Estuaries are mixed waters where fresh water and the sea meet.
The health of an estuary depends on the health of the catchment area and tributaries which will affect the freshwater flow into the estuary. At the ocean end tides play a crucial role as they bring in nutrients, affect sediment dynamics and the open/closed condition of the estuary mouth. Generally the mouth will remain open if it has an impediment such as a rocky outcrop, or rocky bank to one side of it as is the case with the Bushman’s and Kariega Estuaries.
Both the Bushman’s and Kariega Estuaries are permanently open and are of intermediate size. They share many of the same characteristics. Both are freshwater deprived and as such the estuaries are marine dominated systems which function more as an extension of the sea. Their marine nature supports the growth of many salt marshes and eelgrass beds, and with reduced freshwater flow in recent years these habitats have proliferated.
Many of their water characteristics are similar to the ocean, making salinity uniformly marine and during drought years the salinity is often higher. The estuaries differ slightly in oxygen content and turbidity (clarity due to movement of sand). In recent years turbidity in the Bushman’s Estuary has been high due to the increased silt in the water column from the disturbed land in the catchment area. In the Kariega there is better tidal mixing which means the water is well oxygenated. The Kariega Estuary is clearer perhaps due to less degradation of the land in the catchment area.
The Bushman’s River rises on the farm Hoekoe about 60 kms south of Somerset East. It is South Africa’s second longest navigable river with access through Bushman’s and Kenton-on-Sea and various secondary and farm roads to the upper reaches of the estuary. Its catchment area is 2 675 km², it is 292.9 kms in length and its major tributaries are the Bega, Blou, Gxetu, New Year and Steins rivers. The change from river to estuary occurs about 33 kms from the mouth around Harvest Vale.
The upper and middle reaches of the estuary are about 180 m wide and confined to a deeply eroded v-shaped valley with a single mud bottom channel about 2.5m deep. The main channel is slightly deeper in the middle reaches compared to the upper reaches. Towards the ocean the channel splits into several deep, permanently submerged channels which criss-cross the estuary, creating a ‘braided’ pattern that divides the large intertidal salt marshes into ‘islands’.
Over time agriculture surrounding the catchment area and the proliferation of farm dams and weirs has reduced the volume and quality of freshwater inflow. Drought conditions compound this situation. Closer to the mouth the building of boat houses and jetties has affected sedimentation, erosion and flow rates. It is believed that the outflow of high concentrations of brine from the desalination plant near the mouth has made the mouth more saline. Increased urbanisation towards the mouth resulting in septic tank leakage has added nutrients and bacteria to the water. The construction of the R72 bridge on top of a natural sandbar has prevented the removal of fine sediments at spring tides resulting in a build up of sand and mud at the base of the pillars.
With reduced freshwater flow and fewer flooding events the normal flushing of the estuary has not taken place. Consequently sand has built up in the lower reaches and around the mouth of the Bushman’s Estuary. The accumulated sand on the eastern bank is now being blown into the surrounding dunes. This combined with the stabilisation of vegetation along the west bank of the estuary and subsequent inactivation of the Dry Bones Valley bypass system has led to the growth of the dune over the former Bushman’s Mouth parking lot. This is a concern to many people in Kenton and Bushman’s.
The Kariega rises just west of Grahamstown near Howison’s Poort. The estuary is long and sinuous and is navigable by small boats for approximately 18 kms. The catchment area is 685 km² and the river length is 138 kms. The major tributaries are the Assegai and Palmiet rivers. Freshwater inflow has been restricted by the construction of the Settler’s Dam, the Howison’s Poort Reservoir on the Palmiet River and the Mossland Dam upstream from the mouth. The change from river to estuary occurs approximately 24 kms from the mouth.
The upper reaches of the estuary are bordered by game farms and therefore it is the least disturbed area of the estuary in regards to bait collection and fishing. In the middle reaches there are salt marshes and the lower reaches are bordered by sand and mud flats. Near the mouth the estuary travels along the west bank and past the boat launching site before curving to exit on the east bank.
There is much less development along the Kariega Estuary than the Bushman’s and less agricultural activity in the catchment area. There may be some septic tank leakage and storm drain runoff from Kenton-on-Sea west bank. However the impact of people on the Kariega Estuary is less than Bushman’s Estuary.
In February this year Professor Whitfield told us about the importance of estuaries in the life cycles of many fish species. With the use of slides he illustrated that a total of 155 species are associated with South African estuaries; 42 species (27%) can breed in estuaries; 61 species (39%) use estuaries as nurseries and 103 species (66%) completely or partially depend on estuaries.
He then highlighted threats arising from, for example, habitat destruction, fish and bait exploitation, alien invasives, climate change and human impact (coastal developments and pollution). He said that the health of estuaries is also dependent on the wise management of catchment areas, particularly in the provision of freshwater to estuaries.
The condition of both our estuaries is of critical importance not only to the residents and visitors to Kenton, Ekuphumleni, Bushman’s, Marselle and Klipfontein but also to the flora and fauna which live in and alongside the estuaries. Each one of us has a role to play in protecting and keeping these precious resources healthy and thriving.
With support from Estuary Care, the Sustainable Seas Trust (SST) is developing a guide book on our two estuaries. The SST team is keen to have your photographs and anecdotes for possible inclusion in the book. Visit http://www.sst.org.za to find out more.
Sources:
- Unknown
- Samantha Green Sustainable Seas Trust
- Professor Alan Whitfield, Chief Scientist at the South African Institute for Aquatic Biodiversity (SAIAB)
