Watersheds and river basins: Here's why they are vitally important

A proper understanding of watersheds and basins is mission-critical.

By Christopher McFadden
Mar 26, 2022

Earth is unique in the Solar System for being the only planet with liquid water on its surface. And this water is not isolated to just little pockets here and there - it is literally everywhere in some form on Earth's surface.

What's more, this water doesn't just sit around doing nothing - it is constantly moving and changing state. This cycle, called the water cycle, involves the continuous movement of water between the Earth's oceans, atmosphere, and land.

Even on land, water is constantly moving and cycling. Some of this involves watersheds, areas of land that contain streams and rivers, which all drain into the same larger body of water, such as a large river, lake, or ocean.

But what exactly are watersheds and how do they work? Let's find out.

What is a watershed?

Watersheds are areas of land that feed major water bodies like streams, rivers, lakes, and, of course, the ocean. In fact, all lands on Earth are part of one watershed system or another. Each part of a watershed, in essence, feeds water, to a greater or lesser extent, into some common point or points.

By way of example, take the mighty Mississippi River. The Mississippi flows some 2,350 miles from its source at Lake Itasca through the center of the continental United States to the Gulf of Mexico. This river is fed by an enormous watershed that contains all the smaller tributaries that drain water into the river. Altogether, the Mississippi River drainage basin (which is often said to include the Missouri and Ohio Rivers) is the fourth largest drainage basin in the world.

All bodies of water have to be replenished in some form or another or they will, over time, simply dry up. This is why watersheds, otherwise known as catchment areas, are so important for many aspects of life on this planet.

Moreover, watersheds come in many shapes and sizes and are not isolated entities in their own right. A drainage basin is the area of land drained by a river. A watershed is the area surrounding a drainage basin and often marks the boundary between two drainage basins. A drainage basin may in fact consist of a large number of watersheds, which together form a large and complex web.

Minnesota, for example, contains eight major drainage basins and 81 major watersheds. Some of these extend into other states or countries (Canada). From this, you can see that proper watershed management not only affects local communities, but can have wide-ranging implications for residents of other areas, and even other nations.

What are the 5 major types of watersheds?

Watersheds can be classified depending on their relative size, drainage, shape, soil type, climate, and land use pattern.

In terms of size, watersheds tend to fall into one of the following main types:

1. A macro watershed - these are the largest of all and tend to have an area in excess of 50,000 hectares or 500 km2.

2. Sub-watersheds - these are the next largest and tend to fall between 10,000 and 50,000 hectares, or between 100 and 500 km2.

3. Milli-watersheds - these are watersheds with areas between 1,000 hectares (10km2) and 10,000 (100km2) hectares.

4. Micro watersheds- These are the second smallest kind and tend to have an area between 100 (1 km2) and 1000 hectares (10 km2).

5. Mini watersheds - These are the smallest kind and tend to be those with areas less than 100 hectares (1 km2).

What are the main components of watershed management?

As you can appreciate, watersheds are vitally important for all life on land. For this reason, proper management of them is incredibly important for both nature and human civilization.

To this end, most nations will operate some form of watershed management program, but the level of sophistication and regulation can vary widely. However, in most cases, watershed management will encompass some or all of the following:-

Good watershed management will attempt to control runoff and degradation to conserve soil and water as far as reasonably practicable. This is in part to protect natural habitats, but also to maximize the production of human activities on the land, like farming.
--It will also attempt to manage and utilize any runoff water for useful purposes such as irrigation or, indeed, drought prevention.
--They will attempt to protect, conserve, and improve watershed land to maximize efficiency and sustainable production.
--The system will also try to protect and enhance the water resources that come from the watershed.
--Soil erosion will be constantly monitored, with attempts made to reduce the effect of sediment yield on the watershed.
--Any deteriorating lands will be rehabilitated at the earliest opportunity.
--It will have some policy for moderating the peak flood flows at downstream areas.
--Strategies will be found and developed to increase the infiltration of rainwater.
--It will attempt to improve and increase the production of timbers, fodder, and wildlife resource.
----It will also attempt to enhance the groundwater recharge, wherever applicable.

What are some examples of watersheds or basins?

We've already mentioned one of the most famous watersheds above, but there are many others around the world, as you can imagine. Of them, some of the most famous are as follows.

1. The Amazon basin is the lifeblood of the Amazon rainforest

The Amazon basin is the part of South America drained by the Amazon River and its tributaries. It covers an area in the region of something like 2.7 million square miles, or 7 million square kilometers, or about 40 percent of the South American continent. It includes a number of South American countries.

These include parts of Bolivia, Peru, Guyana, Ecuador, Columbia, Venezuela, and, of course, Brazil. Most of the area of the basin is also covered with the Amazon rainforest.

Several large rivers in their own right feed directly into the Amazon. Six of these tributaries—the Japurá (Caquetá in Colombia), Juruá, Madeira, Negro, Purus, and Xingu rivers—are each more than 1,000 miles (1,600 km) long, making them, technically speaking, major tributaries to the Amazon. Other rivers, like the Cassiquiare, form minor tributaries. In turn, each of these rivers has its own watersheds that, taken as a whole, form the entire Amazon basin.

The basin is so large that it has been estimated that somewhere in the region of 20 percent of all freshwater carried to the oceans comes from this basin alone. Another interesting fact is that the Amazon basin formerly flowed west to the Pacific Ocean until the Andes formed, causing the basin to flow eastward towards the Atlantic Ocean.

2. The Congo basin is also enormous

Another of the world's major watersheds is the Congo Basin. Located in Central Africa, sometimes also referred to as west equatorial Africa, it also contains one of the largest tropical rainforests in the world.

The Congo is the Earth's second-largest river by volume, and it drains an area of around 3.7 million square kilometers (1.4 million square miles). Much of the basin is covered by rich tropical rainforests and swamps. Like the Amazon basin, the Congo basin encompasses various African nations, including Cameroon, the Central African Republic, the Democratic Republic of Congo (DRC), Equatorial Guinea, and parts of the Republic of Congo and Gabon.

The tropical rainforest in the basin covers around 178 million hectares. It is home to many species of animals and plants, and the first human inhabitants of the area may have been "pygmies" whose isolation within the vast rainforests of the region has enabled some small enclave tribes to survive to this day relatively untouched by the outside world. The rainforests are also home to many endangered and rare animal species too like the okapi, bonobo chimp, Congo peafowl, and, of course, the western lowland gorilla.

The Congo Basin is vitally important for large parts of the region's agriculture and energy generation. Large parts of the input water for the basin come from the highlands of the East African Rift system which, in turn, feeds the Chambeshi River, the Uele, and Ubangi Rivers in the upper reaches, and the Lualaba River draining wetlands in the middle reaches.

The East African Rift is, geologically speaking, relatively young and, as a consequence, results in relatively large amounts of sediment load in many of the region's rivers. The entire region eventually dumps its enormous quantities of freshwater into the Gulf of Guinea and the Atlantic Ocean.

3. Another important global watershed is the Nile basin

Staying in continental Africa for a moment, another of the world's most important river systems is the Nile Basin. Located in Northern Africa, it is the third-largest drainage basin by area in the world.

Drained, primarily, by the Nile River, it is, arguably, one of the best known in the world. Covering an area of around 1.24 million square miles, or 3.2 million square kilometers, it crosses many arid and highly-populated regions of Africa. The Nile drains an area that includes Burundi, the Central African Republic, Chad, the Democratic Republic of the Congo, South Sudan, Sudan Egypt, Eritrea, Ethiopia, Kenya, Rwanda, Tanzania, and Uganda. The Basin supplies fresh water, electricity, and food fish for the residents of these countries.

Rising in the highlands, in the rivers that feed Lake Victoria and Lake Tana in northwestern Ethiopia, the basin ultimately drains into the Meditteranean Sea famously through the Nile delta.

The Nile River, the major river of the basin, is largely navigable along part of its course but is interrupted by both natural and artificial obstacles, like the Aswan Dam.

All in all, it is estimated that the basin supports somewhere in the region of 270 million people, or about 20 percent of the population of Africa. Some other estimates put the figure at closer to 40 percent.

...continued in Part 2