Red tide is a term used for brownish or reddish areas of ocean, river, or lake water. The color comes from the presence of millions of microscopic organisms in the water. The discolored areas may range from less than a few square yards or meters to more than 1,000 square miles (2,600 square kilometers). They may last from a few hours to several months. Red tides appear in waters in most parts of the world. In the United States, they are often seen off the coasts of Florida, Texas, and southern California.