The South African government intends to develop the Mzimvubu Water Project (MWP) which includes the construction of two dams (the Ntabelanga and Laleni dams) in the Tsitsa River, Eastern Cape province, South Africa. This investment is believed to be important to unlock the economic potential of this rural, poor and underdeveloped area. We consider a range of variables to ascertain what the realistic economic lifespan of each of the dams are, subject to uncertainty and complexity, as the Highlights:  Variability in soil erosion in a catchment cause a dam's actual and predicted economic lifespan to differ significantly.  When no mitigation is assumed the modelled Ntabelanga dam's lifespan declines from 55-68 years to 31-44 years. With mitigation, the anticipated lifespan is 50-65 years.  When no mitigation is assumed the modelled Laleni dam's lifespan declines from 26-33 years to 16-21 years. With mitigation, the anticipated lifespan is 24-30 years.  The state of the catchment has to be improved and maintained through restorative actions together with societal endorsement and behavioural change. Such behavioural change is to be informed by a catchment management plan and the proactive and careful implementation thereof.  The investment in social and natural capital to safeguard the lifespan of the manufactured capital (the dams) and thus the financial capital invested, is of high importance.