The Yellow River has been prone to flooding for centuries, mainly due to the huge buildup of sediment that can fill up the riverbed. When combined with heavy rains, this leads to a drastic rise in the river's level, causing serious flooding in the flat lands that lie along it.
The Yellow River actually gets its name from the huge amounts of yellow loess silt that it contains, which often gives the river a distinctive yellow color. The problem with a river containing large amounts of silt and sediment is that it tends to build up along the banks and other slow-moving areas. In addition to flooding, this can eventually cause the river to change its course as well.
The river has experienced quite disastrous flooding for several centuries, and it's estimated that there have been more than 1,500 floods since the second century B.C. The worst flood, which occurred in 1931, is estimated to have directly or indirectly resulted in the deaths of between 850,000 and 4 million people, making it the worst natural disaster in recorded history.
Recent evidence has pointed to early Chinese attempts to control flooding of the Yellow River as possibly being one of the biggest factors behind these floods, as their attempts resulted in worsening the sedimentation and causing the riverbed to rise.