Mississippi River Restoration and Resilience Initiative Act or the MRRRI Act This bill establishes the Mississippi River Restoration and Resilience Initiative to protect and restore the Mississippi River Corridor. The Environmental Protection Agency (EPA) must establish the initiative and the Mississippi River National Program Office to carry out the initiative. The office must (1) coordinate agency actions to protect and restore the Mississippi River Corridor; (2) develop, implement, and update the initiative, actionable goals, and an action plan required by the bill; (3) make updates and information available on a public website; and (4) submit annual reports to Congress. Additionally, the bill outlines the focus areas that each project carried out under the initiative must address, such as improving water quality and protecting wildlife habitat. The office may make grants to certain nonfederal entities to carry out eligible projects. The bill directs the EPA to include the initiative as a separate budget line item in its annual budget submission. Next, the bill directs the Department of the Interior to coordinate with the office to establish four Mississippi River Corridor research centers within the U.S. Geological Survey (USGS). These research centers must, among other duties, conduct scientific research on the focus areas of the initiative. The bill also directs the USGS to host a science forum to share current science and identify data gaps related to the ecological health of the Mississippi River Corridor. The USGS must, within two years of the forum, develop a science plan for the initiative.