On the 3 river cruises I've done, we never wore truly dressy anything but we did dress a bit more than for other nights. My cruises were fall, winter, and late spring. I know I wore Chicio's black Travelers with pretty jewelry and dressier shoes than those I wore for touring for the Captain's Dinners and I felt I was dressed every bit as well as those around me.
No one wore gowns - a few of the women did wear lovely silk tops and/or jackets. I just wouldn't plan to wear jeans and sneakers and I have no idea how summer might change what some cruisers choose to wear.