can you name a few rivers that add their water to this river?
The Marias River, the Musselshell River,  the Milk River
Missouri River
receiving the Marias River from the west then widening into the Fort Peck Lake reservoir a few miles above the confluence with the Musselshell River. Farther on, the river passes through the Fort Peck Dam, and immediately downstream, the Milk River joins from the north.

Flowing eastward through the plains of eastern Montana, the Missouri receives the Poplar River from the north before crossing into North Dakota where the Yellowstone River, 
where does the missouri river start and stop It rises  in the Rocky Mountains of western Montana, the Missouri flows east and south for before entering the Mississippi River north of St. Louis, Missouri.
wiki:25677388
Missouri River [SEP] Course
Farther on, the river passes through the Fort Peck Dam, and immediately downstream, the Milk River joins from the north. Flowing eastward through the plains of eastern Montana, the Missouri receives the Poplar River from the north before crossing into North Dakota where the Yellowstone River, its greatest tributary by volume, joins from the southwest. At the confluence, the Yellowstone is actually the larger river. The Missouri then meanders east past Williston and into Lake Sakakawea, the reservoir formed by Garrison Dam. Below the dam the Missouri receives the Knife River from the west and flows south to Bismarck, the capital of North Dakota, where the Heart River joins from the west.