Rivers report card: Three years of river health monitoring in Table Mountain National Park
The South African Scoring System (SASS 5) is a biomonitoring tool that uses freshwater macroinvertebrates to assess the ecological state of rivers. The Krom, Silvermine and Disa Rivers historically had SASS 5 data collected by various researchers since the early 1990s. However, regular monitoring by other agencies ceased in 2016 due to capacity constraints. Scientific Services staff started SASS 5 monitoring in Table Mountain National Park in 2018. This article provides feedback on the seasonal monitoring of four rivers from 2019 to 2021. Monitoring continued in the sites with historical data to maintain the dataset, and two additional sites were established in Cecilia Ravine in 2018 to gather baseline data to measure the impacts of invasive alien trees (Upper site), plantations and tourist activities (Upper and Lower sites).
Our findings can be seen in the table below. The Krom and Silvermine Rivers were mostly in a good ecological state from 2019 to 2021, suggestive of healthy, and largely unimpacted, river systems. The upper reaches of the Silvermine falls within the park while more that 50% of the Krom is also conserved in the park. Park management has cleared invasive alien trees from both catchments within the park for the past 30 years. Cecilia Ravine Upper site had seasonal differences in river health with the ecological state deteriorating from good to fair in summer. This is likely a natural fluctuation because water flows are very low in summer, resulting in limited habitats available to invertebrates. The ecological state improves as water flows improve during autumn. In the Cecilia Ravine Lower site, river health in summer further deteriorates to a fair-poor ecological state. This site is impacted by human use, which is allowed in this section of the park. Similar to the site upstream, the ecological state of the river improved in autumn when flows improved downstream. In contrast to the other rivers, the ecological state of the Disa River fluctuated between good, fair and poor with no discernible seasonal pattern. This site is impacted by flow modifications from dams upstream; in some years total flow is abstracted in summer.
Table 1. Seasonal ecological state of the different rivers from 2019 until 2021.
The river health data still need to be compared with water temperature, chemistry and habitat data that were collected simultaneously to the SASS 5 data to provide a better understanding of the causal effects of changes in river health. There is also a need to obtain flow data for the Disa River from the City of Cape Town’s Bulk Water department, and park management would need to lobby the national Department of Water and Sanitation to maintain ecological flows for the river. The appropriate management actions around the Cecilia Ravine Lower site will need to be discussed with park management, and future research and monitoring will need to be designed around the management actions to be undertaken.
Trevor Adams conducts SASS in the Silvermine River in Table Mountain National Park; this river is mostly in a very good ecological state reflecting its unimpacted flow through a protected area
This article was originally published in the 2021/2022 Research Report.
