Surface Diversion:	A man-made structure or installation for diverting water from a stream, river or other surface water body for a beneficial purpose (municipal, industrial, agricultural, hydroelectric generation, etc.). Surface diversions fall into two general categories: "gravity" and "pump".
Diversion Headgate:	A gated structure that controls the flow of water from the surface water source into a gravity conveyance facility (canal, ditch, pipeline, etc.)
Fish Screen (or Fish Guard):	A fish protection device installed at or near a surface water diversion headgate to prevent entrainment, injury or death of targeted aquatic species. Fish screens physically preclude fish from entering the diversion and do not rely on avoidance behavior like electrical or sonic fish barrier technology. Fish screens are categorized by: 1) diversion type (gravity vs. pump), and 2) debris cleaning function ("active" or automatic vs. "passive" or manual cleaning).
Diversion Entrainment:	The voluntary or involuntary movement of fish from the parent water body into the surface diversion.
Screen Entrainment:	The voluntary or involuntary movement of fish through, under or around the fish screen resulting in loss of fish from the population. Entrainment is a function of screen mesh opening size and gaps between the screen frame and canal structure walls.
Impingment:	The involuntary contact and entrapment of fish on the screen surface due to approach velocity exceeding swimming capability.
Screen Orientation:	The orientation of a fish screen relative to the axis of the approaching flow in the diversion canal or pipeline. Screens may be oriented parallel to flow (0o) and at increasing angles up to perpendicular (90o).
Fish Bypass System:	Gravity fish screens installed downstream of the diversion headgate usually require a "fish bypass system" to collect fish from in front of the screen and safely transport them back to the stream. The fish bypass consists of an entrance/flow control section and a fish conveyance channel or pipeline.
Fish Bypass Water:	A portion of the diverted flow used to transport fish from in front of the fish screen back to the stream through the fish bypass system. Fish bypass flow requires positive hydraulic head differential between the water surface at the screen and the water surface at the bypass outfall to the stream.
Approach Velocity:	The component of approaching water velocity passing through the fish screen perpendicular (90o) to the screen surface (usually expressed in feet/sec). Excessive approach velocity results in impingment of aquatic life on the screen surface or entrainment past the screen. Approach velocity is simply a function of diversion flow rate (expressed in cubic feet/sec) and submerged screen area (square feet).
Sweeping Velocity:	The component of approaching water velocity which moves parallel to the screen surface as a function of screen orientation and the amount of fish bypass flow. High sweeping velocity reduces the chance of impingment or screen entrainment.
Flow Conversions:	1 million gallons/day (mgd) = 694 gallons/min (gpm) = 1.55 cubic feet/second (cfs)