 The Rocky Mountains of southwestern Montana at the headwaters of the Missouri River first rose in the Laramide Orogeny , a mountain @-@ building episode that occurred from around 70 to 45 million years ago ( the end of the Mesozoic through the early Cenozoic ) . This orogeny uplifted Cretaceous rocks along the western side of the Western Interior Seaway , a vast shallow sea that stretched from the Arctic Ocean to the Gulf of Mexico , and deposited the sediments that now underlie much of the drainage basin of the Missouri River . This Laramide uplift caused the sea to retreat and laid the framework for a vast drainage system of rivers flowing from the Rocky and Appalachian Mountains , the predecessor of the modern @-@ day Mississippi watershed . The Laramide Orogeny is essential to modern Missouri River hydrology , as snow and ice melt from the Rockies provide the majority of the flow in the Missouri and its tributaries . 
 The Rocky Mountains of southwestern Montana at the headwaters of the Missouri River first rose in the Laramide Orogeny , a mountain @-@ building episode <a href='null_relcl' title='1'><mark>that</mark></a><a href='been_done' title='2'><mark></mark></a> occurred from around 70 to 45 million years ago ( the end of the Mesozoic through the early Cenozoic ) . This orogeny uplifted Cretaceous rocks along the western side of the Western Interior Seaway , a vast shallow sea <a href='null_relcl' title='3'><mark>that</mark></a>stretched from the Arctic Ocean to the Gulf of Mexico , and deposited the sediments <a href='null_relcl' title='4'><mark>that</mark></a>now underlie much of the drainage basin of the Missouri River . This Laramide uplift caused the sea to retreat and laid the framework for a vast drainage system of rivers flowing from the Rocky and Appalachian Mountains , the predecessor of the modern @-@ day Mississippi watershed . The Laramide Orogeny <a href='drop_aux' title='5'><mark>is</mark></a>essential to modern Missouri River hydrology , as snow and ice melt from the Rockies provide the majority of the flow in the Missouri and its tributaries . 
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