The zebra mussel was introduced into the Great Lakes from eastern Europe in the ballast water of ships. First discovered in 1988, huge populations were soon found in the lower Illinois River. This aggressive mussel attaches to just about any hard object, and can seriously reduce native mussels by taking over nearly all available habitat. The zebra mussel will attach to boats and can live for several days out of water. this allows it to be easily transported to new locations.
Their unimpeded growth caused an population explosion of more than 50,000 individuals per square meter in some areas of the river. In 1993, however, the natural phenomena of flooding and a harsh winter caused a collapse of the zebra mussel population of about 99 per cent.