April 23, 2009

Flooding of the Red River and its tributaries over the past week has forced thousands of residents to evacuate their homes for higher ground. This state of emergency in Manitoba begs the questions – why does flooding happen, and why do flooding disasters seem to be getting worse?

First off, it’s important to recognize that flooding is only considered a disaster if it impacts us humans and our belongings. When we aren’t affected by Mother Nature’s whims, we can see natural seasonal flooding as a brilliant means of channeling the Earth’s water to support great biodiversity.

Impact on Wildlife

An example of how seasonal flooding benefits wildlife is the Okavango Delta in Africa.This Delta, located in Botswana, is a nearly flat plain, snuggled between the diverging water branches at the mouth of the Okavango River. One of the world’s largest tropical floodplains is located on this Delta. (A floodplain is a lowland that becomes flooded for several months annually due to the swelling of adjacent rivers).

Did You Know? The flood-prone area on the Delta is divided into zones: permanent swamps, seasonal floodplains and areas only flooded during years of particularly large floods.

Let’s take a look at how wildlife benefits from this flood plain’s flood-cycle. When dry, this floodplain is grassland that supports grazers. As floodwater submerges this grassland, the plant nutrients from the soil and grazer’s dung are drawn out into shallow water. This stimulates plankton growth, which supports a growing population of invertebrates and fish. Later, when the floodplain starts to dry, a wide variety of vegetation flourishes and attracts diverse wildlife.

Causes of River Swelling which leads to Floods

Both the Okavango River and the Red River swell during spring. These two flood zones are each in a “low pressure area.” Wind flows down a pressure gradient from surrounding high atmospheric pressure areas,into this region of low atmospheric pressure. This low pressure is a result of decreased air mass, as the air gets warm and moist – in Africa, tropical thunderstorms are the cause, while in Manitoba, in heat insulation is the reason for spring floods. The influx of wind, coupled with local warm and moist air is a recipe for increased rain that contributes to the volume of the river water. In the case of the Red River, unmelted ice adds to the flood equation.

In spring, snow melts from south to north along the Red River, which flows northward. As temperatures rise in the north, snow rapidly melts,but cannot be absorbed into the still-frozen ground. This extra water source adds to the spring rain storms that raise the River’s water level. River water can also build up behind an unthawed blockage of ice (ice dam), and flood the areas upstream of the jam.After the ice dam breaks apart, the sudden surge of water that breaks through the dam can then flood areas downstream of the jam.

Did You Know? On average, rainfall has been increasing over the years, due to less frequent, but more intense rainstorms. The reason being that, climate change causes the slowing of the water cycle at the atmospheric level (the gaseous envelope surrounding the earth).

As we have seen above, seasonal flooding is a natural phenomenon based on flood-prone climate and geography. Mother Nature has no ‘evil’ intent when floods do affect humans. All we can do is learn from the past, and take proactive preventative measures to lessen the damage of future flooding.

Did You Know? The Red River Floodway Expansion Project is nearing completion. When it is finished, it will protect more than 450,000 Manitobans, over 140,000 homes, and over 8,000 businesses, and prevent more than $12 billion in damages to the provincial economy in the event of a 1-in-700 year flood.

Impact on Humans

Did You Know? Flooding impacts humans long after water levels have dropped.

Excess moisture in affected homes and standing water may contribute to the growth of mold micro-organisms (bacteria, viruses and parasites), which most commonly cause symptoms of gastrointestinal infections (GI) such as; stomach ache, fever,vomiting and diarrhea.

Learn More!

Devonian Times

Okavango Delta Flood

Red River Flood

References (Scientific Papers)

Lindholm, Markus et al. “Food Websand Energy Fluxes on a Seasonal Floodplain: The Influence of FloodSize.” Wetlands Dec. 2007: 775-784.

Ramberg, Lars et al. “Species diversity of the Okavango Delta, Botswana.” Aquatic Sciences 68 (2006): 310-337.

Stephens, Graeme and Ellis, Todd. “Controls of Global-MeanPrecipitation Increases in Global Warming GCM Experiments.” Journal ofClimate 21 (2008): 6141-6155.

Jina is a University of Toronto undergrad studying Life Sciences. In herspare time she strums the guitar or doodles in her sketchbook.

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