Looking for a reference for identifing stream regimes(Intermittent, Perennial)
I'm not sure if this is a standard per se, but I found it useful.  These definitions are from a 1923 publication of the United States Department of the Interior entitled Outline of Ground-Water Hydrology.  Here is the link to the PDF and the specific definitions below can be found on pages 57-58.  Hope this helps.

Perennial:



  A perennial stream, or stretch of a stream, is one which flows
  continuously. Perennial streams are generally fed in part by springs,
  and their upper surfaces generally stand lower than the water table in
  the localities through which they flow.



Intermittent streams are divided into spring-fed intermittent streams and surface-fed intermittent streams.



  A spring-fed intermittent stream, or stretch of a stream, is one that
  flows only at certain times when it receives water from springs. The
  intermittent character of streams of this type is generally due to
  fluctuations of the water table whereby the stream channels stand a
  part of the time below and a part of the time above the water table.
  This is the ordinary type of intermittent stream.


  A surface-fed intermittent stream, or stretch of a stream, is one that
  flows during protracted periods when it receives water from some
  surface source, generally the gradual and long-continued melting of
  snow in a mountainous or other cold tributary area. The term may be
  arbitrarily restricted to streams or stretches of streams that flow
  continuously during periods of at least one month.



Ephemeral:



  An ephemeral stream, or stretch of a stream, is one that flows only in
  direct response to precipitation. It receives no water from springs
  and no long-continued supply from melting snow or other surface
  source. Its stream channel is at all times above the water table. The
  term may be arbitrarily restricted to streams or stretches of streams
  that do not flow continuously during periods of as much as one month.