1. [n]  a final election to resolve an earlier election that did not produce a winner
2. [n]  the occurrence of surplus liquid (as water) exceeding the limit or capacity

RUNOFF is a 6 letter word that starts with R.

1. Water flowing in a river as a result of snow melt or heavy rains.
2. The part of precipitation and snowmelt that reaches streams (and thence the sea) by flowing over the ground.
3. The portion of rain or snow that does not percolate into the ground and is discharged into streams instead.
4. That part of water yield that appears in streams. Expressed in acre feet per year.