Rivers tend to flow downstream, but sometimes they don’t. Sometimes they flow sideways and sometimes they flow upstream! How can this happen?
I’ll come back to this in a moment, but first a little bit of physics, just to put things in perspective. Rivers flow because water falls, due to the effect of gravity. In fact, water in rivers will follow the quickest path downhill (the path of least resistance), normally, unless prevented by some barrier, like a big rock. Then the river either will back up (creating a pool behind the rock) until it flows over the rock or it will flow around the rock. Either way, the water will meander its way downhill, until it eventually meets a lake or the sea, or sometimes just disappears into the ground. Rivers in upland, mountain regions tend to flow fast, make lots of noise and froth a lot, because the slope of the land is steep. Mountain rivers tend also to have rocky bottoms and banks which limit the river to move sideways and widen. On the other hand, rivers in lowland regions tend to flow slowly, because the slope of the land is much less steep. These lowland rivers tend to be wider and deeper and their bottoms and banks are made of sand or clay and can be eroded much easier than rock. Mountain rivers, even after lots of rain, tend to be constrained within gorges of rock, and are only dangerous if you are caught in a gorge when the river is rising (which can happen really fast at times). When a lot of rainfall falls on lowland rivers or flood-waves move down into lowland river reaches, the lower slope here means that there are more ‘choices’ of which direction the river goes and at a certain point, the river will break its banks and head in all sorts of directions. Flooding in very flat country can extend for hundreds of kilometres. But I am getting off the point.
Rivers can flow upstream in several relatively unusual situations. The first is when a tributary stream is running slowly and the mainstem or parent river is rising (perhaps because of upstream releases from a dam or because of rainfall in the upper catchment of the river and not the tributary) so that it ‘backs up’ into the tributary and flows upstream. Imagine holding a clear plastic tube with one end slightly lower than the other. If you added water to the higher end, it would flow out of the lower end. Now imagine if you lowered the same tube (with the ends at different heights) into a bath filled with water. Even with the tube inclined, the bath water will run up the tube, as if water is running uphill. This usually only happens for a short time and is often not very dramatic.
The second situation is when upstream irrigation pumps water at a great rate from a river that is only flowing slowly. In this situation, it is like someone sucking a drink up in a straw: if there is enough pressure, the river can truly flow upstream. Until it reaches the pump, of course.
A third example of upstream river flow is when rivers meet tidal estuaries. In this case, when tides are rising (flood tides), the water can move upstream if the tidal pulse is more substantial than the river flow. One of the most dramatic examples of this is in rivers, like the River Severn in the UK, where the shape of the estuary and river are such that the rising tide is funnelled into a smaller and smaller channel, creating an upstream-moving wave (sometimes 2 m high), which travels at quite a pace for up to 28 km upstream against the current (called ‘bores’).
Source: Project Britain, the River Severn
People actually surf the Severn ‘Bore’! The best time is spring, when the tides tend to be highest. This phenomenon is not uncommon around the world, but occurs only where there are unusually large tides and the right shaped estuaries and rivers.
So, for every generalisation, there is an exception. Rivers flow downstream…mostly.
For more information on the Severn and other tidal bores: Rowbotham, F. 1970. “The Severn Bore”, David and Charles; Lynch, D.K. 1982. Tidal bores. Scientific American, 247 October, pp. 134-143; Witts, C. 1999. “The might Severn Bore”, Rivern Severn Publications, Gloucester, UK. Or check out the Tidal Bore Research Society’s website.