In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County In the summer of 1974, Professor David, a member of the U.S. Water Conservancy Delegation, visited the Hongqi Canal in Linxian County