They vanished from the Thames because up to 1970 pollution in the lower reaches prevented migration.

In some cases, there can be an impermeable substance smeared along the fault line (such as clay) that also acts to prevent migration.

In 2003, a groundwater treatment system was voluntarily installed to prevent possible offsite migration of contaminants.

All oil fields have a geological barrier preventing upward migration of oil.

This was heavily managed, with birds taken into captivity after the breeding season to prevent migration.

Following the end of the breeding season, the birds are taken into cages in late July or early August to prevent migration.

It seems likely that the Settlement Laws were not so much preventing migration as inhibiting distance.

Several of these countries established strict visa requirements to prevent further migration.

We will also need further action on preventing illegal migration, combating the ruthless business of human trafficking and protecting our common external borders.

That is the strength of exchange rates: they can prevent unemployment and migration by cutting their coat according to their cloth.