Sorting in data frames

Hi, 

Is it possible to sort the rows within a data frame, with respect to
the values in one or more of the columns? For example, if I had measured
water temperatures at differenrt depths within a lake, on different
sampling dates and had a dataframe containing sampling dates (x),
sampling depths (y) and data for each day-depth combination (z), would
it be possible to rearrange the data frame so that it is sorted by day
and then by depth? 

Best wishes

Steve Thackeray