I have no solution to your 'r.line' problem, but until their is a fix, may
I suggest using the '-o' option of d.rast, or perhaps r.mapcalc, to allow
you to view both your dem and your streams. For example, in r.mapcalc;
newmap=if(streams,streams,dem)
Scott Tweddale
GIS Lab
Univ. of Illinois at Urbana-Champaign