I spent two days in the Tsitsikamma on a team building trip over the weekend where we stayed at the Tsitsikamma Lodge which is right next door to the Kruis River. What was apparently a gentle meandering stream the day before turned into a raging torrent of water overnight after a big rain storm.
The brown colour of the water is due to tannin being picked up by the rivers in the area as they flow through the Fynbos and forests. So in a way the vegitation acts as a "tea bag" causing the water to change colour. Even though it is brown in colour the water is perfectly clean and is used by locals, visitors and hikers.