About half of the world's rivers don't flow all year round
Rivers seem permanent, yet roughly half of Earth's waterways stop flowing for part of the year, dried by desert seasons or locked in winter ice. Others keep running through droughts, fed from below by groundwater. And despite a common belief, rivers have no preference for flowing north to south; gravity alone decides.
Every drop in a river ultimately fell as precipitation. It arrives as rain running off slopes, as melting snow and glaciers, or by seeping in from aquifers wherever the channel sits lower than the water table, which is why some rivers survive dry spells. Late in summer, when the snow is gone, glacier melt can take over. All the uphill land feeding a river forms its drainage basin, and ridges separate one basin from the next: on the Continental Divide in the Rocky Mountains, rain on one side ends up in the Pacific and on the other in the Atlantic.
Rivers sculpt the land. Tributaries join at confluences; water cutting through hard rock carves valleys and, across plateaus, canyons. The sediment they carry, called alluvium, is sorted as it travels, heavy stones dropping first and silt going farthest, and it ends up in deltas, desert sand and bar islands. A river bends because any obstacle deflects the current against the opposite bank, which erodes into a curve and throws the flow back again. Floods spread mineral-rich silt over floodplains, feeding crops and wildlife.
Not all rivers are visible. Some flow through flooded caves in karst, where dissolving rock hollows out passages, and host microbes that fascinate researchers. Others have been buried by cities: Sunswick Creek in New York City was covered in the 1800s and now survives as a sewer-like pipe. Tidal rivers near estuaries rise and fall with the sea, their brackish water moving upstream or down with the tide.
Rivers sustained the first civilisations and still supply food, transport, drinking water and power from water wheels and dams. That engineering has a cost: dams, canals and levees have destroyed habitats, driven some species extinct and starved rivers of sediment, while shrinking snowfall under climate change leaves less summer flow. Pollution controls, sewage treatment and dam removal have helped some recover.
Source: River
