I got caught in a downpour while climbing a mountain and was amazed to see, as I climbed down, the tiny trickles of water combine to form mini-streams, the dry rocks I had climbed up now hosted sizable waterfalls, and the path which I had followed to the mountain had become a small river.

But it got me wondering about my question, which is the reverse of this. Why, after a long period of dry weather, are many rivers still flowing? I live near a river and it seems that it takes over a week of dry weather before there's any noticeable drop in its level. How does it not simply run out of water? Is it simply that it takes very a long time for the water at its source and the sources of all its tributaries to reach the point I'm observing from?

It seems there's a constant supply of water when I would have expected the network of rivers to start draining, and the drying up starting from the source the moment the rain stopped.

  • Was the mountain located in a dry climate? – gerrit Mar 9 '17 at 16:30
  • @gerrit - Temperate climate, but I'm guessing that the streams on the mountain dried up soon after the rain stopped (I didn't stick around), but I only put that in to give a background on what prompted the question, my main question is about normal rivers. – colmde Mar 10 '17 at 9:48
up vote 6 down vote accepted

The rivers are generally fed by water that percolated into the ground, it takes longer for that water to work its way down to the stream so the streams and rivers keep flowing long after the initial rain. https://water.usgs.gov/edu/rivers-contain-groundwater.html

  • 1
    Excellent answer. Don't think I could say it in any cleaner or more concise way myself. A lot of water runs off mountains when it rains, but a good deal infiltrates and reemerges from springs along the mountainside after slowly working through the mountain. This plus any gradual snowmelt should be the two core reasons for why you see rivers run, even fairly near the source, for weeks/months with minimal rain. – JeopardyTempest Mar 9 '17 at 21:19
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    indeed, and the very few hard rock rivers that do exist are ephemeral for the very reason the poster considered. – John Mar 10 '17 at 1:59

A river's primary of source of water is precipitation, surface water run off, and shallow groundwater discharge within the drainage basin the river is located. The size of the drainage basin really determines if a river may run dry during drought conditions. Some river basins are so large that drought conditions are unlikely to persist over the entire basin at any given moment.

(See image drainage showing Mississippi River drainage ~1/2 of the US).

Mississippi River Drainage basin.

Colorado River Drainage basin Reference.

With the Colorado river, scarce rainfall and over-usage leads to a greatly diminished flow at discharge into the into the Gulf of California (Sea of Cortez).

  • 3
    While relevant in a broader context, I don't believe this explains the OPs observation, which was much more local. For example, even if the entire basin for a small river does not get rainfall for a month, it still won't run dry because of the large volume of water that may be stored underground. I don't have numbers and it's not my field, but I suspect the answer to the more local OP observation should be found in that aspect. – gerrit Mar 9 '17 at 16:32
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    Specifics for any given river will vary. For example, I know of a stream in eastern Wyoming near the Black Hills, SD that cuts down into the Madison Limestone and artisan groundwater flow is a significant source of river water. – Gary Kindel Mar 9 '17 at 17:28

Glaciars and snow in mountains act as natural dams for many rivers. That's why several rivers have an increase of their flow during hot times, when the snow melts and the river moves more water compared to other times of the year.
Since many mountains keep part of their snow and glaciars the whole year, the rivers that start there will have water for a long time.
Finally, snow that falls in mountains is by far more than the rain that falls in the valley below, so each rain you see in the valley is a storm up in the mountains.

In exchange, rivers that depends heavily on rainfall are more prone to dry once the rain stops.

  • Glaciers are not a major source of water in rivers in Africa & Australia. Africa only has 3 glaciers (Mount Kilimanjaro, Mount Kenya, and the Rwenzori), while Australia has no glaciers. – Fred Mar 10 '17 at 12:21
  • Indeed @Fred . Australia is a good example, most rivers in the australian desert have water only by rainfall, so they dry fast, while rivers in the south of Australia are abundant in water thanks to the mountains that keep the water as snow, and also these mountains force the clouds to leave part of their water there. – Santiago Mar 10 '17 at 12:31

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