Floods in the Oldman River Valley at Lethbridge
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O ^ 0 .,_ ( FLOODS IN THE OLDMAN RIVER VALLEY AT LETHBRIDGE Alex Johnston Sir Alexander Gait Museum Lethbridge, Alberta MA INC GB 1989 1399.5 C3 '-r- 'H J64 1989 FLOODS IN THE OLDMAN RIVER VALLEY The Oldman River rises from an alpine lake on Mt. Lyall and flows through the foothilis to the plains of eastern Alberta, draining 6,238 sq. mi. and serving 125,000 people. Along with its principal tributaries, the Oldman River is dependant upon meltwater originating In the Rocky Mountains. Approximately 60 per cent of the Oldman's flow passes through the system from mid-May to Mid-July. The basin supplies water to nine of the province's 13 irrigation districts; two of those nine districts (the Lethbridge Northern and St. Mary's) account for nearly half of the 1.2 million acres of irrigated land in Alberta. The time of deposition of the higher terraces in the Oldman River valley at Lethbridge Is not known with certainty. But it is likely that basal sediments, such as those on which the original Fort Whoop-Up stood, began to be deposited in the late 1500s at the end of the Little Ice Age Climatic Episode [a post-Wisconsin period of mountain glacier-building called the Neoglaciation]. A warming trend at that time resulted in the release of snow and glacial meltwaters. The meltwaters, combined with increased spring rainfall, resultedZm higher flood stages, higher sediment loads, and increased deposition. An archaeological site, this one downriver from Lethbridge northeast of Coaldale, has yielded information on flooding of the Oldman River. The site, known as the Ross Site [after Cleve Ross, the landowner], was excavated by Dr. Richard G. Forbis of the Glenbow Foundation, Calgary, in 1957. Several occupation layers were encountered, two of whio, »vere particularly rich in artifacts. Occupation layers were separated by several inches of sterile sand, which had been deposited by major floods. Forbis counted the sand layers, determined the approximate age of the occupation layers, and concluded that the Oldman River had experienced a major flood about oncfe every 50 years for the previous 400 years. (Nowadays, and for the last 100 years, major floods have occurred about once every 10 to 12 years.) Around the turn of this century, land use patterns in the Oldman River Basin began to change. These changes included grazing by cattle and some sheep and the plowing of prairie grasslands for dryland and irrigated agriculture. The changed land use patterns, combined with apparent climatic changes, altered the hydrological regimes in the Oldman River valley. The net result was higher and more destructive flood stages and a major increase in lateral erosion of the valley floodplain and its terrace deposits. These developments have been studied at the Fort Whoop-Up archaeological site, about five miles upstream from the Lethbridge Nature Reserve. Analysis of Dominion Land Surveyor's notebooks and township plans indicate a major shift occurred in the Oldman River channel at Fort Whoop-Up [and at Lethbridge] in the early 1900s. The earlier active channel lay to the northwest of the fort site and was relatively straight A narrow floodplain terrace had carried the Slideout-Fort Macleod Trail along the south bank of the river until it ascended to the prairie level via the next coulee west of Fort Whoop-Up. By 1926, the active channel had shifted south. It removed the lower floodplain and began to cut at the front of the Whoop-Up terrace. From 1926 to 1981, the edge of the terrace retreated some 400 meters-an average rate of about eight meters per year. Erosion of the riverbank occurred not only by massive undercutting and collap'se during floods, when high velocity flows were involved, but also by peeling back of the riverbank through gravity collapse as a result of soil dessication, freezing/thawing, -cattle trampling and gradual undercutting during high water. Infra-red aerial photography flown during the flood of 1975 indicated very active erosion and collapse of both north and south banks at that time. In 1984, the stockade and other remains of the original Fort Whoop-Up were about 180 metres east of the riverbank edge. Given the average rate of retreat, and the reasonable expectation of a flood of equivalent size to those of 1964 and 1975 occurring every 10 to 12 years, one can predict that the Fort Whoop-Up site will be lost by the turn of the century unless major efforts are made to stabilize the bank and somehow deflect the Oldman River elsewhere. Changes in the river channel occurred at Lethbridge as well, best illustrated by a map of Indian Battle Park in a brochure entitled "The Place Of Our Beginnings." The most important change was a shift of that portion of the river channel downstream from the CP Rail High Level Bridge from the east c bank of the valley to the westward. Slaughterhouse Bottom, the location of rancher O. S. (Hod) Main's 1883 butchering shed, from which he supplied fresh meat to the hamlet of Coalbanks, was destroyed in the process. '\. ^ • A floodplain study of the Oldman River through Lethbridge was undertaken by Alberta Environment because of the 1975 flood and a report was published in 1978. The study concentrated on a reach of 9.4 miles of river valley through the city. The study indicated that a peak flow of 156,000 cubic feet per second (cfs), which would represent a height above normal river levels of 30 feet, could be expected to occur once in 1,000 years-or a 1:1000 year flood level. Similarly, a 1:100 year flood level would be a peak flow of 106,000 cfs and a height above normal of 24 feet, a 1:50 year flood level would involve a peak flow of 91,000 cfs and a height above normal of 22 feet, a 1:20 year flood level would involve a peak flow of 71,000 cfs and a height above normal of about 20 feet, and a 1:10 year flood level would involve a peak flow of 56,000 cfs and a height above normal of about IB feeL [Heights above normal are my estimates, based on historical news reports and technical papers. It should be noted that modern bridge engineers look upon many of these figures as conservative.] In the recent historical pasL floods have occurred at Lethbridge in "the early 18705" (hereafter called 1870), 1887, 1897, 1899, 1902 (three major floods), and 1908 plus about nine years (see table, "Oldman River Floods") since installation of stream gauges around 1909. There was an 1851 winter flood, which the Blackfoot thought unusual enough to record in their winter count for that year. According to contemporary news reports, the floods of 1870, 1887, and 1897 were serious but not spectacular events while the 1899 flood was smaller. The Macleod Gazette of 17 June 1897 reported that the Oldman River was running high enough to cut new channels as it had done In 1887 and in the early 1870s. Also, heights above normal river levels were comparable to heights reached in ^8^'^ and the early 1870s. Estimated peak flows and heights above normal were: 1870, 45,000 cfs and 12.5 feet; 1887, 45,000 cfs and 12.5 feet; 1897, 45,000 cfs and 12.5 feet; and 1899, 40,000 cfs and 11.0 feet. It is unfortunate that hydrological records were not kept in Lethbrjdge until about 1909 and, hence, the figures given up to that time are estimates. However, the estimators In many cases were experienced irrigators who were able to determine the flow of water with a fair degree of accuracy. It is not widely known that there were three serious floods in the Belly (now Oldman) River valley at Lethbridge in 1902. These floods crested on 22 May, 2 June, and 9 July. The year of 1902 was the wettest in our recorded history (27.91 inches of total precipitation or 709 mm), experienced the wettest month (May, 11.27 inches or 286 mm), and the wettest summer (24.45 inches or 621 mm). The only one of these floods listed In the 1978 Lethbridge floodplain study was the 22 May flood. The estimated peak flow on that date was 125,000 cfs. There wasn't much Information in news reports on the 2 June 1902 flood. It was not as bad as the 22 May flood but it likely crested at around 60,000 cfs with a height above normal of around 18 feet. The news reports of the 9 July 1902 flood indicated that It was more serious than the 22 May flood. Reports from the Gait company waterworks pumphouse, then located a few hundred feet north of the Helen Schuler Nature Centre, said that the July flood waters reached a level on the pumphouse motors at least two feel higher than they had during the 22 May flood. -This meant that the July flood probably attained a peak flow of around 140,000 cfs and crested at about 27 feet. The July flood took out another section of the Belly River bridge, destroyed the piling put in to repair the earlier flood damage, while the pile driver was lost in the river... -1 The 1902 floods significantly increased the overall width of the river valley at Lethbridge by lateral erosion of the coulee banks^ Thus, when the 1908 flood occurred the valley carried a much greater volume of water than in 1902^even though the height of the' flood waters above normal was about the same as in July 1902.