Case study: Mlalazi Estuary and Floodplain

Name of the wetland

Mlalazi Estuary and Floodplain

Name of the respondent

Ricky Taylor

Photo 2: A valued habitat in the estuary is the salt-marsh which is in the supratidal margins of the estuary. Photo Ricky Taylor

What you have seen in the wetland – state, change, and drivers

The Mlalazi Estuary has been altered by increasing human impacts since the latter 1800s. It is, however, regarded as an estuary that is in good condition. Conversely, much of its floodplain is severely transformed from its natural state.
The Mlalazi Estuary, and its floodplain, is in the north-east coast of South Africa. The estuary is tidal for about 15 km and is set in a wide floodplain. About half the catchment area is natural, the rest has been transformed by agriculture and tree plantations.
In its natural state the mouth of the estuary would close during extended periods of below-average rainfall which resulted in low river inflows. It could stay closed for several months at a time. During this time water in the estuary would back up behind the beach berm, inundating the lower parts of the floodplain. Breaching would occur naturally when the estuary water level was high enough to overtop the beach berm.
Since the late 1800s the mouth has always been breached artificially whenever the water level in the estuary starts to rise after closure. This is to protect sugar-cane fields planted in the floodplain. This breaching has altered the ecological state of the estuary – which now functions more as a permanently open estuary than one that is intermittently closed. The most obvious change of this altered regime is that mangroves, and their associated fauna, have been able to colonise the estuary – which was formerly mangrove-free.
Most of the present-day estuarine-floodplain was part of the ancestral estuary that has largely filled with sediment in the past 6000 years – hence the small estuary situated in a large floodplain. The natural vegetation of the floodplain consists of salt-marsh in the supratidal areas; saline lawns where soils are salty but mostly dry; reed and sedge swamps where there is an accumulation of fresh water; swamp forest where there is groundwater-fed seepage; and woody thickets in slightly raised sites.
The river entering the estuary comes down as a flood whenever there is a large rainfall event in its catchment area. The ensuing flood fills the floodplain basin to levels of up to 10 m above mean sea level. But this is usually a short-term event – the flood waters rising and diminishing rapidly. Vegetation in the main path of the flood may be scoured by fast-flowing water, while in the backwaters it is killed by the deposition of fine sediments. Each flood manifests itself differently, resulting in a patchwork of vegetation at different successional stages. This dynamic creates ‘unoccupied’ habitat which encourages invasion by alien vegetation.
Flood dynamics were altered in about 1900 when a railway embankment was constructed across the upper reaches of the floodplain and a bridge was built across the estuary. Floods can no longer spread laterally as they used to do in the upper portion of the floodplain. Floods are now contained by this railway embankment and focused to pass under the bridge. This has reduced natural channel switching and avulsion patterns of the flood waters. In addition, much of the floodplain is now cultivated. It is only within the designated Mlalazi Nature Reserve that there is still natural vegetation.

Conservationists have focused much more on the estuary than on the floodplain. The state of the estuary has changed with the colonisation of mangroves. The present state of the estuary is dependent on the mouth being breached before the mangroves are killed by prolonged inundation with the backing up water.

Photo 4: Without mangroves there would be little of the associated mangrove fauna such as these fiddler crabs. These crabs have an obligate marine larval stage. Photo Ricky Taylor.