St. John's Lake is a huge tidal inlet on the Cornish side of the River Tamar, opposite Plymouth. It is an area of saltmarsh formed when the incoming tide meets the fresh river water. It supports the greatest number of estuarine birds in Cornwall and attracts the most species and individual birds in this area. As such it has been designated under the European Directive on Wild Birds as a Special Protection Area. Apart from the resident birds it provides an important 'refueling' stop for birds on migration.