Dic: reaches (as a noun plural)

1. N-PLURAL : usu the adj N of n

The upper, middle, or lower reaches of a river are parts of a river. The upper reaches are nearer to the river's source and the lower reaches are nearer to the sea into which it flows.

• This year water levels in the middle and lower reaches of the Yangtze are unusually high.

2. N-PLURAL : usu the adj N of n

You can refer to the distant or outer parts of a place or area as the far, farthest, or outer reaches.[FORMAL]

• the outer reaches of the solar system.

3. N-PLURAL : usu the adj N of n

You can refer to the higher or lower levels of an organization as its upper or lower reaches.[FORMAL] (= echelons)

• the upper reaches of the legal profession.

.... Cobuild

reach:

an area in which something acts or operates or has power or control

• "the range of a supersonic jet"; "a piano has a greater range than the human voice";
• "the ambit of municipal legislation";
• "within the compass of this article";
• "within the scope of an investigation";
• "outside the reach of the law";
• "in the political orbit of a world power"

ambit, range, scope, compass, orbit

extent - the distance or area or volume over which something extends; "the vast extent of the desert"; "an orchard of considerable extent"

approximate range, ballpark - near to the scope or range of something; "his answer wasn't even in the right ballpark"

confines - a bounded scope; "he stayed within the confines of the city"

contrast - the range of optical density and tone on a photographic negative or print (or the extent to which adjacent areas on a television screen differ in brightness)

internationality, internationalism - quality of being international in scope; "he applauded the internationality of scientific terminology"

latitude - scope for freedom of e.g. action or thought; freedom from restriction

purview, horizon, view - the range of interest or activity that can be anticipated; "It is beyond the horizon of present knowledge"

expanse, sweep - a wide scope; "the sweep of the plains"

gamut - a complete extent or range: "a face that expressed a gamut of emotions"

spectrum - a broad range of related objects or values or qualities or ideas or activities

palette, pallet - the range of colour characteristic of a particular artist or painting or school of art

.... http://www.thefreedictionary.com/reach