These islands are made of reed bundles, and from walking on them appear to be around five feet thick. They are in a shallow part of the lake, and are anchored to the bottom by incorporating live reeds into the bundles. Heavily populated and a major tourist attraction, they are one of only two such occurrences in the world.


See the following link for more details.
http://www.motojournals.com/journals.asp?section=southamerica_part1&page=9