Why was it significant that the Union gained control of the Mississippi River early in the US Civil War?

The control of the river was of strategic importance to both sides during the war. The North had set up a blocade in some major southern cities. To fully contain them they needed to also have control of the river. Roads at this time were not very well maintained. There were some railways, but these were limited. River travel was still the quickest and easiest way to travel during this time. Large amounts of troops and supplies could be transported in this way. The Mississippi also has many tributaries. Who ever controlled it would in turn control these lesser waterways.