Is The Ganga River The Longest River In The Ganga Basin?

Seems like a strange question to ask? The Ganga is no doubt the most important north Indian river in terms of its cultural and religious significance but is it the longest?

A new study published in Current Science (open access) which measured the lengths of various river segments from headwaters to their confluence with the Ganga segment and further to the mouth, finds that it is not. The honor goes to the Tons-Yamuna segments:

The length and discharge data together suggest that there exists a river within the Ganga Basin which is longer than the Ganga River by at least 370 km (Table 3). This is the segment originating from the Banderpunch Mountains (i.e. Tons River). But, is this the main stem in the Ganga basin? It is well known that the main stem of a river sets the base level for its tributaries. Therefore, we measured incision by both the Ganga and the Yamuna rivers upstream of Allahabad up to 100 km. The results showed that the Yamuna River is more incised, thus setting the base level for the Ganga River (Figure 4). This analysis further strengthens the result of the present study that the river segment in the Ganga Basin from the Banderpunch Mountains (source point S8) is the main stem. The total length of the river comes to 2758 km up to its confluence with the Brahmaputra.

Not only is the Ton-Yamuna segment longer than the Alaknanda-Ganga segment up to Allahabad (confluence of Yamuna and Ganga) but the discharge of the Yamuna is significantly more than the Ganga at the confluence.  This is because of the contribution to the Yamuna of the large Chambal river draining the Pre-Cambrian heartland of India. Of interest too is that the Chambal discharges more water than the Yamuna at the confluence of these two rivers.

This kind of work is important because:

This finding has huge implications on the geomorphic study of the Ganga Basin rivers. It would mean that the HFR (the authors have named the newly calculated longest segment as the Himalayan Foreland River for scientific purposes only) sets the base level for the Ganga River. Since several relationships are worked out with the length of a river (e.g. basin area versus stream length, discharge versus stream length, grain size versus stream length, etc.), there is a need to re-evaluate these relationships for the Ganga Basin with this length. Further, changes in these relationships can affect the predictability of river response that can in turn influence any river-related planning in the Ganga Basin.

I have one additional  comment. The authors ask :

The results also raise an important question; in spite of greater length why has this segment not gained importance? This question remains unanswered and the only possible answer could be that rivers in the Ganga Basin attained their present set-up at a much later stage, e.g. the Yamuna River is suggested to have started flowing towards east only during Late Pleistocene.

What could they possibly mean by this? Are they talking  of cultural  significance of these rivers? We are now reasonably sure  that the Yamuna started flowing along its present course as early as 50,000 years ago (1 ,2) much before any of these north Indian rivers came to be deified.