As shippers and other users probably suspected, the July drought dried up the Mississippi and Ohio rivers to all-time lows and severely depressed flow in the Missouri, the U.S. Geological Survey said Friday. The survey's hydrogologists said the Mississippi at Keokuk, Iowa, averaged 10.5 billion gallons per day for the period July 1 through July 15, 74 percent below normal and 13 percent below the lowest previous July in 1936. At Vicksburg, Miss., the river averaged 93 billion gallons per day, 66 percent below normal and 22 percent below the 1934 record for the period. At Louisville, Ky., the Ohio averaged 5 billion gallons per day, 84 percent below normal and 3 percent under the smallest previous July flow recorded in 1930. The flow of the Missouri at Hermann, Mo., was 28.4 billion gallons per day, 42 percent below normal but still 31 percent above the July low-water record of 1936. The St. Lawrence River at Massena, N.Y. was still in the normal range, averaging 157 billion gallons per day or 11 percent below the normal mark. That was 21 percent higher than the July 1964 low-water record.