Floodplains: All you need to know

Floodplains are a fascinating piece of landscape. Problem is, we never talk about it. Rivers tend to hog all the limelight. When it comes to economic needs, the river is studied more closely; what fishes can we derive, what is the discharge of the river, whether a dam can be built across it, what is the hydroelecrtic potential of a river. Rivers provide water to directly to billions of people all over the world and they are important. But floodplains, often overlooked, are no less…

Floodplains are extremely important to humans; ecologically and economically. Floodplains provide the world with $3920 billion of services!

What is a floodplain?

A floodplain has many definitions, depending on the context and the discipline it is used it. The most common definition in literature says that “a floodplain of a river is an area flooded by a 100-year flood” (Bhowmik and Stall 1979). Let me explain that.

Floods are usually characterized by their recurrence period: how long before another flood of the same intensity will occur again. Greater the intensity, longer is the recurrence period. The “100 year flood” in this definition indicates that we consider a flood of such a high intensity that over history, it is statistically expected to occur once every 100 years. When such a flood occurs, if can inundate the entire floodplain. (That does not mean that such floods cannot occur within 100 years; this number is the average over the entire history of the river, which last over thousands of years).

Features of a floodplain

A floodplain has many specific features; features that you do not find associated with any other landscape in the world.

1. It has hill slopes along it’s sides, and the river generally flows in the centre of the floodplain.
2. Along the hill slopes, you have terraces; abandoned floodplains of the past.
3. The active floodplain is inundated every year at the time when discharge of the river is highest. In India, this is usually the summers and the monsoon season.
4. Plenty of water bodies of different types, hosting a variety of life at different stages of succession.
5. The floodplain also has structures called “levees” that line the river channel. These levees are natural depositions of sediments along the river bank by the river. Often, it is an effective flood control measure; it prevents water from the channel to overflow.
6. Floodplains also have fine alluvial soils.

Floodplains are one of the most important landscapes in the world!

Floodplains play a very, very important role in the world, ecologically, anthropologically and economically.

Via ecosystem services, floodplains are of massive economic importance to human beings. It is estimated that out of the 2×10⁶ km² area that floodplains cover in the world, we get close to $3920 billion! This figure does not include the value of grazing land and groundwater recharge they provide. Add that, and the number grows to around $4500 billion. 

Floodplains are technically a safeguard for the areas beyond the reaches of the river. If and when a river floods, the excess water gets contained in the floodplain itself. It keeps it from overflowing beyond the floodplain, and protecting the life and property there. Floodplains nature’s way of controlling floods.

They form because of the sediments deposited by the river as it flows to the mouth of the sea. These sediments are extremely porous; the allow water to percolate into the ground. In the process, they generate a complex system of groundwater environments and recharge aquifers. These aquifers can extend up to kilometers on either side of the river, and provide water to life in those reaches. The entire Northern Plains, for example, has an average depth to water table of 10 meters below ground level. The population along this stretch primarily meets its water needs by extracting ground water. Up to 60% of the water used for agriculture is extracted from the groundwater recharged by the rivers in the floodplain (Shah et al. 2003).

Riverine floodplains are characterized by high biodiversity and productivity. More species of plants and animals occur in floodplains than any other landscape unit in most parts of the world. (Tockner and Stanford 2002). They are also called riparian ecosystems. This diversity arises from the unique conditions in the floodplain. It hosts an environment suitable for both aquatic and terrestrial life forms. The water bodies on the floodplain hosts the aquatic life, and the dry regions host terrestrial life forms. Further, many organisms that survive on both land and water (amphibians) live their lives on floodplains. In Switzerland, 10% of the entire fauna is restricted to river flood plains, although flood plains only cover 0.26% of the country’s surface!

High quality sediments along the floodplain makes it the most fertile regions of the world. Every major civilization from history to present date has been established in floodplains because agriculture can be easily practiced here.

So, the food you eat, the water you drink, the air you breathe; chances are that they have come to you directly or indirectly from a floodplain near you.

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References:

1. Bhowmik N.G, and J.B. Stall, 1979. Hydraulic geometry and carrying capacity of floodplains. University of Illinois Water Resources Center, Research Report 145 UI LU-WRC-79-0145, Champaign, Il. 147 p.
2. K. Tockner, J. A. Stanford, Environ. Conserv. 29, 308 (2002).