There is a number of modules suitable for different applications:

• To compute the flow accumulation, stream network and (sub)-watershed boundaries use (note we set the resolution to 50ft by g.region res=50 to get the results faster)

  r.watershed ch20ftdem accum=ch50f.accum basin=ch50f.basin threshold=50000 stream=ch50f.stream
  

  The module uses SFD-D8 algorithm and finds the route through the pits using a shortest path algorithm. To really see the flowaccumulation map you need to change the color table, for example like this: (note that the negative values are streams that may originate outside the given area)

  r.colors ch50f.accum co=rules
  >-500000 black
  >-100 black
  >-10 grey
  > 0 white
  > 10 yellow
  > 100 cyan
  > 1000 blue
  >300000 0 0 30 
  

  To get a nice stream network for a selected flow accumulation threshold use map algebra

  r.mapcalc acc100="if(abs(ch50f.accum)>100,1,null())"
  

  If you are getting lots of straight lines the resolution of your DEM is too high for D8 compared to the surface complexity - lower the resolution to get more realistic stream network pattern, although some detail will be lost. or you can use r.flow if there are no depressions in the DEM.

  r.watershed ch20ftdem accum=ch20f.accum basin=ch20f.basin threshold=5000 stream=ch20f.stream
  

• To compute flow accumulation on hillslopes, hillslope length and/or flowlines in vector format, especially for smoother, high resolution surfaces use (note that the routing does not flow through depressions, it stops there creating an effective sink):

  r.flow elrst20ft.t50sm skip=4 flout=flowrst lgout=lgdrst20ft dsout=dsdrst20ft
  
  r.flow -u elrst20ft.t50sm skip=4 flout=flowrst.u lgout=lgdrst.u dsout=usdrst
  
  

  The version r.flowmd distributes water to 2 cells rather than a single one

• Compute flow accumulation for massive DEMs (thousands of rows and columns) using SFD and MFD algorithms. The program is experimental and has to be compiled individually - it is under

  /usr4/grass5cvs/grass5sourcecvs/grass50_exp_2003_02_22/src.contrib/DUKE/r.terraflow
  

  r.terraflow -s elevin=elrst20ft.t50 filled=elrst20ft.t50.fill direction=elrst20ft.t50dir swatershed=terrawshd accumulation=terra.accum tci=terra.tci 
  

  MFD flow accumulation for Chagres, Panama
  MFD floods the depressions
  MFD floods the depressions - closer look, note the noise on IFSARE DEMj
  Close look at the comparison between SFD terraflow result and r.watershed

  Compare different algorithms for stream extraction from IFSARE

• Simulation of overland water and sediment flow

  /usr4/grass5cvs/grass5sourcecvs/grass50_exp_2003_02_22/src.contrib/GMSL/r.simwe
  

  r.sim.water
  r.sim.sediment
  

  see GRASS manual for detailed description of commands