You can make a good approximation of the absolute age of a "river" by determining the absolute age of the deepest river deposits in the specific basin.

In the case you have two rivers and you want to determine the relative age, you can also use the ages of the deposits. Additionaly you can evaluate the possible lithology of  each river-branch. For example, you will have entirely different lithology of the deposits, if the source rock is different.

I hope I helped you

Regards

Ioannis Alexandridis 

Ordering  and/or numbering of streams in any basin can be used to determine which river is younger and which one is older...e.g. ist order unbranched streams are older thant 2nd order and 3rd order are younger than 2nd order stream and so on...second point is in analyzing radiogeneic carbon (C-14) of water running in any stream,rivers and canals you will be able to determine its relative (not absolute age) age.However this is limited to "water age" and  not inclue" channel age" itself

You can make a good approximation of the absolute age of a "river" by determining the absolute age of the deepest river deposits in the specific basin.

In the case you have two rivers and you want to determine the relative age, you can also use the ages of the deposits. Additionaly you can evaluate the possible lithology of  each river-branch. For example, you will have entirely different lithology of the deposits, if the source rock is different.

I hope I helped you

Regards

Ioannis Alexandridis 

Thank you very much dear Ioannis T. Alexandridis. Will you please elaborate your first point? whether the deepest river deposits from among the exposed sections in a river basin or using some geophysical method, one has to dig to the deepest possible level to find that deepest river deposit? Do you have any published work in this connection?

In order to determine absolute ages of rivers (deposits), often absolute dating techniques such as C14-dating or OSL/IRSL-dating can be used. Of course depending on estimated age and lithology.

In order to determine relative ages you often can use cross-cut relationships i.e. which river dissects what other river and is thus younger.

The exact answer really depends on the right question. 

Moreover, deepest river deposits are not necessarily the oldest, since incision might occur in the younger branch/system.

Dear  Michael Geoffray, about the elaboration of my first point, I agree with Jan. Additionally, about the question, which is the oldest deposit, it could be indeed outcroped in the surface, but also can be burried in it's original depositional level. A detailed geological mapping with detailed lithology observations combined with geophysical data, or well-log data can help you to distinguish the lowest-oldest deposit.

You could be more sure about the deepest deposit, if this one be systematically and not occasionally observed to be the deepest one.

The method for the determination of the absolute age, is your choise.

About my second point, Jan maybe pointed it out more clearly, but we suggest the same thought.

Dear Jan, your final comment about the deepest deposit is right of course, thus my first answer starts with the term "approximation".

Best regards

Ioannis Alexandridis

Dear Jan Peeters, is there anything wrong with the words in the above question asked by me? I think, you have grasped what I wanted to ask, therefore, please suggest right form for my question so that one one else can question the clarity of the question (it's a humble request, don't take it otherwise please).

By using radiocarbon dating of fluvial archives you get reliable proxies up to around 50,000 years ago (Late Pleistocene). From my point of view the age of a river is determined by the age of the mountains it dissects. However, the river course may have changed and even the flow direction, which is probably the case of the Amazon river.

To assess the problem I think that it is necessary to differentiate between age of the valley or channel from the age of the flow of water now running on the channel. I think you want to find out the age of a sector of the river or the Valley, since the full course of a river is the result of very different processes that have happened in different times and on different rocky substrates. In principle, a channel or a Valley, are younger than younger strata cutting. But there are two problems: on the one hand, this claim is true for the channel, but not for the Valley, which as depression may precede the river and the formation of the channel; on the other hand, this detail only can be seen when it fits in differentiated sediment, however when we face massive outcrops as it is the case of granitic batholiths, this simple assessment is not applicable. Perhaps knowing the chemical elements which transports water, or which could transport in origin, according to their nearest food sources, and its way of reacting with rocks of the riverbed, by isotopic analysis, the age could be assess when water began to flow and opened the channel.

If what is find out is the age of the water flow, is needed to determine the age of the springs that feed the sector and/or of the pass that facilitated the entrance of water to the depression in which now it flows..

Finally, and definitively, in the river there are fossils. They are fossils of plants and animals that can relate to climate changes and species in each period. I think it seem to provide the most reliable way to determine the age of the flow and the channel.

From the point of view of the sediments, terraces and hanging deposits are indicators of the existence of a course whose age is determined by higher deposits. As regards deposits deep, considered as witnesses of antiquity I do not totally agree, may be in some cases, but may also respond to previous courses or to local processes.

For details, see John S Bridge (2003): Rivers and flood plains. Forms, Processes, and Sedimentary Record. Blackwell publishing. See pp 328 ahead. Chapter 9, 10

Dear Luis Vicente Garcia Merino, Sir,

Thank you very much for your answer. Your clear answer helped me a lot. Of course my query is concerned with the age of the river channel through which water flows. In my question above, I wanted to know what is the age of the Ganga River channel (by the Ganga river channel I meant, channel of the river from Gangotri to Bay of Bengal) and whether one of its right hand tributaries which flows through terrain relatively older than the Himalayas and  the Ganga Basin, is older in age than the Ganga River channel (of which it, the Son River, is a tributary)?

The river channels are formed as soon as water starts flowing from the Himalayan Ranges, the 50 million years ago, Ganga's recorded flow is since last 21000 years ago (BP), see http://www.iitk.ac.in/infocell/iitk/newhtml/storyoftheweek51.htm 

For finding the current path and the shifts of the river channel you need to perform the bank studies, boulders/sediments aging studies and morphometric analysis of the basin. The quoted article must answer your basic question.