A river’s life has one purpose: Find a way to flow to the sea with the least amount of effort. This condition of “least effort” is the river’s equilibrium state. Often, this state is not achieved for hundreds or thousands of years since the origination of the river…
Any river in the world starts at a high elevation. From there, they follow the elevation gradient and eventually reach 0 elevation; the sea level. On the way, a river does a lot of work. In the upper reaches of the river, it has high kinetic energy.
The river is young, it has a fair amount of water with it and it has the help of gravity. The river rushes down, forming “rapids” in many places. It also erodes it’s path heavily, creating hills on it’s sides and valleys in it’s path. It travels down steeply, covering short distances for a fall in unit elevation.
Once it reaches a more level plain (as the elevation gradient decreases), the river tends to cool off. It has carried a lot of eroded material with it, and since it doesn’t have the energy (or gravity helping it) to carry the load, it begins to drop it off. This creates the floodplain of that river. As the elevation gradient further decreases, the river moves slowly, and meanders. It travels gently, covering a large distance for every unit fall in elevation.
Eventually, the river meets the sea, and this is around the time when the river is the calmest. It doesn’t erode at all, only deposits sediments. It has literally no energy left, and like an old man embracing death, it meets the good ol’ sea.
This is the equilibrium state of a river. Graphically, we track the path of the river with distance traveled against fall in elevation. The ideal river profile, when it reaches equilibrium state, looks like this-
However, rivers are often not able to reach this state. If they do, they don’t stay in this state for very long. Because there a lot of other things at play here. For example, if the amount of water the river carries in the upstream increases suddenly (increased discharge), then the river would erode more, and there would be a sag in this curve. It would cut into the river bed, and even in sections where it generally travels more distance for fall in a unit elevation, it would travel much less distance. This is called river incision. On the profile, it would look like this-
Similarly, if there was, say a sudden increase in the elevation of the the river path in the middle of it’s path (where generally the slope is gentle), it creates a knick point. This knick point appears like a new origin for the river, right in the middle of it’s path to the sea. The river again begins to incise, and the situation looks something like this-
Fun fact: these knick points in between the channel are generally caused due to fault movement or changes in rock type. In natural conditions, it leads to the formation of waterfalls.
As the river erodes and incises into the channel, this knick point moves further and further backwards on the profile. That is because with erosion, the river eventually reaches it’s base level for that area. A base level is the theoretical level below which a river cannot erode. When discharge increases or a knick point arises, the base level changes, and that is when active erosion restarts. The profile where the knick point is shifting changes over time –
Finally, the knick point either reaches the origin, or it coincides with the base level and the river gets back to it’s equilibrium state.
Fun fact: Increase or decrease in sea level also alters the base level of a river. This also creates a knick point. If this happens, it takes decades for the knick point to reach the origin or coincide with a previous base level upstream.
What does a dam do?
Well, a dam also creates a knick point, because it drastically reduces the flow of water downstream from it.
But here’s where the dam differs from naturally created knick points: A dam creates a knick point that cannot move. So, once a dam is created, the river begins to erode and creates new base levels for itself. But since the knick point cannot move behind, the incision and erosion never stops. This is what the profile will look like in some time-
Basically, the dam can never attain it’s equilibrium that you saw in the first figure. It remains in the dynamic state. It could, however, achieve a pseudo-equilibrium if you consider the dam to be the “new” source of the river (as you can see, the curve from the knick point of the dam is similar to the curve in the ideal river profile). However, even that does not happen.
That is because the discharge from the dam’s reservoir is extremely unpredictable. During times or rain and/or overflow, the dam would release water and increase the stream discharge, flooding the floodplains. When there is shortage of water, it will not release any water and will lead to small streams and erosion.
Dams are an unnatural introduction in the river’s path and its floodplain. It creates changes that effects the river flow in time scales of a few decades, unlike natural changes which take centuries. This is one example in which the entire river flow takes a hit and how it can never be the same again, because there is a dam in the river path.
Can you imagine how the river profile would be if there were multiple dams in the river (for example, a stretch 36 km of the river Ganga has close to 15 dams or other structures obstructing it’s flow)?