"What Is The Ideal Existence for Our Rivers and Oceans?" The Role of Rivers as a Passage Linking Land and Sea

An amazing 38 trillion cubic meters of river water flows into our oceans each year, almost all of which is rainwater, created through the evaporation of our seas. Through the continual circulation of this huge amount of water from land to sea, the many problems carried by rivers are swept straight out to sea without resolve, seriously affecting our oceans and their resources. To return our oceans to the former pleasure of yesteryear is going to require a serious reconsideration of the relationship between them and our rivers.

1. Rivers Fostered by the Ocean

Generally speaking, where there is land there are rivers, and the water in them flows in one direction from land out to sea. It is no secret that water runs from high places to lowlands and therefore painfully obvious that river water runs out to sea. However, as Table 1 points out, it must be remembered that more rain falls on land than water is evaporated from it, or in other words via the earth's atmosphere the oceans supply land with much of its water.
As Table 1 shows, a very large part of the water that exists on earth is seawater, and of the small amount of fresh water that does exist, most of it is in the form of ice and groundwater. Of the resources of fresh water that mankind can get close to, lakes and wetlands are only a tiny 0.007% of the total, and rivers and streams make up an even smaller proportion of just 0.00014% or a minute existence of only 2 trillion cubic meters (2×1012m3). However, when you view the situation from the perspective of the "water cycle" or the way water evaporates to form steam or clouds, is transformed back into rain, and then returns to land or the ocean's surface, the picture dramatically changes. As you can see in Diagram 1, the total amount of water evaporated from earth per year is 496 trillion m3, 425 trillion m3 from the ocean and 71 trillion m3from land, but this is 38 times the value of the existing water in the atmosphere, listed in Table 1 as 13 trillion m3. This therefore means that the water in the atmosphere is completely renewed every 10 days.
Similarly, in contrast to the 2 trillion m3 of water that exists in rivers and streams, 19 times that amount of water, or 38 trillion m3 as shown in Diagram 1, flow out to sea per year, meaning that the water in rivers and streams is on average totally renewed approximately every 20 days.
Although the amount of water in the atmosphere or in rivers and streams is only a very small portion of the total in existence, the most important thing to remember about fresh water resources is the way they are always being replaced. Furthermore, if you retrace the origin of the water that ensures the flow of river water from land out to sea, you will find that it is mostly supplied from the oceans by evaporation.

Diagram 1(Unit:Trillion m3per year)

Illustration by Shuichi Furuoka

2. Rivers as a Passage Linking Land and Sea

Ultimately aiming to exit out into the sea, river water flows from places of high altitude down to those of lower ones under the force of gravity. However, through this process deposits that have fallen away from the terrain in the river basin or chemical substances and waste material created by organic matter or human activity are up taken by the flow and are either dissolved into the water or taken out to sea. With the help of gravity, rivers play the role of a passageway that carries not only water from land, but also sediment and many other substances.
In the workings of rivers that are recently under so much attention, there is also the role of a natural passage for living matter. Of course, since historic times it has been well known that fish come and go between rivers and sea, but we now also know that many plants, birds, insects, frogs and many other larger scale mammals also depend on the rivers as a passageway that links mountain and ocean. In other words for living matter, rivers are the roads that connect the sea with inland regions.
Humans too, use rivers as passageways. Recently, rising expectation for the revival of the river shipping industry, which was degenerated by the road and rail transport industries in the last century, is becoming eminent as the world's attention turns towards efficient energy consumption. Such transportation was very popular during the Meiji Era only 100 years ago, but for reasons such as the current of rivers being too strong, the depth being too shallow or the possible transportation distance being too short, the existence of river shipping fell by the wayside. Today, although there maybe a question mark surrounding the future of commercial river transportation, the revival of river shipping for excursion and tourist purposes is definitely something that should be considered, especially as there are advantages in low energy consumption.

3. River Mouths and Tidal Land

Japan has complex topographic features and very intricate coastlines. The total length of Japan's coastline extends to more than 35,000 kilometers, which far exceeds the 20,000-kilometer length of that of the United States. Furthermore, because Japan has many open and enclosed bays, inland seas and islands that shut out the external forces of tidal currents and ocean waves, many of its waters are very gentle, and moreover, due to the constant outflow of sediment from its rivers, are also relatively shallow.
There are many semi-land regions in Japan that take on the form of tidal lands during low tide, providing the perfect habitat for living matter such as crabs and shellfish, and also excellent breeding grounds in the surrounding shallow waters for the production of seaweed, oysters, scallops and the like. Again, the estuarine waters created by the inflow of freshwater into that of saltwater at river mouths, also provide the environment for other unique living matter such as Japanese Shijimi (a variety of small shellfish).
In recent years, the importance of such tidal lands and river mouths has been strongly advocated from the environmental protection prospective and development projects such as the Isahaya reclamation on the coast of the Ariake Sea, the waste treatment plant planned for the Fujimae Tidal Flats in Nagoya Port and the land filling of the Sanbanze Tidal Lands in Ichikawa City of Tokyo Bay have been aborted and strong public debate on such issues continues.

4. Coastal Erosion and the Retreat of Japan's Coastline

Our coastlines, the line of tangency between land and sea, contain a variety of forms such as reefs, sea cliffs, the muddy shores of enclosed bays and the manmade coastlines of harbors and fishing ports. However, most people associate coastlines with flowing white sand and green palm trees. Different from the slimy character of tidal lands, many of Japan's coastlines used to be a precious place where clean and beautiful swimming areas existed and you could always lie down and get the feel of that beach that everyone associates in their minds.
Unfortunately, these beaches are being abruptly lost through a reduction in sand supply, which has resulted from the construction of sediment controls and dams, and ironically also because of the negative effects of seawalls, that were actually constructed to prevent erosion. There is even data that suggests that 10,000 kilometers of Japanese beach coastline has on average retreated 8 meters in the last 100 years since the Meiji era. In response to this, the flushing out of sediment accumulated in storage reservoirs with the discharge from dams, the construction of slit structures in sediment controls that allow the accumulation of rocks but don't disturb the flow of downstream gravel or sand, which therefore doesn't allow deposits to accumulate, and other similar efforts are presently being made.

5. Restoring the Relationship Between River and Sea to What It Should Be

In the new millennium, a huge effort is required to restore the distorted relationship between river and ocean to its closest natural form. In other words we need to 1. Secure the supply of sand into coastal regions, plan the restoration of beaches with white sand and green trees and increase the opportunities for humans to interact with the ocean in beach areas. 2. Preserve our natural tidal lands and river mouths as much as possible and create some way of making our manmade zones more natural. 3. Make further efforts to improve the water quality of rivers and prevent the dumping and drifting of waste flowing out to sea, in order to clean up the pollution along the coastline and in the coastal zones of Japan.