Have you ever been to Assam? The point is that Brahmaputra has the requisite flow, gradient and speed (around 5 kilometres per hour) to not let the pollutants/effluents settle down, whereas Yamuna's average speed in non-monsoon phase is around 1.4 kilometres per hour, which allows the pollutants to settle down. Additionally, in 2 per cent length of Yamuna passing through Delhi, 75 per cent of the pollution load is added. Assam is just a Lilliput in front of Delhi in terms of environmental footprint produced by industries, population load, urbanisation, drainage issues, sewage, etc.
Therefore, making a small stretch way by the banks of Brahmaputra is no yeoman's task which Arvind Kejriwal couldn't accomplish. Arvind Kejriwal failed on many counts and this one has just been a continuation of the larger failure to deal with our dying rivers.