Much more interesting than you might think:
It’s called IJsseloog, an artificial island used to store polluted silt—smaller than sand but larger than clay—in the middle of the IJssel River in Flevoland, Netherlands. More from the all-knowing Wikipedia:
For now, cleaning the silt is not yet feasible. To avoid nuisance to agriculture, to local residents, and to prevent groundwater contamination, the silt depository was built in the centre of the lake and not on the coast. The water level in the IJsseloog is maintained at 4m below sea level to prevent pollution outflows.
A port was built on the outer edge of the IJsseloog for delivery of silt. Once the depot is full, remediation will be done by decantation. Clean silt will be used to construct new ecological zone, the IJsselmonding. The clean sludge that is released will be used for the construction of a natural IJssel estuary.
When storage is full, the layers of clay and sand will be sealed, and the island itself will be used for recreational purposes. After that process, Nature will take over in the IJsseloog.
(See all Orbital Views here)