the ottawa river starts at lake capimitchigama in quebec. it flows west to lake timiskaming, where it begins to define the border between ontario and quebec. from lake timiskaming the river flows southeast to ottawa and gatineau where it tumbles over the chaudière falls. the rideau and gatineau rivers flow into the ottawa river here. the ottawa river drains into the lake of two mountains and the st. lawrence river at montreal. the total length of the river is 1,271 km.