Why is control of the mississippi river so important during the civil war?

Because they served as avenues of invasion into the south and of course it was the biggest and most important river for subduing the confederacy. 

“destroyers for bases agreement in the destroyers for bases agreement between the united states and the united kingdom on september 2, 1940, fifty caldwell , wickes, and clemson class us navy destroyers were transferred to the toyalbnavy from the united states navy in exchange for land rights on british possessions.” i hope this you .