by Watershed Stewardship Assistant, Conlan Donahue
On June 13th, high streamflow advisories were issued for the Oldman River Basin from Waterton to Lethbridge. Many other parts of Alberta received flood advisories on the same day, such as the Bow River and the Battle River watersheds. Does this mean the rivers and floodplains in the Oldman watershed will receive a flash flood? As with everything else in life, it’s complicated.
Q: Will there be flash floods in the Oldman watershed this year?
A: It’s complicated….
When it comes to flood risk, there are weather-related elements (heavy or prolonged rains, storm surges, sudden snowmelt) and then there are human-driven elements, including how we manage our waterways (dams, levees, reservoirs) and land development.
‘Weather’ or not it will flood
A flash flood is when water spills over the banks and flows over land very suddenly. It is characterized by high water levels and fast flow rates covering river valleys, urban streets, and floodplains. A flash flood is typically created by excessive rainfall in a short period of time; however, it can also be caused by large-scale snow melt or a failed dam/levee. The risk is usually highest in the late winter and early spring. In the Oldman watershed, snowmelt plays a significant role in our varying annual flow rates.
Gladstone Creek, 2013. Photo by Connie Simmons. The 2013 floods in southern Alberta were due to sudden, heavy rainfall at a time when flow was naturally high due to seasonal snowmelt.
The likelihood of a flash flood increases with the presence of heavy, warm rain by melting the snowpack faster. NASA’s current climate models suggest that rising global temperatures will escalate the Earth’s water cycle, increasing evaporation. Increased evaporation can result in larger, more intense storms occurring more often. Storm-affected areas are more likely to see higher precipitation and an increased risk of flooding. Conversely, areas located far away from storm tracks are likely to see less rainfall and are at increased risk of drought.
The human factor
The Oldman watershed is a busy landscape with a lot of human activity—including roads, towns, agricultural lands, dams, irrigation canals, and resource extraction, to name a few. All of these influence the way water moves over the land.
While forested, wetland, and grassy landscapes are like sponges that can absorb water, urban areas with lots of concrete, pavement, and roofs do not hold the water and shed it over land quickly. Human development adds pavement and other hard surfaces, alters natural drainage systems, and sometimes includes building homes on floodplains (low-lying areas that are prone to frequent flooding). As a result, these human-built areas are susceptible to higher flood risk in terms of frequency, severity, and damage.
By building dams, reservoirs, irrigation canals, any other water-related structures, humans have also altered the natural flow of water across the landscape. A 2022 study of seasonal flow patterns of over 2,200 Canadian and American streams found that “human-managed streams—those impacted by developments like dams, canals, or heavy urbanization—had significantly different flow patterns compared to streams in natural watersheds.” On one hand, careful management of these waterways can help reduce flooding downstream by storing and releasing water as needed. On the other hand, these systems can also be overwhelmed by storms and rainfall events, resulting in higher flood impacts.
Oldman Dam release during high water in 2013 (Photo by Connie Simmons). Human-managed streams have altered flow patterns compared to natural streams. Infrastructure like reservoirs and dams can both prevent and exacerbate flood impacts.
Natural processes
Seasonal variation in river flow is a primary force for shaping the environments of our rivers. Flooding is a natural process, and, while it can be exacerbated by, and cause damage to, human infrastructure, there are also important benefits that improve the health of a watershed and its species. As flood water escapes the confines of streambanks and flows over land, it redistributes organic material and living organisms, creating an exchange of nutrient-rich sediment with the floodplains.
A healthy watershed and functional, well-vegetated riparian zones will naturally reduce the risk and severity of floods. The riparian zone is the transition area between the upland and the shoreline of a river, stream or lake. Riparian zones specialize in maintaining natural water levels, regulating temperature, and preventing erosion. OWC plants willows in damaged riparian areas to restore these functions by stabilizing streambanks and jumpstarting the regrowth of riparian vegetation. Healthy riparian zones are especially important during a flood event as the vegetation reduces the force, height, speed, and volume of water rushing downstream. These natural benefits help to mitigate and prevent further damage to developed areas downstream of the water body.
Flooded area in 2013. Photo by Debbie Jo Webster. Healthy wetlands and riparian areas act like sponges and absorb water, releasing it slowly. In this way they help reduce the frequency and severity of floods.
Reducing risk, mitigating impact
So, what can we do to help reduce the impacts of floods? Signing up for an OWC membership and attending a willow-staking event to physically help rebuild our riparian zones is the perfect start!
Flood prevention/mitigation tips for around your house & yard:
Ensure downspouts for eavestroughs divert water away from your house
Install rain barrels
Landscaping to include permeable surfaces like a prairie urban garden, rain garden, dry creekbed, etc.
Prepare a 72-hour emergency kit for your family
More tips are offered here by Climate Check.
Flash flooding can also have serious impacts on rural landowners and their farms by delaying and reducing harvests, interrupting supplies, and causing stress to livestock.
Flood prevention/mitigation tips for rural landowners:
Plant trees / restore riparian areas
Maintain wetlands
Ensure livestock have access to high ground
Create an emergency response plan
Flooding can damage rural infrastructure as well, including fences, roads, bridges, etc. Landowners can mitigate impacts by planting trees, maintaining wetlands, and creating an emergency response plan. Photo by Debbie Jo Webster, 2014.
Cover photo: Flooded Oldman River in 1995.
