From the DNR site
The season is open all year on Mississippi River pools 9, 10, 14, 15, 16, 17, 18 and 19. There is a closed season for all fishing on the Mississippi River in the tailwaters at three navigation dams from December 1 of each year through March 15 of the following year. The closed areas include only the main channel and main channel border areas and do not apply to backwaters and side channels in these zones. These closed fishing areas are:
1. from Dam 11 at Dubuque downstream to the railroad bridge, near river mile 579.9.
2. from Dam 12 at Bellevue downstream to the mouth of Mill Creek, near river mile 556.0 and
3. from Dam 13 at Clinton downstream to the downstream end of Stamp Island near river mile 521.5.
The answer to your question is March 16th