What is Miner's Inch?

Legal Definition
The miner's inch is a unit of flow in terms of volume per unit time, usually in relation to flow of water. The definition of a miner's inch varies by location.

In hydraulic mining and some forms of placer mining, as well as ore dressing, a large and regular supply of water is needed. The miner's inch is a method of measuring the amount of flow a particular water supply system (such as a flume or sluice) is capable of supplying.

The miner’s inch was derived from the amount of water that would flow through a hole of a given area and a given pressure (for example, 4-6 inches of water, or 1-1.5 kPa). The word 'inch' actually refers to the area of the hole in 'square inches'.
-- Wikipedia
Legal Definition
The quantity of water which will escape from a ditch or reservoir through an orifice in its side one inch square, the center of the orifice being six inches below the constant level of the water, equivalent to about 1.6 cubic feet of water per minute. Defined by statute in Colorado as "an inch-square orifice under a five-inch pressure, a five-inch pressure being from the top of the orifice of the box put into the banks of the ditch to the surface of water " Mills' Ann. St. Colo. § 4643. See Longnure v. Smith, 26 Wash. 439, 67 Pac, 246, 58 L. R. A. 308.
-- Black's Law Dictionary