REPORT OF INVESTIGATIONS No. 42 135

Confining bed. A bed which, because of its position and its impermeability or
 low permeability relative to that of the aquifer, gives the water in the
 aquifer either an artesian or subnormal head.

Confluence. The meeting or junction of two or more streams.
Diffuence. Flowing apart. A term used to describe a stream which branches
 in a downstream direction.
Direct surface runoff. The runoff entering stream channels promptly after
 rainfall.
Discharge. Flowing or issuing out. Also used to designate the volume of
 water flowing past a cross section of a stream in a unit of time.
Double-mass curve. A plot of the cumulative values of one variable versus
 the cumulative values of another.
Drainage area. The size of a drainage basin usually expressed in square
 miles.

Drainage basin. An area enclosed by a topographic divide such that direct
 surface runoff from precipitation normally would drain by gravity into
 the river basin.
Drainage divide. The boundary line, along a topographic ridge, separating
 two adjacent drainage basins.
 Drainage system. A surface stream or a body of impounded surface water,
 together with all surface streams and bodies of impounded surface water
 that are tributary to it.
 Effective precipitation. A weighted average of current and antecedent pre-
 cipitation that is "effective" in correlating with runoff.
 Evaporation. The process by which water becomes vapor at a temperature
 below the boiling point, including vaporization from free water surfaces
 and from land surfaces.

 Evapotranspiration. Evaporation plus transpiration.
 Fault. A fracture or fracture zone along which there has been displacement of
 rock material on the two sides relative to one another parallel to the
 fracture.
 Ground water. That part of the subsurface water that is in the zone of
 saturation.

 Hydraulic conveyance. The water-carrying capacity of a stream channel.

 Hydraulic gradient. As applied to an aquifer, it is the rate of change of
 pressure head per unit of distance of flow at a given point and in a given
 direction.

 Hydrograph. A graph showing stage, flow, velocity, or other property of
 water with respect to time.

 Infiltration. See seepage.