystem goods and services delivery to vulnerable users, for example through the dwindling of nursery functions of the delta, which form an important sources of shrimp fisheries in Maputo Bay.

The local users have broad livelihood portfolios including fishing (riverine, wetland and coastal), farming (both wetland and dryland), free-range livestock keeping, gathering of reeds, etc.  

To revert the situation, the potential for improved dynamic environmental flows was assessed through the Eflows/DIDEM project founded on transdisciplinary analysis and modelling of the links between river discharge, salinity, biological components and livelihood strategies.

Methods included 2D-hydrodynamic modelling, using tidal gauge data, supported by an improved digital elevation model combining drone mapping and bathymetric surveys across the delta, a salt intrusion model using depth stratified water density data (dry and wet season) and data from a network of continuous monitoring of conductivity, temperature and depth as well as an analysis of groundwater flows. For the biology, an assessment of mangroves and floodplains, dry and wet season surveys of phyto- and zooplankton, weekly waterbird monitoring analysed through a feeding guild approach and multivariate statistical analysis.

In addition to an analysis of income-generating activities, focus group discussions, in depth interviews and commented field visits, a network of experienced local observers employing mobile phone picture diaries to document their activities. Serious game was employed where participants expounded their zone-specific resource use strategies as a function of different river discharge and rainfall defining optimal and worse case scenarios.  

To prevent salinization, a key factor impacting on livelihood strategies, the floodplains require a minimum flow of 120 m³s^-1 is (varying between 40 and 200 m³s^-1) but, more importantly, a dynamic environmental flow to create hydraulic barrier to salinity during the equinox tides of the dry season would reduce risks for vegetation loss and erosion. Maintaining flood peaks of over 1000 m³s^-1, which inundate the floodplain, is beneficial for the nursery function of the delta for both crustaceans and fish as well as the waterbirds, an important tourism attraction.