JET — A forceful stream of fluid (as water or gas) discharged from a narrow opening or a nozzle.

JET STREAM — A high-speed, meandering wind current, generally moving from a westerly direction at speeds often exceeding 400 kilometers (250 miles) per hour at altitudes of 15 to 25 kilometers (10 to 15 miles). In the Western United States, the jet stream's north-south latitudinal position largely determines the application and intensity of precipitation during the winter months when most rain and snowfall occur.

JETTY — A structure extending into a sea, lake, or river to influence the current or tide, in order to protect harbors, shores, and banks.

JIG — An apparatus for cleaning or separating crushed ore by agitation in water.

JOINT-USE CAPACITY — That reservoir capacity which has been assigned to flood control purposes during certain periods of the year and to other purposes during other periods of the year.

JOINT-USE STORAGE — Reservoir storage space which is used for more than one purpose. The operation may follow a fixed predetermined schedule or may be flexible and subject to adjustment, depending upon particular hydrologic conditions.

JUVENILE WATER — Water brought to the surface or added to underground supplies from magma.