The construction of Lock and Dam No. 12 in 1939 created a series of islands, backwater channels, and backwater lakes between Dubuque and Bellevue, Iowa. However, since the lock and dam construction, river sediments have been slowly filling in the backwater areas, reducing the value of historic fisheries overwintering sites. In addition, the lock and dam also caused changes to the floodplain forest communities. The floodplain forest is now dominated by similar-aged species of cottonwood and silver maple, and this lack of diversity results in reduced habitat value.