Dictionary is a convenient place to define rare or unusual (English) words, that do not justify separate articles. Also for abbreviations.
In my humble opinion.
A leat is a manmade watercourse, usually supplying water to a watermill or its mill pond. 
A sluice is another kind of man-made watercourse, often fitted with a gate. 
At Portmadog there is a tidal sluice with gate.
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