Multiple choice general knowledge science & technology The largest drainage basin in the continental United States is: The Missouri River drainage basin The Columbia River drainage basin The Arkansas River drainage basin The Mississippi River drainage basin Reveal answer Fill a bubble to check yourself D Correct answer Explanation The Mississippi River drainage basin is the largest in the continental US, covering over 1.2 million square miles across 31 states and including major tributaries like the Missouri and Ohio Rivers. The Columbia and Arkansas are smaller systems.