Quoted from: https://www.cesm.ucar.edu/models/cesm1.2/rtm/doc/c11.html 

The RTM was developed to route total runoff from the land surface model to either the active ocean or marginal seas which enables the hydrologic cycle to be closed This is needed to model ocean convection and circulation, which is affected by freshwater input. It also provides another method of diagnosing the performance of the land model because the river flow can be directly compared to gauging station data. Full details of the RTM can be found in the CLM Technical Note..

The RTM is generally run at a time step greater than that of the CLM because of computational constraints. The total runoff from the land model at each time step is now accumulated by the driver until the RTM is invoked. The runoff at the land model resolution is interpolated by the coupler to the resolution of RTM and converted to units of m3 s-1 by multiplying by 1×10−3 A where A is the area (m2) of the RTM grid cell. The RTM grid cells that are at river mouths, hence providing freshwater flux to the ocean, are identified by examining each RTM ocean grid cell and determining if a RTM land grid cell flows to that ocean grid cell. River mouth grid cells are also assigned if any overlapping grid cells at the land model resolution contain land. When used as part of the Community Climate System Model, the ocean freshwater liquid and ice fluxes at the RTM resolution are passed to the flux coupler which distributes the fluxes to the appropriate ocean grid cells. When used with the Community Atmosphere Model or when run offline, RTM serves ponly as a diagnostic tool. The river-routing scheme conserves water globally.

For a complete documentation of namelist settings, see >RTM namelist variables. RTM's build-namelist utility is called by Buildconf/rtm.buildnml.csh. To modify RTM namelist settings, you should add the appropriate keyword/value pair at the end of the $CASEROOT/user_nl_rtm file (see the documentation for each file at the top of that file). To see the result of your change, call preview_namelists and verify that the changes appear correctly in CaseDocs/rof_in.