Eichhornia crassipes, the water hyacinth, is an unwelcome immigrant in South African rivers and lakes as it is in many other parts of the world. The plant is a free-floating perennial aquatic plant or hydrophyte originating from tropical and subtropical South America where the plants are found on water in rain forests as well as in warm temperate desert regions.
In the photo it floats down the Crocodile River between the Kruger National Park and Mjejane Game Reserve, escorted by some of the eponymous river reptiles not concerned with the plant’s progress.
Whereas only a few innocuous looking pieces are seen here, many tons of floating foliage may suffocate aquatic life in many parts, often exceeding other intruders in severity. For this is one of the fastest growing plants known on earth, a pernicious invader reproducing (mainly) from stolons or runners that become independent plants when the runner breaks.
Every newly independent bit floats away without natural enemies to live wherever fresh water may take them. They don't succeed in salt water, but for the sea we have plastic. A thriving acre of water hyacinth may weigh up to 200 tons.
“Normal” propagation from the plant's copious seed production can await favourable germination conditions for years. This compounds the problem. Water surface mats of the leaves expand fast, the break-aways taken downriver and leaving plants behind wherever the flow is slow.
There are various chemical, mechanical and biological campaigns worldwide for gaining control of this weed and minimising its ecological and economic impact (http://plants.ifas.ufl.edu; Wikipedia).