The Earth\'s tides, considered one of the most predictable events on the planet, are actually quite variable over long time periods, U.S. researchers say. Researchers at Oregon State University say tides have changed dramatically over thousands of years because of ice ages, plate tectonics, land uplift, erosion and sedimentation. And they could change again in the future, the researchers said in a release issued by the university. The study, done with computer simulations at a high resolution, was published in the Journal of Geophysical Research. \"Scientists study past sea levels for a range of things, to learn about climate changes, geology, marine biology,\" said David Hill, an associate professor in the OSU School of Civil and Construction Engineering. \"In most of this research it was assumed that prehistoric tidal patterns were about the same as they are today. But they weren\'t, and we need to do a better job of accounting for this.\"\" For example, the researchers say, some tides on the East Coast of the United States may at times have been enormously higher than they are today, with a difference between low and high tide of 10 to 20 feet instead of the current 3-6 foot range. \"Understanding the past will help us better predict tidal changes in the future,\" Hill said. \"And there will be changes, even with modest sea level changes like 1 meter (3+ feet). In shallow waters like the Chesapeake Bay, that could cause significant shifts in tides, currents, salinity and even temperature.\"\"