ST. LOUIS — Beneath the surface of rivers lurks a hazard that isn’t well understood — but could wreak havoc on people and communities near the water. Changes in weather patterns may be unsettling river channels that have been historically sturdy, driving them toward two extremes: accelerated erosion or supercharged flooding. Now a scientist at St. Louis’ Washington University is starting a new experiment that could anticipate — and perhaps even prevent — damage wrought by intense rainfall, an issue that has taken on increased urgency after a record-breaking downpour flooded St. Louis last week…