What is the most populous city west of the Mississippi river?

Answer 1

All of the cities of Iowa, Missouri, Arkansas, Texas,and states farther west.

Minneapolis MN is mostly west of the river, as is St. Cloud, MN.

Shreveport, Lake Charles, and Lafayette, LA are west of the river.

One former city in Illinois is west of the river, the village of Kaskaskia.

Answer 2

Los Angeles, California. The city proper has about 3.8 million residents and the very large metropolitan area has nearly 13 million.