The narrow lake east of IJsselmeer is Lake Ketelmeer. The lake receives water from rivers that carry industrial pollutants from factories upstream. Those polluted sediments have settled to the bottom in a thick layer of contaminated sludge. To restore a normal aquatic environment, this material needs to be removed from the lakebed.
In the middle of the lake, in the 2010 image, a circular feature appears. This artificial island is called the IJsseloog, a repository for contaminated material dredged from the bottom of the lake. Once the repository is full, it will be capped and turned into a nature reserve.