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the map shows a top view of the mississippi river. the gap between lines 1 and 2 shows how much saint anthony falls receded between 1680 and 1766. the gap between lines 3 and 4 shows how much the falls receded between 1856 and 1876. at what rate (on average) did the falls recede during each of these time spans? express your answer in units of feet per year. did the rate increase or decrease from one time span to the next?

hint: determine how many years passed between lines 1 and 2 and between lines 3 and 4 to you find your answers.