Painting river widths according to their contents of water

[Jochen Albrecht]

Dear Netters:
The background of this question isn't really scientific, however, an answer
might teach me to how to use the more complicated raster analysis functions
more efficiently.
With r.watershed I got a map of possible stream flows, with the values of the
streams representing the accumulated sum of water stream upwards. The values
range from somewhat like -40,000 (how do I get negative values?) to +26,000.
If these values are assigned to a 255 colors table, (preferably 255 shades of
blue) I can roughly guess, how the streams compare. What I would really like,
is to broaden the streams according the the amount of water flowing in them,
so that the large streams are 5 or 10 pixels wide. Since the raster map is a
more a less continous surface function of equally distributed rainfall, it is
difficult to delineate the rivers without loosing the actual values. These how-
ever, I need later for the translation from cell/color values into something to
use r.grow with.
Has anybody ever tried something like this before.

           Jochen