Which top-five river system includes the Missouri and Jefferson rivers?

The top-five river system that includes the Missouri and Jefferson rivers is the Mississippi system.

The Missouri joins the Mississippi near St. Louis, Missouri. Farther upstream, the Missouri's headwaters include the Jefferson River, which forms at Three Forks, Montana, where the Jefferson, Madison and Gallatin rivers meet. Tracing the longest connected channel from Montana to the Gulf of Mexico produces a river system roughly 3,700 miles (5,970 km) long.

This combined waterway is commonly ranked the world's fourth-longest river system, after the Nile, Amazon and Yangtze. The Mississippi itself is shorter than the Missouri when measured as a named main stem, but the broader system takes its familiar name from the Mississippi below their confluence.

A common mix-up is calling the Missouri the answer simply because it is the longest tributary. The question asks for the system that incorporates both rivers, so Mississippi is the canonical quiz answer.