In an estuary, the meeting place of a river and a tidal sea, harsh conditions can adversely effect commercial shipping. For example, near the Amazon delta is the estuary for the Para River in which the incoming tide can build up to a flood wave of water. As the wave moves inland, the estuary becomes narrower and the wave becomes a wall of water that can grow to hieghts of 5 or 10 meters. This daily tidal wave can cause significant damage along the river bank and is accompanied by a frightening sound called the pororoca (thundering water). Needless to say, few merchants are anxious to operate ships in such an estuary.
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