Final answer:
The Assiniboine River flows eastward from Saskatchewan to Manitoba, where it joins the Red River of the North, playing a vital role in irrigation and agriculture in Canada's southern Prairie Provinces.
Step-by-step explanation:
The river that flows eastward from Saskatchewan to Manitoba and then joins the Red River of the North is the Assiniboine River. The Assiniboine River is a significant part of the Canadian river system and provides a natural border between the provinces as it winds its way across the prairies.
This river is also an essential resource for agriculture, as some of Canada's best farmland can be found along its banks, especially in the southern Prairie Provinces.