barrier (made of concrete, earth, etc) built across a river to hold back the water and form a reservoir, prevent flooding, etc 水坝; 水堤; 水闸. 
reservoir formed by such a barrier 水坝围成的水库.> dam v
1 [Tn, Tn.p] ~ sth (up) build a dam across (a river, valley, etc) 建水坝於（河﹑ 谷等）.
2 (phr v) dam sth up (fig 比喻) hold back (emotions, etc) 抑制（情感等）: to dam up one's feelings 抑制住自己的感情.