The write-up on the Mound City in the Dictionary of American Fighting Ships at:

http://www.history.navy.mil/danfs/m15/mound_city.htm

doesn't indicate that the Mound City sank. It does state:

The "Mound City and Carondelet grounded near the wing dams across Red River 10 May (1864). Hauled across the upper falls above the obstructions by throngs of straining soldiers, the ships were able to get free. Mound City spent the remainder of the year guarding Indianola."