The U.S. Geological Survey has published a 24-page report that describes how the agency measured phosphorus loads for 23 major tributaries to all five Great Lakes, for water years 2014-2018. (Unlike calendar years, water years are 12-month periods that begin on October 1, according to the water cycle.) The estimated loads of phosphorus at the 23 sites were summed to determine, for each site, both the annual amount of phosphorus entering the Lakes and the cumulative amount for those years.
The Great Lakes Restoration Initiative supports phosphorus monitoring efforts to help track changes in loads as a result of agricultural best management practices and other nutrient reduction efforts.
Data also help track U.S. progress within Annex 4 of the Great Lakes Water Quality Agreement with Canada.
Comparison of phosphorus measurements among the 23 tributary sites allowed hydrologists to determine which tributaries contributed the largest annual and cumulative loads of phosphorus into the Lakes. USGS continues to measure phosphorus loads for each year.
Knowing where most phosphorus is entering the Lakes helps determine where control actions should be implemented to most effectively reduce phosphorus influx. High levels of phosphorus contribute to harmful algal blooms and hypoxic, or low oxygen areas, which both have negative impacts on Great Lakes food webs.