In the Spring of 2008, we installed a bog to help filter the water of the pond.
Beginning with a cement-block wall, rubber lining was added over the existing liner and filled with a thin layer of gravel. We added tubing with holes distributed throughout, and then filled in the rest with pea-sized gravel.
A pipe line was run to the upper pond to create a siphon to feed the bog.
The bog is wonderful! I love the extra variety of plants I can keep thriving without watering them all summer long!