The Mississippi River ranks as the third longest river in the world, and it is eighth in terms of average water volume discharged. The river and its tributaries collect runoff and sediment from 31 states, making it the second largest watershed in the world. The sediment-laden Mississippi has, over thousands of years, built a vast wetland system along the lowland coast of Louisiana. Due to the delta-forming processes of the river, the nutrient-rich salt-water and fresh-water marshes and estuaries are nursery grounds for one of the most productive fisheries in the world. Louisiana's wetlands also buffer New Orleans and other coastal towns from erosion and high-velocity hurricane winds, and serve as important habitat for 15 million migratory birds and many other wildlife species.