they will move to cleaner water when available - muddy water clogs their gill-rakers. With all the recent rain up through Iowa, my guess is the water got too muddy for a while. Just check the prediction graph for Altonfrom this website.
http://www.crh.noaa.gov/ahps2/hydrograph.php?wfo=lsx&gage=alni2&view=1,1 ,1,1,1,1,1,1