Paleo-drainage of the Oldman River


The Oldman River we see today, has changed drastically from its paleo father that existed millions of years ago. The Oldman River today drains primarily to the north east and empties itself into the South Saskatchewan River, which eventually drains into Hudson's Bay.  The river valleys of today are steep walled valleys which can exceed 150m deep (Stalker, 1988). Its drainage pattern is very complex with with "horse shoe turns" where it will suddenly be flowing south and turn in the opposite direction to the north. The path of the Oldman River seems to chaotically change its path in all directions as it flows to the South Saskatchewan River. The Oldman River of yesteryear, the ancestral Oldman, which existed some 2 million years ago, drained almost exclusively from east to west. The river drained across the prairies, in a regular fashion, to Manitoba where it entered the Missouri watershed. What caused these drastic changes in the Oldman River?

Glaciation and the Oldman River:

The answer to the question, what caused the drastic changes in the drainage of the Oldman River, is glaciation- ice margins of the glaciation that occurred in the Pleistocene era. 15,000 years ago the Laurentide ice sheet covered most of North America (see map) (visit link) This was the end of the last glaciation period called the "Pleistocene" which started about 2 million years ago and ended some 12,000 years ago. During this period, the Laurentide ice sheets had many periods of advancements and many periods of recession. The area that the Oldman River basin lies in was never static and was often at the edge or "margin" of  ice sheets. According to Stalker and Barendegt, in their paper, "The origins of valleys of Lethbridge" there are four major events that shaped the current drainage of the Oldman River (near Lethbridge).

    1. Formation of Glacial lake Lethbridge...
Stalker and Barendregt, suggested that during one of the last advances of the Laurentide ice sheet, it happened to halt its advance near the region of Lethbridge. (see diagram). As the Oldman River, Belly River and the St. Mary's River ran into this glacial "dam" a large lake was formed called Glacial Lake Lethbridge. As the three rivers flowed into this glacial lake, their velocities were significantly decreased and as a result dropped their sediment load into the lake. More recent research by Barendregt and others, suggest that the formation of Glacial Lake Lethbridge occurred as the ice retreated from its maximum and stood still at the Lethbridge end moraine.

    2. Glacier Advancement....
Soon after the Glacier Lake Lethbridge was formed, the Laurentide ice sheet in this region began a second of its final advances in this region. The advance of the glacier continued until the areas of just north of Lethbridge and approximately 50 km south of Lethbridge were engulfed glacial lake Lethbridge. (See diagram) As a result of this advancement, The Oldman River and its tributaries were forced to unload their suspended sediments further to the south, leaving sediment beds as far west as as Kipp in this region. For the ancestral Oldman River, this meant a filling in of its ancient river valley to current level, forever changing this paleo-river.  As the lake drained, the modern Oldman River began to forge its paths throughout southern Alberta.

    3. Draining of Glacial Lake Lethbridge
About 12,000-8,000 years before present, the glacier that covered much of this area began its  final recession. As the Glacier retreated, so did the glacial damming of glacial Lake Lethbridge. Eventually and moderately, the lake drained itself into outlets such as Chin Coulee. The Oldman River began to recarve out some of its lost territory in this region. Though the valleys of its predecessor father were forever lost, as its valleys were filled in with the stagnating sediments of Glacial Lake Lethbridge, the new Oldman River would have hundreds of meters of sediment to forge a new valley through.

    4.  Full Glacial retreat...
 With the glacial lake Lethbridge drained, and the glacier beginning to retreat to the north and east, the newly reborn Oldman River in this region,  was free to undercut new valleys. This occurred with a few changes. As the glacier retreated to the north and east, it significantly lowered the surrounding landscape by gouging it out with its rapidly retreating margin. Couple this with deposition of sediment, and you can account for the seemingly chaotic current path of the Oldman River. However the path of new Oldman River was forever changed. Instead of flowing in easterly fashion across the prairies to the Missouri river basin, the lowering of the land towards the southeast by the glacier, forced the Oldman river to flow in a northeastern fashion into the South Saskatchewan River Basin.

Conclusions :
What should you take away from this synopsis of Barendregt's and Stalker's article, if you want to understand how the current course of the Oldman river? First note that Barendregt's and Stalker's article account for the current flow around Lethbridge, by using the advancing and retreating of the Laurentide ice sheet in this region. It should be noted that events such as the one described in the article were happening all over this region of Southern Alberta. That the formation of glacier margins and the south easterly glacier retreat accounts somewhat for the seemingly unusual direction of the Oldman River.  Have a look at the maps page, can you understand how the current drainage patterns of other river might be accounted for?

Special thanks to R. Barendregt, and M. Stalker for giving such an
amazing detailed account of the history of the Oldman river.
1.    "The Palliser Triangle" Barendegt et al.
        1988, University of Lethbridge.
2.    Beatty, C.B. (1975).  The Landscapes of Southern Alberta.
       The University of Lethbridge  Printing Services.