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.
4. Ob-Irtysh is a large basin you may never have heard of
Yet another of the world's most important basins in the Ob-Irtysh basin. It includes the watersheds of both the Ob and Irtysh rivers, hence the compound name, and covers large regions of the Russian Federation, Mongolia, Kazakhstan, and the People's Republic of China.
Of the two main rivers, the Ob is smaller and could be considered, on the grander scheme of things, actually a tributary for the larger Irtysh river. The basin covers an area of roughly 1.14 million square miles, or 3 million square kilometers, and is the foremost drainage basin in Asia. The Irtysh river's primary source lies in the Mongolian Altai in Dzungaria (just north of Xingjiang, PRC), close to the border of Mongolia.
The Ob river is one of the largest rivers in Asia and, when the Irtysh is included, forms the seventh-longest river system in the world. Both the Ob and Irtysh eventually dump their freshwater into the Kara Sea, a subsidiary of the Arctic Ocean, via the Gulf of Ob. This just so happens to be the world's longest estuary too.
With regards to the Ob, the largest city along its course is Novosibirsk, which is the most important city in all of Siberia. It is also the third-largest in Russia. Both the Ob and Irtysh are vitally important sources of freshwater for the region, and also provide very important economic activities including energy generation, agriculture, and freight transport.
Historically, the main rivers of the basin have been contaminated with nuclear waste dumped from Soviet nuclear testing that has, sadly, affected native plants, animals, and humans throughout the region.
5. The Rio de la Plata Basin is another vitally important river basin
Moving over to South America again, another of the world's most important drainage basins is the Rio de la Plata Basin. More commonly known as the River Plate Basin, it covers an area of roughly 1.2 million square miles (3.1 million square kilometers), this watershed gets its name from the main drainage river for the region, the mighty Rio de la Plata ("River of Silver", or "Silver River" in Spanish).
The basin drained by the main tributaries of the Río de la Plata (Uruguay, Paraná, and Paraguay) covers approximately one-fifth of South America. Where the rivers join, they form the widest estuary in the world, which drains into the Atlantic Ocean.
The basin is bounded by the Brazilian highlands to the north and Andes Mountains to the West. The entire system drains into the Atlantic Ocean at a rate between 15 and 26 thousand cubic meters per second.
The entire basin also acts as an important recharge zone of the Guarani Aquifer - one of the largest of its kind in the world. Like the vast majority of major watersheds and basins around the world, its waters are tapped by various human activities including drinking water, agricultural irrigation, power generation, etc.
It stretches over various South American countries including, Bolivia, Brazil, Paraguay, Uruguay, and Argentina. Second, only in size to the Amazon Basin in South America, it covers roughly a quarter of the entire landmass of the continent.
From the conflicts of the Spanish and Portuguese conquests to more modern naval conflicts like the famous "Battle of the River Plate", the region is, sadly, more than familiar with human conflict.
6. The Yenisey Basin is the largest that drains into the Arctic Ocean
The Yenisey, or the Yenisei, is another of the world's largest and most important basins. Situated across the Russian Federation and Mongolia, its main river, the Yenisey, is the largest of the major Siberian rivers.
The basin covers an area of about 965 thousand square miles or 2.5 million square kilometers. The Yenisey rises in northern Mongolia and is fed by several other large tributary rivers like the Angara, Podkamennaya Tunguska, and Nizhnyaya Tunguska. The entire basin, via the Yenisey, drains into the Yenisey Gulf in the Kara Sea.
A major and important feature of the upper part of the Yenisei River Basin is Lake Baikal, considered the deepest and oldest lake in the world.
The waters of the basin include many species of fish, some of which are unique to the region like the gobionine cyprinid and grayling. The region is also home to the Taimyr reindeer herd which is, by all accounts, the largest of its kind in the world.
This herd migrates to winter grazing grounds along the Yenisey every year and is a real sight to behold on the move.
7. The Amur, or Heilong Jiang basin, is home to some of the world's most endangered big cats
Yet another of the world's most important basins is the Amur, or Heilong Jiang, river basin. The former is the Russian name for the river and the latter is modern Chinese for "Black Dragon River".
Stretching across the borders of the People's Republic of China, the Russian Federation, and Mongolia, its main drainage river, the Amur/Heilong is the world's tenth longest river.
This river forms the main boundary between China and Russia, and the entire basin has an area of around 716 thousand square miles or 1.85 million square kilometers.
The basin is home to some interesting endemic lifeforms including, but not limited to, the Kaluga (a large predatory species of sturgeon), northern snakehead, yellow cheek fish, and softshell turtle. The Kaluga can reach lengths in excess of 18 feet (5.6 meters).
The river, by which the watershed derives its name, is historically very important for the region and is an important symbol of Chinese-Russian relations stretching back centuries. For example, it played a role in border conflicts during the Sino-Soviet split between the 1950s and 1960s, when ideological differences led to serious degradation in relations between the two nations.
The basin is also home to some endangered species of big cats like the Amur tiger and Amur leopard. According to the World Wide Fund for Nature (WWF), the incredibly biologically diverse region is under increasing pressure from economic development and illegal wildlife production.
8. The Yangtze river basin is one of the world's biological treasure troves
Famed for its enormous biodiversity, the Yangtze river basin is another of the world's largest and most important basins. Named after the mighty river Yangtze (the world's third longest river, and the longest in Asia), the entire basin covers an area of roughly 700 thousand square miles. To put that into perspective, that is around 4 times the size of California!
The Yangtze rises on the Tibetan plateau, and flows, more or less, due east until it reaches the East China Sea. The entire basin from which it is fed accounts for roughly one-fifth of the entire area of China, and is also home to something like a third of China's entire population.
Due to the river's enormous size and length, the river, and by extension the entire basin, has played a vitally important role in Chinese history and economics for millennia. For thousands of years, the river has been used as a source of potable water, industrial process, military use, and, of course, energy production.
The Three Gorges Dam, for example, is situated on the Yangtze and is the largest hydroelectric dam in the world. Interestingly, the area around where the dam is built has evidence of human habitation as long ago as 27,000 years!
The Yangtze delta, one of the largest in the world, is vitally important for China's economy today, accounting for around 20 percent of its entire gross domestic product (GDP).
9. The Huang He basin is also huge
And lastly on our list of notable global drainage basins, is the Huang He, or Yellow River, basin. Situated primarily in the People's Republic of China, its main river from which the basin derives its name is the second-longest in China and sixth-longest river in the world.
The basin covers an area of roughly 307 thousand square miles, about 795 thousand kilometers squared, and ultimately drains in the Bohai Sea by the Shandong Province city of Dongying. Waterways within the basin run generally west to east and cross various Chinese provinces including Qinghai, Gansu, Sichuan, Shaanxi, Ningxia, Shanxi, Henan, Shandong, Hebei, and some parts of the autonomous region of Inner Mongolia.
The Yellow River basin is not only fascinating from a geographical point of view but also historically, as it is thought to be the cradle of the ancient Chinese civilization.
The basin, and its rivers, are constantly plagued by floods that can be incredibly devastating. Some have even taken the lives of many millions of local inhabitants, including the 1332-33 flood during the Yuan Dynasty, the 1887 flood during the Qing Dynasty, and the 1931 flood during the Republic of China.
The floods are caused by the large amounts of fine-grained sediment carried by the river from the Loess plateau that constantly deposits itself at the bottom of the Yellow River's bed. It is, in fact, this high sediment content that gives the river its characteristic color and, hence, its name.
Previous flooding events have been so severe that they have changed the course of the river several times from all generally north-easterly, except during the Jin-Yuan and Ming-Qing dynasties between the 12th and 19th centuries when it flowed more due easterly. In more recent times, the threat from devastating flooding has been partially controlled through the construction of dams and better river management.
Why do we need to study watersheds?
We hope, by now, that you've gained some appreciation for the importance of watersheds to human civilization and nature. Without water, there can, quite literally, be no life, so it is crucial that we understand what watersheds are and how to reduce, as far as reasonably practicable, our impact on them.
Watersheds and drainage basins are vital for many foundational parts of modern human society from growing food, industry, drinking water, power generation, sanitation, etc. In fact, studying the health of a watershed is usually a good proxy for assessing how well managed the land of the watershed is.
If the water that flows through it is heavily polluted, for example, it usually indicates some very serious problem with some aspect of the ecosystem upstream. If the water has a high silt content after the rain this may indicate that the soil is eroding somewhere in the water basin that needs attention.
Rivers may contain high levels of chemical pollutants that, obviously, indicate someone, somewhere, is releasing large amounts of them upstream. If the discharge varies a lot in short intervals, then the runoff from the land's surface is probably not well managed and there may be improperly designed developments.
Watersheds in any region can have one, or more, of these issues, and for this reason, among others, it is vitally important for authorities to understand the "hows" and "whys" of watershed issues to better understand the watershed's ecology, or "ecological function". A well monitored and managed (as best humans can manage an enormous natural process) is good for not just human beings, but the ecosystem that supports our civilizations too.
If watersheds become too badly damaged, polluted, or disturbed by things like dams or artificial tapping from industry, farming, or energy generation it can, and will, have enormous unforeseen impacts that could culminate in events like floods or large water bodies, like lakes, completely drying up! This is not just hypothetical, it has happened in the past like, for example, the devastating impact of Soviet "planning" that led to the loss of the Aral Sea - once the fourth largest lake in the world.
We all, every one of us, depend on healthy watersheds, consciously or otherwise, and so all have a vested interest in ensuring they remain clean, healthy, and sustainable. For this reason, the study of watersheds is a mission-critical activity and offers ways to not only maintain or rehabilitate them but also look for ways to potentially improve them too.
Pure, clean water is not just important today but, if some experts are to be believed, will become increasingly so in the future.