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a river in the USA, in the states of Missouri and Arkansas; a right tributary of the Mississippi River. The St. Francis measures 720 km long and drains an area of approximately 22,600 sq km. Originating on the Ozark Plateaus, it flows through the Mississippi Valley in its middle and lower courses. High water is in the spring; the water is extremely shallow in the autumn. The mean annual flow rate is 167 cu m per sec. The St. Francis is navigable for small vessels for 240 km from the mouth. It is used for irrigation (the Wappapello Reservoir).