Why does a river meet the sea?

Ocean currents

This time series of SeaWiFS recordings shows how the flow of the Amazon changes with the tide.

Strong phytoplankton growth, which is still visible despite the great distance from the river mouth, is the strongest evidence of the huge masses of water and suspended matter that the runoff carries with it. The nutrients present in the runoff and the decaying vegetation fertilize the surrounding water and favor the growth of pythoplankton, which leads to a high concentration of chlorophyll in the water. The seawater is diluted by the runoff, the salinity is reduced. A layer forms on the surface that absorbs heat because of the light-absorbing suspended matter. The combination of all these factors, as well as the speed and large amount of runoff, means that the river water mixes slowly with the seawater of the Atlantic, which is why its effects can be seen over a large area.

1) What do you think other drains would also flow mainly in a northerly direction? Why?

2) What could the knowledge of the direction of flow be useful for?

3) Look at the animation above. What could be the reason for the frequent changes in direction of the runoff within a year?

↓ Click here for answers. ↑ Click here to hide the answer notes.
  1. No, the direction of other discharges is very likely to be influenced by the respective local currents.

    Try to find other large rivers and determine the direction of their drainage.

  2. Knowing the direction of the runoff could help track debris entering the river or find plentiful fishing grounds.

    Do you know any other reasons? (There are quite a few.)

  3. During the year, the direction of discharge changes as frequently as the changes in currents, trade winds and tides. The volume of the runoff also affects the direction; for example, when the river is in high water, the fresh water will advance further east.

MODIS AQUA recordings. On the left the chlorophyll concentration, on the right the discharge plume in real colors. Areas with the highest concentrations of chlorophyll are yellow and correspond to the runoff plume from the Amazon Delta.
Source: NASA Earth Observatory