Assiniboine River

Image:Winnipeg Forks - Red & Assiniboine rivers.jpg and Assiniboine rivers in Downtown Winnipeg ]]

The Assiniboine River rises in eastern Saskatchewan east of the community of Kelvington on the upper prairie level above the Manitoba Escarpment. The Assiniboine River flows through three basic zones with different channel characteristics. Upstream of Brandon, the main stem of the river and its most important tributaries flow within a very large valley. The valley was cut by huge glacial melt water flows at the end of the last glaciation. The floor of this spillway valley provides a natural floodplain for the river and the valley provides a significant storage volume making the construction of the Shellmouth Dam north of Russell both technically and economically viable. The major tributaries in this reach are the Qu'Appelle, Shell and Little Saskatchewan Rivers.

The glacial flows created a large delta east of Brandon extending almost to Portage la Prairie. The river has eroded down through sediments of the delta cutting a narrow valley through these sediments as it drops through a vertical distance of about {{convert|150|m}} to the Lake Agassiz – Red River Plain. In this valley, the river is confined with a narrow valley floor. The Souris River is the primary tributary contributing flow to the Assiniboine in this reach.

Near Portage la Prairie the river emerges from the delta reach onto the relatively flat Red River plain (the floor of former Glacial Lake Agassiz) and at this point it can flow in any direction from roughly northwest to roughly southeast. The gradient of the river channel within the delta reach to the west is relatively high, so the river water velocities are fairly high and the waters of the river carry significant amounts of sediment. The gradient in the flat Red River plain is much less and the velocity of the river water flowing over this plain is much lower. Therefore, the sediments carried by the river waters as they flow through the delta reach are deposited onto the plain. The Assiniboine winds its way east eventually joining the Red River at "The Forks" in Winnipeg, Manitoba.

Today, Assiniboine Herald at the Canadian Heraldic Authority is named after the river. Asessippi Provincial Park, an RV park was built on the east shore of the Shellmouth Reservoir.{{relevance inline|date=July 2023}}

The Assiniboine River has changed course numerous times over past centuries. It has followed its modern course for approximately 700 years. The Assiniboine River formerly met the Red River near the present-day mouth of the La Salle River. A Government of Manitoba report following the 2011 Assiniboine River Flood found that the flood "could have resulted in the river flowing east by a different route, possibly joining the Red River south of Winnipeg, or potentially even flowing north to Lake Manitoba as it did thousands of years ago," without the flood control infrastructure currently in place.{{cite report |url=https://content.gov.mb.ca/mit/wm/assiniboine_lakemb_lsm_report_nov2013.pdf |title=2011 Flood: Technical Review of Lake Manitoba, Lake St. Martin and Assiniboine River Water Levels |date=October 2013 |publisher=Government of Manitoba |page=113 }}

Notable tributaries of the Assiniboine River include:

• Whitesand River, which joins it near Kamsack, Saskatchewan
• Lilian River, which joins it at Sturgis, Saskatchewan
• Souris River, which joins it near Wawanesa, Manitoba
• Birdtail Creek which joins at the Birdtail Sioux First Nation
• Little Saskatchewan River, which joins west of Brandon
• Qu'Appelle River, which joins near the site of the Hudson's Bay Company historic Fort Ellice site

{{see also|2011 Assiniboine River Flood|2014 Assiniboine River Flood}}

Image:2005 Winnipeg Flood.JPG Marina in Winnipeg]]There are three hydrometric stations on the river that have been taking measurements since 1913. The Assiniboine River near Headingley has an average discharge of {{convert|45|m3/s|cuft/s}}.{{Cite web|url=http://atlas.nrcan.gc.ca/site/english/learningresources/facts/rivers.html |title=Rivers of Canada |author=Atlas of Canada |author-link=Atlas of Canada |access-date=2008-02-15 |archive-url=https://web.archive.org/web/20080125080501/http://atlas.nrcan.gc.ca/site/english/learningresources/facts/rivers.html |archive-date=January 25, 2008 |url-status=dead }} One millimeter of runoff from half the watershed would take 70 hours to drain at flow rates of {{convert|360|m3/s|cuft/s}}. The following discharge rates were recorded during the 1995 flood:{{Cite web|url=http://ccrs.nrcan.gc.ca/radar/spaceborne/radarsat1/action/canada/assin/index_e.php|title=The Assiniboine River bursts its banks|author=Natural Resources Canada|author-link=Natural Resources Canada|access-date=2008-02-15|archive-url=https://web.archive.org/web/20080203095148/http://ccrs.nrcan.gc.ca/radar/spaceborne/radarsat1/action/canada/assin/index_e.php|archive-date=3 February 2008|url-status=dead}}

It is prone to spring flooding. Some 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. Despite these efforts, in May 2011 it was necessary to breach one of the dikes beside the river to relieve flood stresses east of Portage la Prairie. A Manitoba-wide state of emergency was declared in the wake of one in three hundred-year floods on the Assiniboine River at Brandon.{{cite web|url=https://winnipeg.ctvnews.ca/evacuees-wait-to-return-home-as-brandon-faces-one-in-300-year-flood-1.641933|title=Evacuees wait to return home as Brandon faces one-in-300-year flood|website=CTV News|access-date=27 November 2020|date=10 May 2011}}{{Cite web|url=https://www.google.com/hostednews/canadianpress/article/ALeqM5iwiWqdGZXp9QEP_dojKRmMHf3NmA?docId=6799222|title=(The Canadian Press)}}{{dead link|date=June 2024|bot=medic}}{{cbignore|bot=medic}}{{cite web|url=http://www.cjob.com/News/Local/Story.aspx?ID=1414873 |title=Provincial State of Emergency Declared|website=CJOB 68 |access-date=27 November 2020 |url-status=dead |archive-url=https://web.archive.org/web/20120331014105/http://www.cjob.com/News/Local/Story.aspx?ID=1414873 |archive-date=March 31, 2012|date=May 9, 2011}} Below are the actual observed flow rates for major floods at different locations along the river:

• Note: Flows in 1882 occurred before any flood protection measures such as the Shellmouth Reservoir and Portage Diversion were built.

Looking specifically at the Assiniboine River at Portage La Prairie, where maximum river flows occur prior to historical spillovers (prior to construction of the Portage Diversion and the Lower Assiniboine River Dikes) into the watersheds of Lake Manitoba and the La Salle River, the top 10 calculated natural peak flow rates before construction of the current flood infrastructure are:

Fish species commonly found in the river include walleye, yellow perch, northern pike, mooneye, burbot, channel catfish, brown bullhead, rock bass, white sucker, shorthead redhorse, and common carp.

• List of longest rivers of Canada
• List of rivers of Manitoba
• List of rivers of Saskatchewan
• Assiniboine River fur trade

{{Reflist}}

• [https://web.archive.org/web/20151103002043/http://www.publications.gov.sk.ca/details.cfm?p=11115 Fish Species of Saskatchewan]
• [https://web.archive.org/web/20111113181911/http://esask.uregina.ca/entry/assiniboine_river.html Encyclopedia of Saskatchewan]
• {{Commons category-inline}}

{{Authority control}}

Category:Rivers of Manitoba

Category:Rivers of Saskatchewan

Category:Tributaries of Hudson Bay

Category:Tributaries of the Red River of the North

Category:Assiniboia, Winnipeg