WATER LEVELS in the Lake Michigan-Lake Huron basin fell one inch between June and July when they usually increase. Mariners will need to exercise caution because water levels are forecasted to range from one to 10 inches below the levels noted on nautical chart over the next six months, according to an Army Corps of Engineers report released last week. In July, water levels were 21 inches below the long term average level recorded between 1918 and 2011. It’s possible levels could fall to new record lows by the end of this year if current trends of below average precipitation continue.