The simplest dam is made by piling mixed sizes of boulders across a stream bed. Water flows through this dam, but a pool several feet deep can be formed; maintenance may be neces-sary after floods. Boards do not rot underwater and, when strongly braced, can be used for a dam. The wings of a timber dam should extend at least 4 ft into the side banks, and ¾ in. rebar should be used to fasten timbers together every 6 ft and near every joint. Rebar can also be driven 5 ft into the stream bed, or the timbers can be cabled to upstream trees. An earthen dam will fail quickly if it is overtopped during a flood; the overflow pipe or spillway must be adequate. An earthen dam will also soon be eroded by seepage through the dam; a clay core is important.