Spring floods and freshets

Since the dawn of civilization, people have been settling on the banks of rivers. When a river overflows, the fertile sludge in the water comes onto the land. It fertilizes the soil and helps people grow bigger crops. But rivers overflowing don't always bring good news, as they often cause floods. In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter on better weather forecasting you will learn what are spring floods and freshets and how they work.

The life of rivers

A river is a large stream of water. Water enters rivers in several ways: groundwater, rain, melting snow and glaciers. But if you look closely, the river's life is almost entirely tied to the weather. Rain is weather, and snowfall and snowmelt are, too. Even groundwater is made up of rainwater seeping deep into the soil.

The weather, in turn, is very changeable. First, there are seasons on Earth. Second, individual days can be very different from the norm. For example, July is the hottest month in the northern hemisphere, but a single day in July can be very cold, more like September.

It is the same with rivers. Every river has its own regime, primarily dependent on the time of year. For example, in temperate latitudes, rivers are covered with ice in the winter, which melts in the spring. And there are rivers in Central Asia, Australia, and the African savannahs that dry out. During the dry season, they are only a bed of muddy puddles; but during the rainy season, they become babbling brooks.

But it's important to remember that this is just an average multiyear pattern. If abnormal weather sets in, the river will react unusually, too.

A river in Gambia. Dan Roizer / Unsplash

How to measure a river

We know very well that weather is measured using many parameters, including temperature, pressure, and precipitation. Similarly, rivers have several characteristics that can be measured, like flow speed and chemical composition. The most important thing for us now is discharge. If we imagine a river flowing into a giant bucket, discharge is the rate at which the bucket will be filled, measured in cubic meters per second.

You can calculate the average discharge over a year and then compare it to the flow we observe on a particular day. This characteristic is called the stream flow. Stream flow can be:

Low, i.e., there is less water in the river now than the annual average
Medium
High, i.e., there is more water in the river now than the annual average

Water availability is closely related to the height of water in a river. If the water availability is too low, the river can dry up; if the water content is too high, it overflows, and there is a threat of flooding. Both situations are dangerous for people who live near rivers.

High stream flow

Every river in the world has a season when the amount of water in it dramatically increases. This is called spring flood. All rivers can be divided into two main groups:

Rivers that overflow due to melting snow in the spring or summer (they are located in temperate zones or in the mountains);
Rivers that overflow during the rainy season (these are in monsoon areas).

Overflows are part of a river's annual regime, like the change between summer and winter. It can be very strong or almost imperceptible, but it is always there.

Therefore, floods can be expected in advance. In temperate latitudes, floods can be predicted by the amount of snowfall, even from the middle of winter. Forecasting floods in monsoon areas is more tricky, but some advancements were made here, too.

But the water level in the river doesn't only rise in the "right" season. For example, there may be heavy rains in temperate latitudes in summer and fall. Or, in winter, there may be a thaw, and some snow will melt early. The water level of rivers will rise, but this cannot be called a flood. These cases are called freshets. Freshets can happen at any time of the year, and in a particular year, they may not happen at all. Unlike floods, freshets are much harder to predict in advance. You have to deal with precipitation forecasts (one of the most inaccurate forecasts) and consider the complex features of rainwater entering the river through the soil.

Freshets can also be very powerful, and are often more dangerous and destructive than floods.

A road blocked by a flood. Phillip Flores / Unsplash

Floods

Seasonal water level rises and overflows can lead to floods, which is why the World Meteorological Organization considers them among the most dangerous natural phenomena.

Floods caused by water level rises and overflows affect almost every country in Europe, along with Southeast Asia, China, and the United States. Flood records are part of the history of many old cities, and you can often find plaques marking record water levels during floods.

During spring floods, ice jams can lead to severe flooding. These are clusters of ice that block the river bed, creating a dam that causes the water level upstream to rise dramatically. To control floods, it is essential to detect and remove these blockages in time (e.g., by blowing up ice with dynamite).

Act during floods and high water events in much the same way as you would during a tsunami. If you live near a major river during heavy rains or melting snow, watch for reports from authorities and follow instructions. If the water is rising higher, and there are no official instructions, move away from the river to an elevated location as soon as possible. Most importantly, do not try to cross the floodwaters on foot or by car. Even a seemingly small current can knock you off your feet.

By following these simple rules, you are sure to keep yourself safe from the elements.

Text: Eugenio Monti, a meteorologist and a climatologist

Cover photo: Josh Wedgwood / Unsplash