Mowbray Project

By far the most significant success of the LMP over the past year has been the rehabilitation of a series of bioretention ponds nestled between the River Park complex, Liesbeek Parkway and the N2.

These retention ponds, constructed in 1997 had fallen into a state of utter disrepair due to siltation, a lack of maintenance and the invasion of homeless individuals. So undesirable and overgrown was the section of the trail that it was barely used by members of the public as it posed significant security risks. Furthermore, water quality analysis of the stagnant remnants of water within the ponds revealed a highly polluted environment in terms of heavy metal counts, lack of oxygen and other variables associated with water quality. Years of attempts to engage with the River Park and City of Cape Town officials and councillors had proved fruitless and as such the LMP took it upon itself to remedy the situation.

Firstly, the infestation of Typha capenisis was dealt with by manual means, clearing a large amount of biomass from the ponds, other problematic plant species, including listed invasives were subsequently removed in February. In order to rehabilitate the system though, restored flow was required. By digging a trench between the first and second ponds, silt that had impeded the flow of water was removed and for the first time in 15 years, water entered and exited the ponds from and to the Liesbeek. Almost immediately water quality improved as the polluted water was flushed and fresh river water permeated the ponds. Much time has been devoted to the ponds this year and previously degraded sections have been entirely replanted with indigenous vegetation, including but not limited to 20 indigenous trees.

The results of the work have been clear for all to see. From a biological perspective, a variety of bird species have inhabited the ponds with many of them breeding. This spring alone has seen the successful rearing of African Black Duck, Yellow Billed Duck, Moorhen and Cape Weaver chicks. Aquatic invertebrate and vertebrate species have also returned to the ponds marking a clear improvement of health and a significant increase in habitat for all of the above mentioned species and more. In addition to the success in terms of functional ecology, the social enjoyment of the trail has been enhanced with a clear increase in walkers, runners and cyclists now enjoying the stretch without concern for their own security.

More work is required and ongoing at these retention ponds with the intention being that they act as a ‘bioretention facility’, thus cleaning the river and storm-water entering the system. As the only retention ponds along the Liesbeek, it is viewed as a critical case study for what can be achieved along the river in its entirety. Currently the LMP is using the ponds to propagate Palmiet reeds which will have uses along the whole river in both their potential in cleaning water as well as preventing erosion.