The degree to which fresh water and saltwater mix in an estuary is measured using isohalines. Isohalines are areas in the water that have equal salt concentrations, or salinities. In estuaries, salinity levels are generally highest near the mouth of a river where the ocean water enters, and lowest upstream where fresh water flows in. To determine isohalines, scientists measure the water's salinity at various depths in different parts of the estuary. They record these salinity measurements as individual data points. Contour lines are drawn connecting data points that have the same salinity measurements. These contour lines showing the boundaries of areas of equal salinity, or isohalines, are then plotted onto a map of the estuary. The shape of the isohalines tells scientists about the type of water circulation in that estuary.