SELECT statement not working as intended

   I'm trying to query the table to extract the single highest value of a
chemical by location and date. This statement gives me all the values per
stream, site, and date:

SELECT str_name, site_id, sample_date, max(quant) FROM chemistry WHERE hydro
= 'Humboldt' group by str_name, sample_date, site_id order by str_name,
site_id;

   I'm not seeing why this doesn't work; the top few lines of output are:

  CalifCrk    | CalCrk         | 1996-10-18  |    188
  CalifCrk    | CalCrk         | 1996-08-23  |    183
  CalifCrk    | CalCrk         | 1996-07-29  |    201
  CalifCrk    | CalCrk         | 1996-09-27  |    185

when what I want is only the third line.

   A clue to the correct syntax is solicited. I'm sure it's something simple
that I'm just not seeing.

Rich