Maintaining and Enhancing Hydroelectricity and River Restoration Act of 2021 This bill allows a new tax credit for maintaining and enhancing hydroelectric dams that is equal to 30% of the basis of any qualified dam safety, environmental, and grid resilience enhancement property placed in service in a taxable year. It also allows a 30% credit for qualified obsolete river obstruction removal expenditures for demolishing and removing certain nonpowered or hydroelectric dams.