Slack water is the time during which no appreciable current in flowing in a body of water. Slack water usually occurs during both high and low tide, when the direction of the currents are reversing. Because slack water is usually determined by the tides, it can be accurately calculated in most regions.
Slack water is of particular interest to scuba divers. The absence of a current means that less effort is required to remain at a given site, and there is less likelyhood of drifting away from the boat. Slack water can also provide better visibility, as there is no current to stir up sand and silt from the bottom. It is very common for dive operators to plan their dives around slack water.
Puddle is used to separate the water of a canal from porous strata, and is placed in ditches to prevent water from spreading away from the canal, and to stablilize embankments.
Water passages are made from the upper reach to the lock chamber, and from the lock chamber to the lower reach, with a gate, called a paddle, that is raised or lowered by a rack and pinion to open or close the passages.
The amount of water required for the passage of one boat is determined by the largest lock, so the rise of the locks on a canal is made as uniform as possible.
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