It is a typical meandering river with a single main channel embanked within a flat, shallow valley at some places, and a steep valley at other places.
It rises near the community of Preeceville in eastern Saskatchewan and then southeastward to "The Forks" in Winnipeg, Manitoba where it flows into the Red River. Some of flood flows can be diverted into Lake Manitoba at Portage la Prairie. In 1967, the Shellmouth Dam was built in Shellmouth to help reduce flood peaks and to supplement flows during dry periods. The Portage Diversion was completed in 1970. There are three hydrometric stations placed on the river that have been taking measurements since 1913. Today, Assiniboine Herald at the Canadian Heraldic Authority is named after the river.
Tributaries include the Souris River which joins it near Wawanesa, the Birdtail River which joins at the Birdtail Sioux First Nation, the Little Saskatchewan which joins west of Brandon and the Qu'Appelle River which joins near the site of historic Fort Ellice.
The following discharge rates were recorded during the 1995 flood:
|Location|| Peak flow, 1995|
| Mean flow, April '95|
| Mean flow, May '95|
| Max flow, date|
April 29, 1922
May 07, 1923
April 27, 1916