Why is Mississippi so backwards

Can rivers flow backwards?

On July 26, 2006 it was that time again: the Havel changed its course - and suddenly flowed backwards between the cities of Brandenburg and Rathenow. The drought in Brandenburg, which lasted several weeks, caused some of its tributaries to partially dry up and the water table to drop considerably. Instead of the usual 58 cubic meters of water per second in the direction of the Elbe, suddenly only twelve were moving towards the source.

But it is not just the lack of precipitation that is to blame for this ghostly journey, although that summer - at least until the beginning of August - it rained very little locally there. More important is the low difference in altitude that the river overcomes between its source in the M├╝ritz National Park Mecklenburg-Western Pomerania and the confluence with the Elbe in Saxony-Anhalt: it is only 40.6 meters over a distance of about 325 kilometers. The Havel is thus a typical lowland river that gently meanders through its wide floodplains at a mostly very low speed. If the amount of water in it is too low, the electricity is sometimes still, as happened at times on the Spree in the Spreewald in the summer of 2006.

This article is contained in Spectrum - Die Woche, August 26, 2006

On the Havel, however, there was also a special effect: so that the Elbe-Havel Canal remained navigable despite the general lack of water, more water was withdrawn from the river than was beneficial - only because of this measure was the reversal perfect.

However, there are also cases in which the flow direction rotates at least temporarily and in sections for natural reasons. A well-known example concerns the Amazon, whose enormous water masses seem to push steadily into the Atlantic year in and year out. During full and new moon, when the sun, moon and earth are in a row, so-called spring tides also push water masses from the sea into the mouth of the jungle stream twice a day. A wave called "Pororoca" then builds up to a height of four meters and rolls upstream against the current. It reaches top speeds of up to 50 kilometers per hour and, depending on the level in the Amazon and depending on the strength of the spring tide, runs up to 300 kilometers inland.

The wave heralds its arrival with an infernal noise that the local Tupi Indians call "poroc-poroc" - hence the name. Warned by the noise, they withdraw from the river, because the power of the Pororoca uproots trees, undermines river banks and can also sweep away huts. Some daring surfers use it today as a permanent surf and sometimes ride it for miles, but they always run the risk of being bitten by snakes or injured by drifting tree trunks.

Incidentally, similar phenomena occur in all parts of the world. The greatest superficial reversal of the current, for example, chases up the Chinese river Qiantang: It can reach a height of up to 8.9 meters and accelerates at up to 40 kilometers per hour. In Severn, UK, the phenomenon can be observed 250 times a year. And in Lupar Benak, Malaysia, in 1949, the English writer Somerset Maugham almost drowned when he ventured too close to the waters while visiting the local Rajah.