My suggestion.

Definition:
1. a barrier to obstruct the flow of water, esp. one of earth, masonry, etc., built across a stream or river.
2. a body of water confined by a dam.
3. any barrier resembling a dam.

Dependendo do contexto.

# lake used to store water for community use
wordnet.princeton.edu/perl/webwn

# A place where anything is kept in store; especially, a place where water is collected and kept for use when wanted, as to supply a fountain, a canal, or a city by means of aqueducts;
en.wiktionary.org/wiki/reservoir

# A body of water collected and stored in an artificial lake behind a dam.
www.streamnet.org/pub-ed/ff/Glossary/glossarydam.html

# Lake-like body of water created by damming of stream or river.
fishandgame.idaho.gov/fish/glossary/

# a storage area for fresh water supplies
www.nhcs.k12.in.us/grade5/science/ch8c.htm

# a natural or artificial lake that is a source or store of water.
fishresource.com/resource/glossary.asp

# An artificial lake into which water flows and is stored for future use.
www.alabamapower.com/hydro/glossary.asp

# A man-made facility for the storage, regulation, and controlled release of water. Types of reservoirs include flood control, water supply, and power generation.
www.basaltriverinfo.net/glossary.htm

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Note added at 30 mins (2008-12-01 16:04:59 GMT) Post-grading
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Açude pode ser o lago ou reservatório de água (este sentido);

Ou a barreira (represa) usada para conter a água.