[twitter][/twitter]

Where a river enters the sea its flow is temporarily blocked off in a way by the in/upcoming high tide. The flow of the river slows down, water level builds up. Very.. very basic, but hope this helps.

Indeed ^^^, in fact under normal conditions where we are in Pathum Thani the flow actually reverses slightly when the tide spring tides peak, you can see the water hyacinth moving upstream.